Glossary of genetics (0–L)

This glossary of cell and molecular biology is a list of definitions of terms and concepts commonly used in the study of cell biology, molecular biology, and related disciplines, including genetics, microbiology, and biochemistry.[1] It is split across two articles:

This page, Glossary of genetics (0–L), lists terms beginning with numbers and with the letters A through L.
Glossary of genetics (M–Z) lists terms beginning with the letters M through Z.

The glossary is intended as introductory material for novices; for more specific and technical detail, see the article corresponding to each term. Overlapping and related glossaries include Glossary of evolutionary biology, Glossary of virology, and Glossary of chemistry.

0–9

3' untranslated region (3'-UTR)
3'-end: One of two ends of a single linear strand of DNA or RNA, specifically the end at which the chain of nucleotides terminates at the third carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 3' carbon is not attached to another nucleotide via a phosphodiester bond; in vivo, the 3' carbon is often still bonded to a hydroxyl group). By convention, sequences and structures positioned nearer to the 3'-end relative to others are referred to as downstream. Contrast 5'-end.
5' cap: A specially altered nucleotide attached to the 5'-end of some primary RNA transcripts as part of the set of post-transcriptional modifications which convert raw transcripts into mature RNA products. The precise structure of the 5' cap varies widely by organism; in eukaryotes, the most basic cap consists of a methylated guanine nucleoside bonded to the triphosphate group that terminates the 5'-end of an RNA sequence. Among other functions, capping helps to regulate the export of mature RNAs from the nucleus, prevent their degradation by exonucleases, and promote translation in the cytoplasm. Mature mRNAs can also be decapped.
5' untranslated region (5'-UTR)
5-bromodeoxyuridine: See bromodeoxyuridine.
5'-end: One of two ends of a single linear strand of DNA or RNA, specifically the end at which the chain of nucleotides terminates at the fifth carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 5' carbon is not attached to another nucleotide via a phosphodiester bond; in vivo, the 5' carbon is often still bonded to a phosphate group). By convention, sequences and structures positioned nearer to the 5'-end relative to others are referred to as upstream. Contrast 3'-end.
5-methyluracil: See thymine.

A

A chromosome
acentric: (of a linear chromosome or chromosome fragment) Having no centromere.[2]
acetylation
acetyl-CoA: A molecule that participates in many biochemical reactions in protein, carbohydrate, and lipid metabolism, notably the citric acid cycle.
acetyltransferase
acrocentric: (of a linear chromosome or chromosome fragment) Having a centromere positioned very close to one end of the chromosome, as opposed to at the end or in the middle.[2]
action potential: The local change in voltage that occurs when the membrane potential of a specific location along the membrane of a cell rapidly depolarizes, such as when a nerve impulse is transmitted between neurons.
activation: See upregulation.
activator: A type of transcription factor that increases the transcription of a gene or set of genes. Most activators work by binding to a specific sequence located within or near an enhancer or promoter and facilitating the binding of RNA polymerase and other transcription machinery in the same region. See also coactivator; contrast repressor.
active site
active transport: Transport of a substance (such as a protein or drug) across a cell membrane against a concentration gradient. Unlike passive transport, active transport requires an expenditure of energy.
adenine (A): A purine nucleobase used as one of the four standard nucleobases in both DNA and RNA molecules. Adenine forms a base pair with thymine in DNA and with uracil in RNA.
adenosine (A): One of the four standard nucleosides used in RNA molecules, consisting of an adenine base with its N₉ nitrogen bonded to the C₁ carbon of a ribose sugar. Adenine bonded to deoxyribose is known as deoxyadenosine, which is the version used in DNA.
adenosine diphosphate (ADP)
adenosine monophosphate (AMP)
adenosine triphosphate (ATP): An organic compound derived from adenine that functions as the major source of energy for chemical reactions inside living cells. It is found in all forms of life and is often referred to as the "molecular currency" of intracellular energy transfer.
A-DNA: One of three main biologically active structural conformations of the DNA double helix, along with B-DNA and Z-DNA. The A-form helix has a right-handed twist with 11 base pairs per full turn, only slightly more compact than B-DNA, but its bases are sharply tilted with respect to the helical axis. It is often favored in dehydrated conditions and within sequences of consecutive purine nucleotides (e.g. GAAGGGGA); it is also the primary conformation adopted by double-stranded RNA and RNA-DNA hybrids.[3]
affected relative pair: Any pair of organisms which are related genetically and both affected by the same trait. For example, two cousins who both have blue eyes are an affected relative pair since they are both affected by the allele that codes for blue eyes.
allele: One of multiple alternative versions of an individual gene, each of which is a viable DNA sequence occupying a given position, or locus, on a chromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.
allosome: Any chromosome that differs from an ordinary autosome in size, form, or behavior and which is responsible for determining the sex of an organism. In humans, the X chromosome and the Y chromosome are sex chromosomes.
alpha helix (α-helix): A common structural motif in the secondary structures of proteins consisting of a right-handed helix conformation resulting from hydrogen bonding between amino acid residues which are not immediately adjacent to each other.
alternative splicing: A regulated phenomenon of eukaryotic gene expression in which specific exons or parts of exons from the same primary transcript are variably included within or removed from the final, mature messenger RNA transcript. A class of post-transcriptional modification, alternative splicing allows a single gene to code for multiple protein isoforms and greatly increases the diversity of proteins that can be produced by an individual genome. See also RNA splicing.
amber: One of three stop codons used in the standard genetic code; in RNA, it is specified by the nucleotide triplet UAG. The other two stop codons are named ochre and opal.
amino acid: Any of a class of organic compounds whose basic structural formula includes a central carbon atom bonded to amine and carboxyl functional groups and to a variable side chain. Out of nearly 500 known amino acids, a set of 20 are coded for by the standard genetic code and incorporated into long polymeric chains as the building blocks of peptides and hence of polypeptides and proteins. The specific sequences of amino acids in the polypeptide chains that form a protein are ultimately responsible for determining the protein's structure and function.
amino terminus: See N-terminus.
aminoacyl-tRNA synthetase: Any of a set of enzymes which catalyze the transesterification reaction that results in the attachment of a specific amino acid (or a precursor) to one of its cognate transfer RNA molecules, forming an aminoacyl-tRNA. Each of the 20 different amino acids used in the genetic code is recognized and attached by its own specific synthetase enzyme, and most synthetases are cognate to several different tRNAs according to their specific anticodons.
aminoacyl-tRNA (aa-tRNA): A transfer RNA to which a cognate amino acid is chemically bonded; i.e. the product of a transesterification reaction catalyzed by an aminoacyl-tRNA synthetase. Aminoacyl-tRNAs bind to the aminoacyl site of the ribosome during translation.
amplicon: Any DNA or RNA sequence or fragment that is the source and/or product of an amplification reaction. The term is most frequently used to describe the numerous copied fragments that are the products of the polymerase chain reaction or ligase chain reaction, though it may also refer to sequences that are amplified naturally within a genome, e.g. by gene duplication.
amplification: The replication of a biomolecule, in particular the production of one or more copies of a nucleic acid sequence, known as an amplicon, either naturally (e.g. by spontaneous duplications) or artificially (e.g. by PCR), and especially implying many repeated replication events resulting in thousands, millions, or billions of copies of the target sequence, which is then said to be amplified.
anaphase: The stage of mitosis and meiosis that occurs after metaphase and before telophase, when the replicated chromosomes are segregated and each of the sister chromatids are moved to opposite sides of the cell.
anaphase lag: The failure of one or more pairs of sister chromatids or homologous chromosomes to properly migrate to opposite sides of the cell during anaphase of mitosis or meiosis due to a defective spindle apparatus. Consequently, both daughter cells are aneuploid: one is missing one or more chromosomes (creating a monosomy) while the other has one or more extra copies of the same chromosomes (creating a polysomy).
aneucentric: (of a linear chromosome or chromosome fragment) Having an abnormal number of centromeres, i.e. more than one.[4]
aneuploidy: The condition of a cell or organism having an abnormal number of one or more particular chromosomes (but excluding abnormal numbers of complete sets of chromosomes, which instead is known as euploidy).
annealing: The hybridization of two single-stranded nucleic acid molecules containing complementary sequences, creating a double-stranded molecule with paired nucleobases. The term is used in particular to describe steps in certain laboratory techniques such as the polymerase chain reaction, where double-stranded DNA molecules are repeatedly denatured into single strands by heating and then exposed to cooler temperatures, causing the strands to reassociate with each other or with complementary primers. The exact temperature at which annealing occurs is strongly influenced by the length and specific sequence of the individual strands.
anticodon: A series of three consecutive nucleotides within a transfer RNA which complement the three nucleotides of a codon within an mRNA transcript. During translation, each tRNA recruited to the ribosome contains a single anticodon triplet that pairs with one or more complementary codons from the mRNA sequence, allowing each codon to specify a particular amino acid to be added to the growing peptide chain. Anticodons containing inosine in the first position are capable of pairing with more than one codon due to a phenomenon known as wobble base pairing.
antioncogene: A gene which helps to regulate cell growth and suppress tumors when functioning correctly, such that its absence or malfunction can result in uncontrolled cell growth and possibly cancer.[5] Compare oncogene.
antiparallel: The orientation of two strands of a double-stranded nucleic acid (and more generally any pair of biopolymers) which are parallel to each other but with opposite directionality. For example, the two complementary strands of a DNA molecule run side-by-side but in opposite directions, with one strand oriented 5'-to-3' and the other 3'-to-5'.
antisense: See template strand.
antisense RNA (asRNA): A single-stranded non-coding RNA molecule containing an antisense sequence that is complementary to a sense strand, such as a messenger RNA, with which it readily hybridizes, thereby inhibiting the sense strand's further activity (e.g. translation into protein). Many different classes of naturally occurring RNA such as siRNA function by this principle, making them potent gene silencers in various gene regulation mechanisms. Synthetic antisense RNA has also found widespread use in gene knockdown studies, and in practical applications such as antisense therapy.
anucleate
apoptosis: A highly regulated form of programmed cell death that occurs in multicellular organisms.
aptamer
artificial gene synthesis: A set of laboratory methods used in the de novo synthesis of a gene (or any other nucleic acid sequence) from free nucleotides, i.e. without relying on an existing template strand.
asynapsis: The failure of homologous chromosomes to properly pair with each other during meiosis.[4] Contrast synapsis and desynapsis.
attached X: A single monocentric chromosome containing two or more physically attached copies of the normal X chromosome as a result of either a natural internal duplication or any of a variety of genetic engineering methods. The resulting compound chromosome effectively carries two or more doses of all genes and sequences included on the X, yet functions in all other respects as a single chromosome, meaning that haploid 'XX' gametes (rather than the ordinary 'X' gametes) will be produced by meiosis and inherited by progeny. In mechanisms such as genic balance in which the sex of an organism is determined by the total dosage of X-linked genes, an abnormal 'XXY' zygote, fertilized by one XX gamete and one Y gamete, will develop into a female.
autosome: Any chromosome that is not an allosome and hence is not involved in the determination of the sex of an organism. Unlike the sex chromosomes, the autosomes in a diploid cell exist in pairs, with the members of each pair having the same structure, morphology, and genetic loci.
autozygote: A cell or organism that is homozygous for a locus at which the two homologous alleles are identical by descent, both having been derived from a single gene in a common ancestor.[4] Contrast allozygote.

B

B chromosome: Any supernumerary nuclear DNA molecule which is not a duplicate of nor homologous to any of the standard complement of normal "A" chromosomes comprising a genome. Typically very small and devoid of structural genes, B chromosomes are by definition not necessary for life. Though they occur naturally in many eukaryotic species, they are not stably inherited and thus vary widely in copy number even between closely related individuals.[4]
back mutation: A mutation that reverses the effect of a previous mutation which had inactivated a gene, thus restoring wild-type function.[6] See also reverse mutation.
backcrossing: The breeding of a hybrid organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of selective breeding, with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC.
bacterial artificial chromosome (BAC)
balancer chromosome
base: An abbreviation of nitrogenous base and nucleobase.
base pair (bp): A pair of two nucleobases on complementary DNA or RNA strands which are bonded to each other by hydrogen bonds. The ability of consecutive base pairs to stack one upon another contributes to the long-chain double helix structures observed in both double-stranded DNA and double-stranded RNA molecules.
baseline: A measure of the gene expression level of a gene or genes prior to a perturbation in an experiment, as in a negative control. Baseline expression may also refer to the expected or historical measure of expression for a gene.
basic local alignment search tool (BLAST): A computer algorithm widely used in bioinformatics for aligning and comparing primary biological sequence information such as the nucleotide sequences of DNA or RNA or the amino acid sequences of proteins. BLAST programs enable scientists to quickly check for homology between two or more sequences by directly comparing the nucleotides or amino acids present at each position within each sequence; a common use is to search for matches between a specific query sequence and a digital sequence database such as a genome library, with the program returning a list of sequences from the database which resemble the query sequence above a specified threshold of similarity. Such comparisons can permit the identification of an organism from an unknown sample or the inference of evolutionary relationships between genes, proteins, or species.
B-DNA: The "standard" or classical structural conformation of the DNA double helix in vivo, thought to represent an average of the various distinct conformations assumed by very long DNA molecules under physiological conditions.[3] The B-form double helix has a right-handed twist with a diameter of 23.7 ångströms and a pitch of 35.7 ångströms or about 10.5 base pairs per full turn, such that each nucleotide pair is rotated 36° around the helical axis with respect to its neighboring pairs. See also A-DNA and Z-DNA.
bidirectional replication: A common mechanism of DNA replication in which two replication forks move in opposite directions away from the same origin; this results in a bubble-like region where the duplex molecule is locally separated into two single strands.[4]
binary fission
binding site
bivalent
blot: Any of a variety of molecular biology methods by which electrophoretically or chromatographically resolved DNA, RNA, or protein are transferred from a support medium such as a polyacrylamide gel onto a carrier such as an immobilizing paper or membrane matrix. Some methods involve the transfer of molecules by capillary action (e.g. Southern and northern blotting), while others rely on the transport of charged molecules by electrophoresis (e.g. western blotting). The transferred molecules are then visualized by colorant staining, autoradiography, or the selective labelling of molecules containing specific sequences or epitopes with hybridization probes or antibodies bound to a chemiluminescent reporter.[4]
blunt end
bromodeoxyuridine (BUDR, BrdU)

C

cadastral gene: A regulatory gene that restricts the expression of other genes to specific tissues or body parts in an organism, typically by producing gene products which variably inhibit or permit transcription of the other genes in different cell types.[4] The term is used most commonly in plant genetics.
cadherin
Calvin cycle
candidate gene: A gene whose location on a chromosome is associated with a particular phenotype (often a disease-related phenotype), and which is therefore suspected of causing or contributing to the phenotype. Candidate genes are often selected for study based on a priori knowledge or speculation about their functional relevance to the trait or disease being researched.
canonical sequence: See consensus sequence.
carboxyl terminus: See C-terminus.
CCAAT box: A highly conserved regulatory DNA sequence located approximately 75 base pairs upstream (i.e. -75) of the site of the start of transcription for many eukaryotic genes.[2]
cDNA: See complementary DNA.
cell
cell biology: The branch of biology that studies the structures, functions, processes, and properties of biological cells, the self-contained units of life common to all living organisms.
cell compartmentalization: The subdivision of the interior of a cell into distinct, usually membrane-bound compartments, including the nucleus and organelles (endoplasmic reticulum, mitochondria, chloroplasts, intracellular vesicles, etc.), a defining feature of the Eukarya.[7]
cell counting
cell culture: The process by which living cells are grown or maintained under carefully controlled conditions, generally outside of their natural environment. Optimal growth conditions vary widely for different cell types but usually consist of a suitable vessel (e.g. a culture tube or Petri dish) containing a specifically formulated substrate or growth medium that supplies all of the nutrients essential for life (amino acids, carbohydrates, vitamins, minerals, etc.) plus any desirable growth factors and hormones, permits gas exchange (if necessary), and regulates the environment by maintaining consistent physico-chemical properties (e.g. pH, osmotic pressure, and temperature). Some cell types require a solid surface to which they can adhere in order to reproduce, while others can be grown freely floating in a liquid or gelatinous suspension. Most cells have a genetically determined reproduction limit, but immortalized cells will reproduce indefinitely if provided with optimal conditions.
cell cycle
cell division
cell fusion
cell membrane
cell signaling
cell wall
cell-free DNA (cfDNA): Any DNA molecule that exists outside of a cell or nucleus, freely floating in an extracellular fluid such as blood plasma.
cellular
cellular differentiation
cellular reprogramming: The conversion of a fully differentiated cell from one tissue-specific cell type to another. This involves dedifferentiation to a pluripotent state; an example is the conversion of mouse somatic cells to an undifferentiated embryonic state, which relies on the transcription factors Oct4, Sox2, Myc, and Klf4.[8]
centimorgan (cM): A unit for measuring genetic linkage defined as the distance between chromosomal loci for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. Though not an actual measure of physical distance, it is used to infer the actual distance between two loci based on the apparent likelihood of a crossover occurring between them.
central dogma of molecular biology: A generalized framework for understanding the flow of genetic information between macromolecules within biological systems. The central dogma outlines the fundamental principle that the sequence information encoded in the three major classes of biopolymer—DNA, RNA, and protein—can only be transferred between these three classes in certain ways, and not in others: specifically, information transfer between the nucleic acids and from nucleic acid to protein is possible, but transfer from protein to protein, or from protein back to either type of nucleic acid, is impossible and does not occur naturally.
centriole: A cylindrical organelle composed of microtubules, present only in certain eukaryotes. A pair of centrioles migrate to and define the two opposite poles of a dividing cell where, as part of a centrosome, they initiate the growth of the spindle apparatus.
centromere: A specialized DNA sequence within a chromosome that links a pair of sister chromatids. The primary function of the centromere is to act as the site of assembly for kinetochores, protein complexes which direct the attachment of spindle fibers to the centromere and facilitate segregation of the chromatids during mitosis or meiosis.
centromeric index: The proportion of the total length of a chromosome encompassed by its short arm, typically expressed as a percentage; e.g. a chromosome with a centromeric index of 15 is acrocentric, with a short arm comprising only 15% of its overall length.[4]
centrosome
cfDNA: See cell-free DNA.
Chargaff's rules: A set of axioms which state that, in the DNA of any chromosome, species, or organism, the total number of adenine (A) residues will be approximately equal to the total number of thymine (T) residues, and the number of guanine (G) residues will be equal to the number of cytosine (C) residues; accordingly, the total number of purines (A + G) will equal the total number of pyrimidines (T + C). These observations illustrate the highly specific nature of the complementary base-pairing that occurs in all duplex DNA molecules: even though non-standard pairings are technically possible, they are exceptionally rare because the standard ones are strongly favored in most conditions. Still, the 1:1 equivalence is seldom exact, since at any given time nucleobase ratios are inevitably distorted to some small degree by unrepaired mismatches, missing bases, and non-canonical bases. The presence of single-stranded DNA polymers also alters the proportions, as the sequence of an individual strand may contain any number of any of the bases.
charged tRNA: A transfer RNA to which an amino acid has been attached; i.e. an aminoacylated tRNA. Uncharged tRNAs lack amino acids.[4]
chDNA: See chloroplast DNA.
chiasma: A cross-shaped junction that forms the physical point of contact between two non-sister chromatids belonging to homologous chromosomes during synapsis. As well as ensuring proper segregation of the chromosomes, these junctions are also the breakpoints at which chromosomal crossover may occur during mitosis or meiosis, which results in the reciprocal exchange of DNA between the synapsed chromatids.
chimerism: The presence of two or more populations of cells with distinct genotypes in an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate zygotes; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from grafting or errors in cell division. It is similar to but distinct from mosaicism.
chloroplast
chloroplast DNA (cpDNA, chDNA, ctDNA): The set of DNA molecules contained within chloroplasts, a type of photosynthetic plastid organelle located within the cells of some eukaryotes such as plants and algae, representing a semi-autonomous genome separate from that within the cell's nucleus. Like other types of plastid DNA, cpDNA usually exists in the form of small circular plasmids.
chondriome: The complete set of mitochondria or of mitochondrial DNA within a cell, tissue, organism, or species.
chromatid: One copy of a newly copied chromosome, which is joined to the original chromosome by a centromere.
chromatin: A complex of DNA, RNA, and protein found in eukaryotic cells that is the primary substance comprising chromosomes. Chromatin functions as a means of packaging very long DNA molecules into highly organized and densely compacted shapes, which prevents the strands from becoming tangled, reinforces the DNA during cell division, helps to prevent DNA damage, and plays an important role in regulating gene expression and DNA replication.
chromatin immunoprecipitation (ChIP)
chromocenter: A central amorphous mass of polytene chromosomes found in the nuclei of cells of the salivary glands in Drosophila larvae and resulting from the fusion of heterochromatic regions surrounding the centromeres of the somatically paired chromosomes, with the distal euchromatic arms radiating outward.[4]
chromomere: A region of a chromosome that has been locally compacted or coiled into chromatin, conspicuous under a microscope as a "bead", node, or dark-staining band, especially when contrasted with nearby uncompacted strings of DNA.
chromosomal crossover
chromosomal duplication: The duplication of an entire chromosome, as opposed to a segment of a chromosome or an individual gene.
chromosome: A nuclear DNA molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within the nucleus of cells and, in most eukaryotes, are composed of long strands of DNA coiled with packaging proteins which bind to and condense the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur during cell division. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA.
chromosome condensation: The process by which eukaryotic chromosomes become shorter, thicker, denser, and more conspicuous under a microscope during prophase due to systemic coiling and supercoiling of chromatic strands of DNA in preparation for cell division.
chromosome segregation: The process by which sister chromatids or paired homologous chromosomes separate from each other and migrate to opposite sides of the dividing cell during mitosis or meiosis.
chromosome walking: See primer walking.
cilium
circular DNA: Any DNA molecule, single-stranded or double-stranded, which forms a continuous closed loop without ends; e.g. bacterial chromosomes, mitochondrial and plastid DNA, as well as many other varieties of extrachromosomal DNA, including plasmids and some viral DNA. Contrast linear DNA.
circulating tumor DNA (ctDNA): Any extracellular DNA fragments derived from tumor cells which are circulating freely in the bloodstream.
cis: On the same side; adjacent to; acting from the same molecule. Contrast trans.
cis-acting: Affecting a gene or sequence on the same nucleic acid molecule. A locus or sequence within a particular DNA molecule such as a chromosome is said to be cis-acting if it influences or acts upon other sequences located within short distances (i.e. physically nearby, usually but not necessarily downstream) on the same molecule or chromosome; or, in the broadest sense, if it influences or acts upon other sequences located anywhere (not necessarily within a short distance) on the same chromosome of a homologous pair. Cis-acting factors are often involved in the regulation of gene expression by acting to inhibit or to facilitate transcription. Contrast trans-acting.
cis-dominant mutation: A mutation occurring within a cis-regulatory element (such as an operator) which alters the functioning of a nearby gene or genes on the same chromosome. Cis-dominant mutations affect the expression of genes because they occur at sites that control transcription rather than within the genes themselves.
cisgenesis
cis-regulatory element (CRE): Any sequence or region of non-coding DNA which regulates the transcription of nearby genes (e.g. a promoter, operator, silencer, or enhancer), typically by serving as a binding site for one or more transcription factors. Contrast trans-regulatory.
cisterna: Any of a class of flattened, membrane-bound vesicles or saccules of the smooth and rough endoplasmic reticulum and the Golgi apparatus. By traveling through one or more cisternae, each of which contains a distinct set of enzymes, newly created proteins and polysaccharides undergo chemical modifications such as phosphorylation and glycosylation, which are used as packaging signals to direct their transport to specific destinations within the cell.[9]
cistron
citric acid cycle
classical genetics: The branch of genetics based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of molecular biology. Contrast molecular genetics.
clonal selection
cloning: The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of asexual reproduction, and cells that undergo mitosis produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in molecular cloning, copies of DNA fragments or other molecules.
closed chromatin: See heterochromatin.
coactivator: A type of coregulator that increases the expression of one or more genes by binding to an activator.
coding strand: The strand of a double-stranded DNA molecule whose nucleotide sequence corresponds directly to that of the RNA transcript produced during transcription (except that thymine bases are substituted with uracil bases in the RNA molecule). Though it is not itself transcribed, the coding strand is by convention the strand used when displaying a DNA sequence because of the direct analogy between its sequence and the codons of the RNA product. Contrast template strand; see also sense.
codon: A series of three consecutive nucleotides in a coding region of a nucleic acid sequence. Each of these triplets codes for a particular amino acid or stop signal during protein synthesis. DNA and RNA molecules are each written in a language using four "letters" (four different nucleobases), but the language used to construct proteins includes 20 "letters" (20 different amino acids). Codons provide the key that allows these two languages to be translated into each other. In general, each codon corresponds to a single amino acid (or stop signal). The full set of codons is called the genetic code.
codon usage bias: The preferential use of a particular codon to code for a particular amino acid rather than alternative codons that are synonymous for the same amino acid, as evidenced by differences between organisms in the frequencies of the synonymous codons occurring in their coding DNA. Because the genetic code is degenerate, most amino acids can be specified by multiple codons. Nevertheless, certain codons tend to be overrepresented (and others underrepresented) in different species.
coenocyte: A multinucleate mass of cytoplasm bounded by a cell wall and resulting from continuous cytoplasmic growth and repeated nuclear division without cytokinesis, found in some species of algae and fungi, e.g. Vaucheria and Physarum.[7]
coenzyme A
cofactor: Any non-protein organic compound that is bound to an enzyme. Cofactors are required for the initiation of catalysis.
comparative genomic hybridization (CGH)
complementarity: A property of nucleic acid biopolymers whereby two polymeric chains or "strands" aligned antiparallel to each other will tend to form base pairs consisting of hydrogen bonds between the individual nucleobases comprising each chain, with each type of nucleobase pairing almost exclusively with one other type of nucleobase; e.g. in double-stranded DNA molecules, A pairs only with T and C pairs only with G. Strands that are paired in such a way, and the bases themselves, are said to be complementary. The degree of complementarity between two strands strongly influences the stability of the duplex molecule; certain sequences may also be internally complementary, which can result in a single strand binding to itself. Complementarity is fundamental to the mechanisms governing DNA replication, transcription, and DNA repair.
complementary DNA (cDNA): DNA that is synthesized from a single-stranded RNA template (typically mRNA or miRNA) in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is produced both naturally by retroviruses and artificially in certain laboratory techniques, particularly molecular cloning. In bioinformatics, the term may also be used to refer to the sequence of an mRNA transcript expressed as its DNA coding strand counterpart (i.e. with thymine replacing uracil).
complementation
compound X: See attached X.
conditional expression: The controlled, inducible expression of a transgene, either in vitro or in vivo.
congression: The movement of chromosomes to the spindle equator during the prometaphase and metaphase stages of mitosis.[4]
consensus sequence: A calculated order of the most frequent residues (of either nucleotides or amino acids) found at each position in a common sequence alignment and obtained by comparing multiple closely related sequence alignments.
conservative replication: A hypothetical mode of DNA replication in which the two parental strands of the original double-stranded DNA molecule ultimately remain complemented to each other at the end of the replication process, and the two daughter strands end up forming their own separate molecule; hence one molecule is composed of both of the starting strands while the other is composed entirely of newly synthesized strands. This is in contrast to semiconservative replication, in which each molecule is a hybrid of one old and one new strand.See also dispersive replication.
conserved sequence: A nucleic acid or protein sequence that is highly similar or identical across many species or within a genome, indicating that it has remained relatively unchanged through a long period of evolutionary time.
conspecific: Belonging to the same species.
constitutive expression: 1. The continuous transcription of a gene, as opposed to facultative expression, in which a gene is only transcribed as needed. A gene that is transcribed continuously is called a constitutive gene.; 2. A gene whose expression depends only on the efficiency of its promoter in binding RNA polymerase,[4] and not on any transcription factors or other regulatory elements which might promote or inhibit its transcription.
contig: A continuous sequence of genomic DNA generated by assembling cloned fragments by means of their overlapping sequences.[6]
copy DNA (cDNA): See complementary DNA.
copy error: A mutation resulting from a mistake made during DNA replication.[4]
copy-number variation (CNV): A phenomenon in which sections of a genome are repeated and the number of repeats varies between individuals in the population, usually as a result of duplication or deletion events that affect entire genes or sections of chromosomes. Copy-number variations play an important role in generating genetic variation within a population.
coregulator: A protein that works together with one or more transcription factors to regulate gene expression.
corepressor: A type of coregulator that reduces (represses) the expression of one or more genes by binding to and activating a repressor.
cosmid
cpDNA: See chloroplast DNA.
CpG island: A region of a genome in which CpG sites occur repetitively or with high frequency.
CpG site: A sequence of DNA in which a cytosine nucleotide is immediately followed by a guanine nucleotide on the same strand in the 5'-to-3' direction; the "p" in CpG refers simply to the intervening phosphate group linking the two consecutive nucleotides.
CRISPR gene editing
crossing over: See chromosomal crossover.
crosslink: An abnormal chemical bond between two or more nucleobases on opposite strands of a double-stranded DNA molecule (interstrand), or between bases on the same strand (intrastrand), specifically via the formation of covalent bonds that are stronger than the hydrogen bonds of base pairing. Crosslinks can be generated by a variety of exogenous and endogenous agents, and tend to interfere with normal cellular processes such as DNA replication and transcription. They are common targets of DNA repair pathways.
cryptic unstable transcript (CUT)
ctDNA: 1. An abbreviation of circulating tumor DNA.; 2. An abbreviation of chloroplast DNA.
C-terminus: The end of a linear chain of amino acids (i.e. a peptide) that is terminated by the free carboxyl group (–COOH) of the last amino acid to be added to the chain during translation. This amino acid is said to be C-terminal. By convention, sequences, domains, active sites, or any other structure positioned nearer to the C-terminus of the polypeptide or the folded protein it forms relative to others are described as downstream. Contrast N-terminus.
C-value: The total amount of DNA contained within a haploid nucleus (e.g. a gamete) of a particular organism or species, expressed in number of base pairs or in units of mass (typically picograms); or, equivalently, one-half the amount in a diploid somatic cell. For simple diploid eukaryotes the term is often used interchangeably with genome size, but in certain cases, e.g. in hybrid polyploids descended from parents of different species, the C-value may actually represent two or more distinct genomes contained within the same nucleus. C-values apply only to genomic DNA, and notably exclude extranuclear DNA.
C-value enigma: A term used to describe a diverse variety of questions regarding the immense variation in nuclear C-value or genome size among eukaryotic species, in particular the observation that genome size does not correlate with the perceived complexity of organisms, nor necessarily with the number of genes they possess; for example, many single-celled protists have genomes containing thousands of times more DNA than the human genome. This was considered paradoxical until the discovery that eukaryotic genomes consist mostly of non-coding DNA, which lacks genes entirely. The focus of the enigma has since shifted to understanding why and how genomes came to be filled with so much non-coding DNA, and why some genomes have a higher gene content than others.
cyclic adenosine monophosphate (cAMP)
cyclosis: See cytoplasmic streaming.
cytidine (C, Cyd): One of the four standard nucleosides used in RNA molecules, consisting of a cytosine base with its N₉ nitrogen bonded to the C₁ carbon of a ribose sugar. Cytosine bonded to deoxyribose is known as deoxycytidine, which is the version used in DNA.
cytochemistry
cytogenetics: The branch of genetics that studies how chromosomes influence and relate to cell behavior and function, particularly during mitosis and meiosis.
cytokine
cytokinesis: The final stage of cell division in both mitosis and meiosis, usually immediately following the division of the nucleus, during which the cytoplasm of the parent cell is cleaved and divided approximately evenly between the two daughter cells. In animal cells, this process occurs by the closing of a microfilament contractile ring in the equatorial region of the dividing cell. Contrast karyokinesis.
cytology: The study of the morphology, processes, and life history of living cells, particularly by means of light and electron microscopy.[7] The term is also sometimes used as a synonym for the broader field of cell biology.
cytolysis: See lysis.
cytometer
cytoplasm
cytoplasmic streaming: The flow of the cytoplasm inside a cell, driven by forces exerted upon cytoplasmic fluids by the cytoskeleton. This flow functions partly to speed up the transport of molecules and organelles suspended in the cytoplasm to different parts of the cell, which would otherwise have to rely on passive diffusion for movement. It is most commonly observed in very large eukaryotic cells, for which there is a greater need for transport efficiency.
cytosine (C): A pyrimidine nucleobase used as one of the four standard nucleobases in both DNA and RNA molecules. Cytosine forms a base pair with guanine.
cytosol: The soluble phase of the cytoplasm, in which small particles such as ribosomes, proteins, nucleic acids, and many other molecules are suspended or dissolved, excluding larger structures and organelles such as mitochondria, chloroplasts, lysosomes, and the endoplasmic reticulum.[7]

D

de novo mutation: A spontaneous mutation in the genome of an individual organism that is new to that organism's lineage, having first appeared in a germ cell of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.[4]
de novo synthesis: The assembly of a synthetic nucleic acid sequence from free nucleotides without relying on an existing template strand, i.e. de novo, by any of a variety of laboratory methods. De novo synthesis makes it theoretically possible to construct completely artificial molecules with no naturally occurring equivalent, and no restrictions on size or sequence. It is performed routinely in the commercial production of customized, made-to-order oligonucleotide sequences such as primers.
degeneracy: The redundancy of the genetic code, exhibited as the multiplicity of different codons that specify the same amino acid. For example, in the standard genetic code, the amino acid serine is specified by six unique codons (UCA, UCG, UCC, UCU, AGU, and AGC). Codon degeneracy accounts for the existence of synonymous mutations.
deletion: A type of mutation in which one or more nucleotides are removed from a nucleic acid sequence.
denaturation: The process by which nucleic acids or proteins lose their quaternary, tertiary, and/or secondary structures, either reversibly or irreversibly, through the application of some external chemical or mechanical stress, e.g. by heating, agitation, or exposure to a strong acid or base, all of which can disrupt intermolecular forces such as hydrogen bonding and thereby change or destroy chemical activity. Denatured proteins may be both a cause and a consequence of cell death. Denaturation may also be a normal process; the denaturation of double-stranded DNA molecules, for example, which breaks the hydrogen bonds between base pairs and causes the separation of the duplex molecule into two single strands, is a necessary step in DNA replication and transcription and hence is routinely performed by enzymes such as helicases. The same mechanism is also fundamental to laboratory methods such as PCR.
deoxyadenosine: One of the four standard deoxyribonucleosides used in DNA molecules, consisting of an adenine base with its N₉ nitrogen bonded to the C₁ carbon of a deoxyribose sugar. Adenine bonded to ribose forms an alternate compound known simply as adenosine, which is used in RNA.
deoxycytidine: One of the four standard deoxyribonucleosides used in DNA molecules, consisting of a cytosine base with its N₉ nitrogen bonded to the C₁ carbon of a deoxyribose sugar. Cytosine bonded to ribose forms an alternate compound known simply as cytidine, which is used in RNA.
deoxyguanosine: One of the four standard deoxyribonucleosides used in DNA molecules, consisting of a guanine base with its N₉ nitrogen bonded to the C₁ carbon of a deoxyribose sugar. Guanine bonded to ribose forms an alternate compound known simply as guanosine, which is used in RNA.
deoxythymidine: See thymidine.
deoxyribonuclease (DNase): Any of a class of nuclease enzymes which catalyze the hydrolytic cleavage of phosphodiester bonds in DNA molecules, thereby severing strands of deoxyribonucleotides and causing the degradation of DNA polymers into smaller components. Compare ribonuclease.
deoxyribonucleic acid (DNA): A polymeric nucleic acid molecule composed of a series of deoxyribonucleotides, each of which incorporates one of four canonical nucleobases: adenine (A), guanine (G), cytosine (C), and thymine (T). DNA is most often found in double-stranded form, which consists of two complementary antiparallel nucleotide chains in which each of the nucleobases on each individual strand is paired via hydrogen bonding with one on the opposite strand; this structure commonly occurs in the shape of a double helix. DNA can also exist in single-stranded form. By storing and encoding genetic information in the sequence of its nucleotides, DNA serves as the universal molecular basis of biological inheritance and the fundamental template from which all proteins, cells, and living organisms are constructed.
deoxyribonucleotide: A nucleotide containing deoxyribose as its pentose sugar component, and the monomeric subunit of deoxyribonucleic acid (DNA) molecules. Deoxyribonucleotides canonically incorporate any of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Compare ribonucleotide.
deoxyribose: A monosaccharide sugar derived from ribose by the loss of a single oxygen atom. D-deoxyribose, in its pentose ring form, is one of three primary functional groups of deoxyribonucleotides and hence of deoxyribonucleic acid (DNA) molecules.
desmosome
destination vector
desynapsis: The failure of homologous chromosomes that have synapsed normally during pachynema to remain paired during diplonema. Desynapsis is usually caused by the improper formation of chiasmata.[4] Contrast asynapsis.
developmental biology
diakinesis: In meiosis, the fifth and final substage of prophase I, following diplonema and preceding metaphase I. During diakinesis, the chromosomes are further condensed, the two centrosomes reach opposite poles of the cell, and the spindle apparatus begins to extend from the poles to the equator.[4]
dicentric: (of a linear chromosome or chromosome fragment) Having two centromeres instead of the normal one.[2]
dinucleotide: Any two nucleotides which are immediately adjacent to each other on the same strand of a nucleic acid molecule.
diploid: (of a cell or organism) Having two homologous copies of each chromosome. Contrast haploid and polyploid.
diplonema: In meiosis, the fourth of the five substages of prophase I, following pachynema and preceding diakinesis. During diplonema, the synaptonemal complex disassembles and the paired homologous chromosomes begin to separate from one another, though they remain tightly bound at the chiasmata where crossover has occurred.
directionality: The end-to-end chemical orientation of a single linear strand or sequence of a nucleic acid polymer or a polypeptide. The nomenclature used to indicate nucleic acid directionality is based on the chemical convention of identifying individual carbon atoms in the ribose or deoxyribose sugars of nucleotides, specifically the 5' carbon and 3' carbon of the pentose ring. The sequence of nucleotides in a polymeric chain may be read or interpreted in the 5'-to-3' direction – i.e. starting from the terminal nucleotide in which the 5' carbon is not connected to another nucleotide, and proceeding to the other terminal nucleotide, in which the 3' carbon is not connected to another nucleotide – or in the opposite 3'-to-5' direction. Most types of nucleic acid synthesis, including DNA replication and transcription, work exclusively in the 5'-to-3' direction, because the enzymes involved can only catalyze the addition of free nucleotides to the open 3'-end of the previous nucleotide in the chain. Because of this, the convention when writing any nucleic acid sequence is to present it in the 5'-to-3' direction from left to right. In double-stranded nucleic acids, the two paired strands must be oriented in opposite directions in order to base-pair with each other. Polypeptide directionality is similarly based on labeling the functional groups in amino acids, specifically the amino group, which forms the N-terminus, and the carboxyl group, which forms the C-terminus; amino acid sequences are assembled in the N-to-C direction during translation, and are almost always written in the same direction.
direct repeat: Any two or more repetitions of a specific sequence of nucleotides occurring in the same orientation (i.e. in precisely the same order and not inverted) and on the same strand, either separated by intervening nucleotides or not. An example is the sequence TACCGnnnnnnTACCG, in which TACCG occurs twice, though separated by six nucleotides that are not part of the repeated sequence. A direct repeat in which the repeats are immediately adjacent to each other is known as a tandem repeat.
dispersive replication
distance measure: Any quantity used to measure the dissimilarity between the gene expression levels of different genes.[10]
DNA: See deoxyribonucleic acid.
DNA barcoding: A method of taxonomic identification in which short DNA sequences from one or more specific genes are isolated from unidentified samples and then aligned with sequences from a reference library in order to uniquely identify the species or other taxon from which the samples originated. The sequences used in the comparison are chosen carefully from genes that are both widely conserved and that show greater variation between species than within species, e.g. the cytochrome c oxidase gene for eukaryotes or certain ribosomal RNA genes for prokaryotes. These genes are present in nearly all living organisms but tend to evolve different mutations in different species, such that a unique sequence variant can be linked to one particular species, effectively creating a unique identifier akin to a retail barcode. DNA barcoding allows unknown specimens to be identified from otherwise indistinct tissues or body parts, where identification by morphology would be difficult or impossible, and the library of organismal barcodes is now comprehensive enough that even organisms previously unknown to science can often be phylogenetically classified with confidence. The simultaneous identification of multiple different species from a mixed sample is known as metabarcoding.
DNA condensation: The process of compacting very long DNA molecules into densely packed, orderly configurations such as chromosomes, either in vivo or in vitro.
DNA fingerprinting
DNA microarray: A high-throughput technology used to measure expression levels of mRNA transcripts or to detect certain changes in nucleotide sequence. It consists of an array of thousands of microscopic spots of DNA oligonucleotides, called features, each containing picomoles of a specific DNA sequence. This can be a short section of a gene or any other DNA element, and is used as a probe to hybridize a cDNA, cRNA or genomic DNA sample (called a target) under high-stringency conditions. Probe-target hybridization is usually detected and quantified by fluorescence-based detection of fluorophore-labeled targets.
DNA polymerase: Any of a class of enzymes that synthesizes DNA molecules from individual deoxyribonucleotides. DNA polymerases are essential for DNA replication and usually work in pairs to create identical copies of the two strands of an original double-stranded molecule. They build long chains of DNA by adding nucleotides one at a time to the 3'-end of a DNA strand, usually relying on the template provided by the complementary strand to copy the nucleotide sequence faithfully.
DNA repair: The set of processes by which a cell identifies and corrects structural damage or mutations in the DNA molecules that encode its genome. The ability of a cell to repair its DNA is vital to the integrity of the genome and the normal functionality of the organism.
DNA replication: The process by which a DNA molecule copies itself, producing two identical copies of one original DNA molecule.
DNA sequencing: The process of determining, by any of a variety of different methods and technologies, the order of the bases in the long chain of nucleotides that constitutes a sequence of DNA.
DNA turnover: Any mechanism by which DNA sequences are exchanged non-reciprocally (e.g. via gene conversion, transposition, or unequal crossing-over) that causes continual fluctuations in the copy number of DNA motifs during an organism's lifetime. Such mechanisms are often major drivers of speciation between populations.[11]
DNA-binding domain (DBD)
DNA-binding protein (DBP): Any polypeptide or protein containing one or more domains capable of interacting chemically with one or more parts of a DNA molecule, and consequently having a specific or general affinity for single- and/or double-stranded DNA. DNA-binding activity often depends on the presence and physical accessibility of a specific nucleobase sequence, and mostly occurs at the major groove, since it exposes more of the functional groups that uniquely identify the bases. Binding is also influenced by the spatial conformation of the DNA chain and the occupancy of other proteins near the binding site; many proteins cannot bind to DNA without first undergoing structural changes induced by interactions with other molecules.
DNase: See deoxyribonuclease.
donor vector
dosage compensation: Any mechanism by which organisms neutralize the large difference in gene dosage caused by the presence of differing numbers of sex chromosomes in the different sexes, thereby equalizing the expression of sex-linked genes so that the members of each sex receive the same or similar amounts of the products of such genes. An example is X-inactivation in female mammals.
double helix: The shape most commonly assumed by double-stranded nucleic acid molecules, resembling a ladder that has been twisted upon its long axis, with the rungs of the ladder consisting of paired nucleobases. This secondary structure is the most energetically stable conformation of the double-stranded forms of both DNA and RNA under most naturally occurring conditions, arising as a consequence of the primary structure of the phosphodiester backbone and the stacking of the nucleotides bonded to it. In B-DNA, the most common DNA variant found in nature, the double helix has a right-handed twist with about 10 base pairs per full turn, and the molecular geometry results in an alternating pattern of "grooves" of differing widths (a major groove and a minor groove) between the parallel backbones.
double-strand break (DSB): The loss of continuity of the phosphate-sugar backbone in both strands of a double-stranded DNA molecule, in particular when the two breaks occur at sites that are directly across from or very close to each other on the complementary strands.[11] Contrast single-strand break.
double-stranded: Composed of two antiparallel, complementary nucleic acid molecules or strands (either DNA–DNA, RNA–RNA, or a DNA–RNA hybrid) which are held together by hydrogen bonds between the complementary nucleobases of each strand, known as base pairing. Compare single-stranded.
double-stranded DNA (dsDNA): Any DNA molecule that is composed of two antiparallel, complementary deoxyribonucleotide polymers, known as strands, which are bonded together by hydrogen bonds between the complementary nucleobases. Though it is possible for DNA to exist as a single strand, it is generally more stable and more common in double-stranded form. In most cases, the complementary base pairing causes the twin strands to coil around each other in the shape of a double helix.
double-stranded RNA (dsRNA): Any RNA molecule that is composed of two antiparallel, complementary ribonucleotide polymers, known as strands, which are bonded together by hydrogen bonds between the complementary nucleobases. Though RNA usually occurs in single-stranded form, it is also capable of forming duplexes in the same way as DNA; an example is an mRNA transcript pairing with an antisense version of the same transcript, which effectively silences the gene from which the mRNA was transcribed by preventing translation. As in dsDNA, the base pairing in dsRNA usually causes the twin strands to coil around each other in the shape of a double helix.
downregulation: Any process, natural or artificial, which decreases the level of gene expression of a certain gene. A gene which is observed to be expressed at relatively low levels (such as by detecting lower levels of its mRNA transcripts) in one sample compared to another sample is said to be downregulated. Contrast upregulation.
downstream: Towards or closer to the 3'-end of a chain of nucleotides, or to the C-terminus of a peptide chain. Contrast upstream.
dsDNA: See double-stranded DNA.
dsRNA: See double-stranded RNA.
duplex: See double-stranded.
duplication: The production of a second copy of part or all of a nucleotide sequence or amino acid sequence, either naturally or artificially, and the retention of both copies; especially when both the copy and the original sequence are retained in situ within the same molecule, often but not necessarily adjacent to each other. See also gene duplication, chromosomal duplication, and repeat.
dyad: See sister chromatids.

E

electron transport chain
electroporation: A molecular biology technique in which a strong electric field is applied to living cells in order to temporarily increase the permeability of their cell membranes, allowing exogenous nucleic acids, proteins, or chemical compounds to easily pass through the membrane and thereby enter the cells. It is a common method of achieving transformation and transfection.
elongation: The linear growth of a nucleic acid polymer by the sequential addition of individual nucleotide monomers to a nascent strand, e.g. during transcription or replication, especially when it occurs by complementary pairing with a template strand. The term is often used to describe steps in certain laboratory techniques such as the polymerase chain reaction.
embryo
emergenesis: The quality of genetic traits that results from a specific configuration of interacting genes, rather than simply their combination.
endocytosis
endomembrane: Any membrane surrounding an intracellular organelle or vesicle, e.g. that of the endoplasmic reticulum, Golgi apparatus, lysosome, vacuole, nucleus (the nuclear envelope), etc.[7]
endonuclease: Any enzyme whose activity is to cleave phosphodiester bonds within a chain of nucleotides, including those that cleave relatively nonspecifically (without regard to sequence) and those that cleave only at very specific sequences (so-called restriction endonucleases). When recognition of a specific sequence is required, endonucleases make their cuts in the middle of the sequence. Contrast exonuclease.
endoplasmic reticulum (ER)
endosome
enhancer: A region of DNA near a gene that can be bound by an activator to increase gene expression or by a repressor to decrease expression.
enhancer RNA (eRNA): A subclass of long non-coding RNAs transcribed from regions of DNA containing enhancer sequences. The expression of a given eRNA generally correlates with the activity of the corresponding enhancer in enhancing transcription of its target genes, suggesting that eRNAs play an active role in gene regulation in cis or in trans.
enucleate: To artificially remove the nucleus from a cell, e.g. by micromanipulation in the laboratory or by destroying it through irradiation with ultraviolet light, rendering the cell anucleate.[7]
envagination
epigenetics
epigenome
episome: 1. Another name for a plasmid, especially one that is capable of integrating into a chromosome.; 2. In eukaryotes, any non-integrated extrachromosomal circular DNA molecule that is stably maintained and replicated in the nucleus simultaneously with the rest of the host cell. Such molecules may include viral genomes, bacterial plasmids, and aberrant chromosomal fragments.
epistasis: The collective action of multiple genes interacting during gene expression. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific phenotypic traits.
ergosome: See polysome.
euchromatin: A relatively open, lightly compacted form of chromatin in which DNA is only sporadically bound in nucleosomes and thus broadly accessible to binding and manipulation by proteins and other molecules. Euchromatic regions of a genome are often enriched in genes and actively undergoing transcription, in contrast to heterochromatin, which is relatively gene-poor, nucleosome-rich, and less accessible to transcription machinery.
euploidy: The condition of a cell or organism having an abnormal number of complete sets of chromosomes, possibly excluding the sex chromosomes. Euploidy differs from aneuploidy, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.
evolution: The change in the heritable characteristics of biological populations over successive generations. In the most traditional sense, it occurs by changes in the frequencies of alleles in a population's gene pool.
excision: The enzymatic removal of a polynucleotide sequence from one or more strands of a nucleic acid, or of a polypeptide sequence from a protein, typically implying both the breaking of the polymeric molecule in two locations and the subsequent rejoining of the two breakpoints after the sequence between them has been removed. The term may be used to describe a wide variety of processes performed by distinct enzymes, including most splicing and DNA repair pathways.[4]
exocytosis
exome: The entire set of exons within a particular genome, including untranslated regions of mature mRNAs as well as coding regions.
exon: Any part of a gene that encodes a part of the final mature messenger RNA produced by that gene after introns have been removed by alternative splicing. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts.
exon skipping
exonuclease: Any enzyme whose activity is to cleave phosphodiester bonds within a chain of nucleotides, including those that cleave only upon recognition of a specific sequence (so-called restriction exonucleases). Exonucleases make their cuts at either the 3' or 5'-end of the sequence (rather than in the middle, as with endonucleases).
exosome: 1. (protein complex) An intracellular multi-protein complex which serves the function of degrading various types of RNA molecules.; 2. (vesicle) A type of membrane-bound extracellular vesicle produced in many eukaryotic cells by the inward budding of an endosome and the subsequent fusion of the endosome with the plasma membrane, causing the release of the vesicle into various extracellular spaces, including biological fluids such as blood and saliva, where they may serve any of a wide variety of physiological functions, from waste management to intercellular signaling.
exosome complex: An intracellular multi-protein complex which serves the function of degrading various types of RNA molecules.
expression vector: A type of vector, usually a plasmid or viral vector, designed specifically for the expression of a transgene insert in a target cell, rather than for some other purpose such as cloning.
extein: Any part of an amino acid sequence which is retained within a precursor polypeptide, i.e. not excised by post-translational protein splicing, and is therefore present in the mature protein, analogous to the exons of RNA transcripts. Contrast intein.
extracellular: Outside the plasma membrane of a cell or cells; i.e. located or occurring externally to a cell. Contrast intracellular; see also intercellular.
extracellular matrix (ECM)
extrachromosomal DNA: Any DNA that is not found in chromosomes or in the nucleus of a cell and hence is not genomic DNA. This may include the DNA contained in plasmids or organelles such as mitochondria or chloroplasts, or, in the broadest sense, DNA introduced by viral infection. Extrachromosomal DNA usually shows significant structural differences from nuclear DNA in the same organism.

F

facultative expression: The transcription of a gene only as needed, as opposed to constitutive expression, in which a gene is transcribed continuously. A gene that is transcribed as needed is called a facultative gene.
fatty acid
fermentation
filopodium
five-prime cap: See 5' cap.
five-prime end: See 5'-end.
five-prime untranslated region: See 5' untranslated region.
fixation: The process by which a single allele for a particular gene with multiple different alleles increases in frequency in a given population such that it becomes permanently established at 100% frequency – that is, the only allele at that locus within the population's gene pool. In the absence of mutation and heterozygote advantage, any given allele is eventually destined to become either permanently fixed over all other variants or completely lost from the population, though how long this takes depends on selection pressures and chance fluctuations in allele frequencies.
fluorescence in situ hybridization (FISH)
forward genetics: An experimental approach in molecular genetics in which a researcher starts with a specific known phenotype and attempts to determine the genetic basis of that phenotype by any of a variety of laboratory techniques, commonly by inducing random mutations in the organism's genome and then screening for changes in the phenotype of interest. Observed phenotypic changes are assumed to have resulted from the mutation(s) present in the screened sample, which can then be mapped to specific genomic loci and ultimately to one or more specific candidate genes. This methodology contrasts with reverse genetics, in which a specific gene or its gene product is individually manipulated in order to identify the gene's function.
forward mutation
frameshift mutation: A type of mutation in a nucleic acid sequence caused by the insertion or deletion of a number of nucleotides that is not divisible by three. Because of the triplet nature by which nucleotides code for amino acids, a mutation of this sort causes a shift in the reading frame of the nucleotide sequence, resulting in the sequence of codons downstream of the mutation site being completely different from the original.
Functional Genomics Data (FGED) Society: An organization that works with others "to develop standards for biological research data quality, annotation and exchange" as well as software tools that facilitate their use.[12]

G

G banding: A technique used in cytogenetics to produce a visible karyotype by staining the condensed chromosomes with Giemsa stain. The staining produces consistent and identifiable patterns of dark and light "bands" in regions of chromatin, which allows specific chromosomes to be easily distinguished.
G1
G2
gamete: A haploid cell that is the meiotic product of a progenitor germ cell and the final product of the germ line in sexually reproducing multicellular organisms. Gametes are the means by which an organism passes its genetic information to its offspring; during fertilization, two gametes (one from each parent) are fused into a single diploid zygote.
gDNA: See genomic DNA.
gene: Any segment or set of segments of a nucleic acid molecule that contains the information necessary to produce a functional RNA transcript in a controlled manner. In living organisms, genes are often considered the fundamental units of heredity and are typically encoded in DNA. A particular gene can have multiple different versions, or alleles, and a single gene can result in a gene product that influences many different phenotypes.
gene dosage: The number of copies of a particular gene present in a genome. Gene dosage directly influences the amount of gene product a cell is able to express, though a variety of controls have evolved which tightly regulate gene expression. Changes in gene dosage caused by mutations include copy-number variations.
gene duplication: A type of mutation defined as any duplication of a region of DNA that contains a gene. Compare chromosomal duplication.
gene expression: The set of processes by which the information encoded in a gene is used in the synthesis of a gene product, such as a protein or non-coding RNA, or otherwise made available to influence one or more phenotypes. Canonically, the first step is transcription, which produces a messenger RNA molecule complementary to the DNA molecule in which the gene is encoded; for protein-coding genes, the second step is translation, in which the messenger RNA is read by the ribosome to produce a protein. The information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however; broader definitions encompass a huge variety of other ways in which genetic information can be expressed.
Gene Expression Omnibus (GEO): A database of high-throughput functional genomics and gene expression data derived from experimental chips and next-generation sequencing and managed by the National Center for Biotechnology Information.[13][14]
gene fusion: The union, either by natural mutation or by recombinant laboratory techniques, of two or more previously independent genes that code for different gene products such that they become subject to control by the same regulatory systems. The resulting hybrid sequence is known as a fusion gene.[4]
gene mapping: Any of a variety of methods used to precisely identify the location of a particular gene within a DNA molecule (such as a chromosome) and/or the physical or linkage distances between it and other genes.
gene of interest (GOI): A gene being studied in a scientific experiment, especially one that is the focus of a genetic engineering technique such as cloning.
gene product: Any of the biochemical material resulting from the expression of a gene, most commonly interpreted as the functional mRNA transcript produced by transcription of the gene or the fully constructed protein produced by translation of the transcript, though non-coding RNA molecules such as transfer RNAs may also be considered gene products. A measurement of the quantity of a given gene product that is detectable in a cell or tissue is sometimes used to infer how active the corresponding gene is.
gene regulation: The broad range of mechanisms used by cells to control the activity of their genes, especially to allow, prohibit, increase, or decrease the production or expression of specific gene products, such as RNA or proteins. Gene regulation increases an organism's versatility and adaptability by allowing its cells to express different gene products when required by changes in its environment. In multicellular organisms, the regulation of gene expression also drives cellular differentiation and morphogenesis in the embryo, enabling the creation of a diverse array of cell types from the same genome.
gene silencing: Any mechanism of gene regulation which drastically reduces or completely prevents the expression of a particular gene. Gene silencing may occur naturally during either transcription or translation. Laboratory techniques often exploit natural silencing mechanisms to achieve gene knockdown.
gene therapy: The insertion of a functional or wild-type gene or part of a gene into an organism (especially a patient) with the intention of correcting a genetic defect, either by direct substitution of the defective gene or by supplementation with a second, functional version.[11]
gene trapping: A high-throughput technology used to simultaneously inactivate, identify, and report the expression of a target gene in a mammalian genome by introducing an insertional mutation consisting of a promoterless reporter gene and/or a selectable genetic marker flanked by an upstream splice site and a downstream polyadenylated termination sequence.
generation: 1. In any given organism, a single reproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of the progeny of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particular lineage or for a population or species as a whole.; 2. In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a given common ancestor by virtue of the same number of reproductive events having occurred between them and the ancestor.[11]
genetic background
genetic code: A set of rules by which information encoded within nucleic acids is translated into proteins by living cells. These rules define how sequences of nucleotide triplets called codons specify which amino acid will be added next during protein synthesis. The vast majority of living organisms use the same genetic code (sometimes referred to as the "standard" genetic code) but variant codes do exist.
genetic disorder: Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism's genome which are congenital (present at birth) and not acquired later in life. Causes may include a mutation to one or more genes, or a chromosomal abnormality such as an aneuploidy of a particular chromosome. The mutation responsible may occur spontaneously during embryonic development or may be inherited from one or both parents, in which case the genetic disorder is also classified as a hereditary disorder. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simply increase the risk of the organism developing it.
genetic distance: A measure of the genetic divergence between species, populations within a species, or individuals, used especially in phylogenetics to express either the time elapsed since the existence of a common ancestor or the degree of differentiation in the DNA sequences comprising the genomes of each population or individual.
genetic engineering: The direct, deliberate manipulation of an organism's genetic material using any of a variety of biotechnology methods, including the insertion or removal of genes, the transfer of genes within and between species, the mutation of existing sequences, and the construction of novel sequences using artificial gene synthesis. Genetic engineering encompasses a broad set of technologies by which the genetic composition of individual cells, tissues, or entire organisms may be altered for various purposes, commonly in order to study the functions and expression of individual genes, to produce hormones, vaccines, and other drugs, and to create genetically modified organisms for use in research and agriculture.
genetic marker: A specific, easily identifiable, and usually highly polymorphic gene or other DNA sequence with a known location on a chromosome that can be used to identify the individual or species possessing it.
genetic recombination: Any reassortment or exchange of genetic material within an individual organism or between individuals of the same or different species, especially that which creates genetic variation. In the broadest sense, the term encompasses a diverse class of naturally occurring mechanisms by which nucleic acid sequences are copied or physically transferred into different genetic environments, including homologous recombination during meiosis or mitosis or as a normal part of DNA repair; horizontal gene transfer events such as bacterial conjugation, viral transduction, or transformation; or errors in DNA replication or cell division. Artificial recombination is central to many genetic engineering techniques which produce recombinant DNA.
genetic redundancy: The redundant encoding of two or more distinct gene products that ultimately perform the same biochemical function. Mutations in one of these genes may have a smaller effect on fitness than might be expected, since the redundant genes often compensate for any loss of function and obviate any gain of function.
genetic regulatory network (GRN): A graph that represents the regulatory complexity of gene expression. The vertices (nodes) are represented by various regulatory elements and gene products while the edges (links) are represented by their interactions. These network structures also represent functional relationships by approximating the rate at which genes are transcribed.
genetic testing: A broad class of various procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or ancestry, diagnose vulnerabilities to heritable diseases, or detect mutant alleles associated with increased risks of developing genetic disorders. Genetic testing is widely used in human medicine, agriculture, and biological research.
genetically modified organism (GMO): Any organism whose genetic material has been altered using genetic engineering techniques, particularly in a way that does not occur naturally by mating or by natural genetic recombination.
genetics: The field of biology that studies genes, genetic variation, and heredity in living organisms.
genome: The entire complement of genetic material contained within the chromosomes of an organism, organelle, or virus. The term is also used to refer to the collective set of genetic loci shared by every member of a population or species, regardless of the different alleles that may be present at these loci in different individuals.
genome size: The total amount of DNA contained within one copy of a genome, typically measured by mass (in picograms or daltons) or by the total number of base pairs (in kilobases or megabases). For diploid organisms, genome size is often used interchangeably with C-value.
genomic DNA (gDNA): The DNA contained in chromosomes, as opposed to the extrachromosomal DNA contained in separate structures such as plasmids or organelles such as mitochondria or chloroplasts.
genomic imprinting: An epigenetic phenomenon that causes genes to be expressed in a manner dependent upon the particular parent from which the gene was inherited. It occurs when epigenetic marks such as DNA or histone methylation are established or "imprinted" in the germ cells of a parent organism and subsequently maintained through cell divisions in the somatic cells of the organism's progeny; as a result, a gene in the progeny that was inherited from the father may be expressed differently than another copy of the same gene that was inherited from the mother.
genomics: An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire genomes, as opposed to individual genes.
genotoxicity: The ability of certain chemical agents to cause damage to genetic material within a living cell (e.g. through single- or double-stranded breaks, crosslinking, or point mutations), which may or may not result in a permanent mutation. Though all mutagens are genotoxic, not all genotoxic compounds are mutagenic.
genotype: The entire complement of alleles present in a particular individual's genome, which gives rise to the individual's phenotype.
genotyping: The process of determining differences in the genotype of an individual by examining the DNA sequences in the individual's genome using bioassays and comparing them to another individual's sequences or a reference sequence.
germ cell: Any cell that gives rise to the gametes of a sexually reproducing organism. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from somatic cells, which are entirely separate from the germ line.
germ line: 1. In multicellular organisms, the subpopulation of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from somatic cells, which cannot pass on their genetic material except to their own immediate mitotic daughter cells. Cells of the germ line are called germ cells.; 2. The lineage of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.
glucogenic amino acid: Any amino acid that can be converted into glucose via gluconeogenesis, as opposed to the ketogenic amino acids, which can be converted into ketone bodies. In humans, 18 of the 20 amino acids are glucogenic; only leucine and lysine are not. Five amino acids (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both glucogenic and ketogenic.
gluconeogenesis (GNG): The chain of metabolic reactions that results in the generation of glucose from some non-carbohydrate carbon substrates, including the glucogenic amino acids. It is one of two primary pathways used by most animals to maintain blood sugar levels (the other being glycogenolysis), especially during periods of fasting, starvation, and intense exercise.
glycogen
glycogenolysis
glycolysis
glycoside: Any chemical compound in which a carbohydrate molecule is covalently bonded to another molecule containing a hydroxyl group (including other carbohydrates) via one or more C
_–O glycosidic bonds. When both molecules are carbohydrates, the glycoside is a disaccharide or polysaccharide.[7]
glycosidic bond
glycosylation: The attachment of an oligosaccharide (e.g. glucose) to an asparagine residue within a peptide or protein by covalent bonding, a process which takes place in or near the rough endoplasmic reticulum.[7]
Goldberg-Hogness box: See TATA box.
Golgi apparatus
gRNA: See guide RNA.
guanine (G): A purine nucleobase used as one of the four standard nucleobases in both DNA and RNA molecules. Guanine forms a base pair with cytosine.
guanine-cytosine content: The proportion of nitrogenous bases in a nucleic acid that are either guanine (G) or cytosine (C), typically expressed as a percentage. DNA and RNA molecules with higher GC-content are generally more thermostable than those with lower GC-content due to molecular interactions that occur during base stacking.[15]
guanosine (G, Guo): One of the four standard nucleosides used in RNA molecules, consisting of a guanine base with its N₉ nitrogen bonded to the C₁ carbon of a ribose sugar. Guanine bonded to deoxyribose is known as deoxyguanosine, which is the version used in DNA.
guide RNA (gRNA)

H

hairpin: See stem-loop.
haploid: (of a cell or organism) Having one copy of each chromosome, with each copy not being part of a pair. Contrast diploid and polyploid.
Hayflick limit
helicase: Any of a class of ATP-dependent motor proteins that move directionally along the DNA backbone and catalyze the separation of the two complementary strands of double-stranded molecules, permitting a wide variety of vital processes to take place, e.g. transcription, replication, and repair.[11]
hemizygous: In a diploid organism, having just one allele at a given genetic locus (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a chromosome is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is deleted, or when a gene is located on a sex chromosome in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all X-linked genes are said to be hemizygous because there is only one X chromosome and few of the same genes exist on the Y chromosome.
heredity: The storage, transfer, and expression of molecular information in biological organisms,[11] as manifested by the passing on of phenotypic traits from parents to their offspring, either through sexual or asexual reproduction. Offspring cells or organisms are said to inherit the genetic information of their parents.
heterochromatin
heterochromosome: See allosome.
heterodimer: A protein or protein domain composed of two different polypeptides which are paired with each other in the quaternary structure of a multimeric complex.[4] Contrast homodimer.
heterogeneous nuclear RNA (hnRNA)
heterokaryon: A multinucleate cell containing nuclei with different genotypes, resulting from the fusion of two or more genetically distinct cells, either naturally (e.g. in certain types of sexual reproduction) or artificially (e.g. in genetic engineering).[7]
heterologous expression: The expression of a foreign gene or any other foreign DNA sequence within a host organism which does not naturally contain the same gene. Insertion of foreign transgenes into heterologous hosts using recombinant vectors is a common biotechnology method for studying gene structure and function.
heterozygous: In a diploid organism, having two different alleles at a given genetic locus. In genetics shorthand, heterozygous genotypes are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a dominant allele) and a lowercase letter (indicating a recessive allele), such as "Aa" or "Bb". Contrast homozygous.
high-throughput
histology: The study or analysis of the microscopic anatomy of biological tissues or of cells within tissues, particularly by making use of specialized techniques to distinguish structures and functions based on visual morphology and differential staining. In practice the term is sometimes used more broadly to include cytology.
histone: Any of a class of highly alkaline proteins responsible for packaging nuclear DNA into structural units called nucleosomes in eukaryotic cells. Histones are the chief protein components of chromatin, where they associate into eight-membered complexes which act as "spools" around which the linear DNA molecule winds. They play a major role in gene regulation and expression.
histone core: The complex of eight histone proteins around which double-stranded DNA wraps within a nucleosome. The canonical histone octamer consists of two each of histones H2A, H2B, H3, and H4, which pair with each other symmetrically to form a ball-shaped cluster around which DNA winds through interactions with the histones' surface domains, though variant histones may replace their analogues in certain contexts.
hnRNA: See heterogeneous nuclear RNA.
holocentric: (of a linear chromosome or chromosome fragment) Having no single centromere but rather multiple kinetochore assembly sites dispersed along the entire length of the chromosome. During cell division, the chromatids of holocentric chromosomes move apart in parallel and do not form the classical V-shaped structures typical of monocentric chromosomes.
homeobox: Any of a class of DNA sequences approximately 180 base pairs in length occurring near the 3'‐end of certain eukaryotic genes and encoding a 60-amino acid domain which is capable of binding to DNA or RNA via a characteristic helix-turn-helix motif in proteins containing it. Homeobox-containing genes are translated into homeodomain-containing proteins, which commonly regulate transcription or translation by binding to other genes or messenger RNAs containing specific homeobox responsive elements. The products of many homeotic genes, exemplified by the Hox genes, are of critical importance in developmental pathways.[4]
homeobox responsive element (HRE): Any DNA or RNA sequence that is specifically recognized and bound by the homeodomain of a homeodomain-containing protein.
homeodomain: A type of nucleic acid-binding domain, typically 60 amino acids in length, found in certain eukaryotic proteins, characterized by a highly conserved helix-turn-helix motif that binds with strong affinity to the backbone of specific recognition sequences in DNA or RNA molecules. A protein may have one or more homeodomains, generally situated towards the C-terminus of the polypeptide containing it. Many homeodomain-containing proteins function as transcription factors by binding to sequences within promoters and blocking or recruiting other proteins, such as RNA polymerase, to the transcription initiation complex. Homeodomains are the translated versions of homeoboxes, though the terms are often used interchangeably.
homodimer: A protein or protein domain composed of two identical polypeptides which are paired with each other in the quaternary structure of a multimeric complex.[4] Contrast heterodimer.
homologous chromosomes: A set of two matching chromosomes, one maternal and one paternal, which pair up with each other inside the nucleus during meiosis. They have the same genes at the same loci, but may have different alleles.
homologous recombination: A type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical ("homologous") molecules of DNA, especially that which occurs between homologous chromosomes. The term may refer to the recombination that occurs as a part of any of a number of distinct cellular processes, most commonly DNA repair or chromosomal crossover during meiosis in eukaryotes and horizontal gene transfer in prokaryotes. Contrast nonhomologous recombination.
homozygous: In a diploid organism, having two identical alleles at a given genetic locus. In genetics shorthand, homozygous genotypes are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast heterozygous.
horizontal gene transfer (HGT): Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g. bacterial conjugation.
housekeeping gene: Any constitutive gene that is transcribed at a relatively constant level across many or all known conditions and cell types. The products of housekeeping genes typically play critical roles in the maintenance of cellular integrity and basic metabolic function. It is generally assumed that their expression is unaffected by experimental or pathological conditions.
Hox genes: A subset of highly conserved homeobox-containing genes which are essential for the proper organization of the body plan in developing animal embryos, ensuring that the correct structures are formed in the correct places. Hox genes are usually arranged on a chromosome in tandem arrays and are expressed sequentially during development, with the sequence of gene activation corresponding to their physical arrangement within the genome and/or the physical layout of the tissues in which they are expressed along the organism's anterior–posterior axis.[4]
Human Genome Project (HGP): A collaborative international scientific research project with the goal of sequencing all of the chromosomal DNA and identifying and mapping all of the genes within human cells, and ultimately of assembling a complete reference genome for the human species. The project was launched in 1990 by a consortium of federal agencies, universities, and research institutions and was declared complete in 2003. Because each individual human being has a unique genome, the finished reference genome is a mosaic of sequences obtained by sampling DNA from thousands of individuals across the world and does not represent any one individual.
hyaloplasm: See cytosol.
hybrid: The offspring that results from combining the qualities of two organisms of different genera, species, breeds, or varieties through sexual reproduction. Hybrids may occur naturally or artificially, as during selective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in speciation.
hybridization: 1. The process by which a hybrid organism is produced from two organisms of different genera, species, breeds, or varieties.; 2. The process by which two or more single-stranded nucleic acid molecules with complementary nucleotide sequences pair with each other in solution, creating double-stranded or triple-stranded molecules via the formation of hydrogen bonds between the complementary nucleobases of each strand. In certain laboratory contexts, especially ones in which long strands hybridize with short oligonucleotide primers, hybridization is often referred to as annealing.; 3. A step in some experimental assays in which a single-stranded DNA or RNA preparation is added to an array surface and anneals to a complementary probe.
hybridization probe
hypoxanthine (I): A naturally occurring non-canonical purine nucleobase that is used in some RNA molecules and pairs with standard nucleobases in a phenomenon known as wobble base pairing. Its nucleoside form is known as inosine, which is the reason it is commonly abbreviated with the letter I in sequence reads.

I

idiochromosome: See allosome.
idiogram: A diagrammatic or schematic karyotype of the entire set of chromosomes within a cell or genome, in which annotated illustrations depict each chromosome in its most idealized form (e.g. with straight lines and obvious centromeres) so as to facilitate the easy identification of sequences, structural features, and physical distances, which may be less apparent in photomicrographs of the actual chromosomes.
idiomere: See chromomere.
immunohistochemistry (IHC)
immunostaining
in silico: (of a scientific experiment or research) Conducted, produced, or analyzed by means of computer modeling or simulation, as opposed to a real-world trial.
in situ: (of a scientific experiment or biological process) Occurring or made to occur in a natural, uncontrolled setting, or in the natural or original position or place, as opposed to in a foreign cell or tissue type or in an artificial environment.
in situ hybridization
in vitro: (of a scientific experiment or biological process) Occurring or made to occur in a laboratory vessel or other controlled artificial environment, e.g. in a test tube or a petri dish, as opposed to inside a living organism or in a natural setting.
in vivo: (of a scientific experiment or biological process) Occurring or made to occur inside the cells or tissues of a living organism; or, in the broadest sense, in any natural, unmanipulated setting. Contrast ex vivo and in vitro.
indel: A term referring to either an insertion or a deletion of one or more bases in a nucleic acid sequence.
inducer: A protein that binds to a repressor (to disable it) or to an activator (to enable it).
inducible gene: A gene whose expression is either responsive to environmental change or dependent on its host cell's position within the cell cycle.
inheritance: See heredity.
initiation codon: See start codon.
inosine
insertion: A type of mutation in which one or more bases are added to a nucleic acid sequence. Contrast deletion.
insertion sequence (IS): Any nucleotide sequence that is inserted naturally or artificially into another sequence. The term is used in particular to refer to the part of a transposable element that codes for those proteins directly involved in the transposition process, e.g. the transposase enzyme. The coding region in a transposable insertion sequence is usually flanked by short inverted repeats, and the structure of larger transposable elements may include a pair of flanking insertion sequences which are themselves inverted.
insertional mutagenesis: The alteration of a DNA sequence by the insertion of one or more nucleotides into the sequence, either naturally or artificially. Depending on the precise location of the insertion within the target sequence, insertions may partially or totally inactivate or even upregulate a gene product or biochemical pathway, or they may be neutral, leading to no substantive changes at all. Many genetic engineering techniques rely on the insertion of exogenous genetic material into host cells in order to study gene function and expression.[4]
insulator: A specific DNA sequence that prevents a gene from being influenced by the activation or repression of nearby genes.
integron: A mobile genetic element consisting of a gene cassette containing the gene for a site-specific recombinase, integrase-specific recognition sites, and a promoter that governs the expression of one or more genes conferring adaptive traits on the host cell. Integrons usually exist in the form of circular episomal DNA fragments, through which they facilitate the rapid adaptation of bacteria by enabling horizontal gene transfer of antibiotic resistance genes between different bacterial species.[4]
intein: Any sequence of one or more amino acids within a precursor polypeptide that is excised by protein splicing during post-translational modification and is therefore absent from the mature protein, analogous to the introns spliced out of RNA transcripts.[4] Contrast extein.
intercalating agent: Any chemical compound (e.g. ethidium bromide) that disrupts the alignment and pairing of bases in the complementary strands of a DNA molecule by inserting itself between the bases.[2]
intercalation: The insertion, naturally or artificially, of chemical compounds between the planar bases of a DNA molecule, which generally disrupts the hydrogen bonding necessary for base pairing.
intercellular: Between two or more cells. Contrast intracellular; see also extracellular.
intercistronic region: Any DNA sequence that is located between the stop codon of one gene and the start codon of the following gene in a polycistronic transcription unit.[4] See also intergenic region.
intercross: A cross in which both the male and female parents are heterozygous at a particular locus.[4]
intergenic region (IGR): Any sequence of non-coding DNA that is located between functional genes.
intergenic spacer (IGS): See spacer.
interkinesis: The abbreviated pause in activities related to cell division that occurs during meiosis in some species, between the first and second meiotic divisions (i.e. meiosis I and meiosis II). No DNA replication occurs during interkinesis, unlike during the normal interphase that precedes meiosis I and mitosis.[4]
internal ribosome entry site (IRES): A sequence present in some messenger RNAs that permits recognition by the ribosome and thus the initiation of translation even in the absence of a 5' cap, which in eukaryotes is otherwise required for assembly of the initiation complex. IRES elements are often located in the 5' untranslated region, but may also be found in other positions.
interphase: All stages of the cell cycle excluding cell division. A typical cell spends most of its life in interphase, during which it conducts everyday metabolic activities as well as the complete replication of its genome in preparation for mitosis or meiosis.
intracellular: Within a cell or cells; i.e. inside the plasma membrane. Contrast intercellular and extracellular.
intragenic region: See intron.
intragenic suppression
intragenomic conflict
intron: Any nucleotide sequence within a functional gene that is removed by RNA splicing during post-transcriptional modification of the mRNA primary transcript and is therefore absent from the final mature mRNA. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts. Contrast exon.
intron intrusion
intron-mediated recombination: See exon shuffling.
invagination: The infolding of a membrane toward the interior of a cell or organelle, or of a sheet of cells toward the interior of a developing embryo, tissue, or organ, forming a distinct membrane-lined pocket. In the case of individual cells, the invaginated pocket may proceed to separate from the source membrane entirely, creating a membrane-bound vesicle within the cell, as in endocytosis.[7]
inverted repeat: A nucleotide sequence followed downstream on the same strand by its own reverse complement. The initial sequence and the reverse complement may be separated by any number of nucleotides, or may be immediately adjacent to each other; in the latter case, the composite sequence is also called a palindromic sequence. Inverted repeats are self-complementary by definition, a property which involves them in many biological functions and dysfunctions. Contrast direct repeat.
in-frame: 1. (of a gene or sequence) Read or transcribed in the same reading frame as another gene or sequence; not requiring a shift in reading frame to be intelligible or to result in a functional peptide.; 2. (of a mutation) Not causing a frameshift.[4]
ionophore: Any chemical compound or macromolecule that facilitates the movement of ions across biological membranes, or more specifically, any chemical species that reversibly binds electrically charged atoms or molecules. Many ionophores are lipid-soluble proteins that catalyze the transport of monovalent and divalent cations across the hydrophobic lipid bilayers surrounding cells and vesicles.[7]
isochore: A large region of genomic DNA with a relatively homogeneous composition of base pairs, distinguished from other regions by the proportion of pairs that are G-C or A-T. The genomes of most plants and vertebrates are composed of different classes of GC-rich and AT-rich isochores.[4]
isochromosome: A type of abnormal chromosome in which the arms of the chromosome are mirror images of each other. Isochromosome formation is equivalent to simultaneous duplication and deletion events such that two copies of either the long arm or the short arm comprise the resulting chromosome.
isomeric genes: Two or more genes that are equivalent and redundant in the sense that, despite coding for distinct gene products, they each result in the same phenotype when set within the same genetic background. If several isomeric genes are present in a single genotype they may be either cumulative or non-cumulative in their contributions to the phenotype.[11]

J

jumping gene: See transposable element.
junctional diversity
junk DNA: Any DNA sequence that appears to have no known biological function, or which acts in a way that has no positive or a net negative effect on the fitness of the genome in which it is located. The term was once more broadly used to refer to all non-coding DNA, though much of this was later discovered to have a function; in modern usage it typically refers to broken or vestigial sequences and selfish genetic elements, including introns, pseudogenes, intergenic DNA, and fragments of transposons and retroviruses, which together constitute a large proportion of the genomes of most eukaryotes. Despite not contributing productively to the host organism, these sequences are able to persist indefinitely inside genomes because the disadvantages of continuing to copy them are too small to be acted upon by natural selection.
junk RNA: Any RNA-encoded sequence, especially a transcript, that appears to have no known biological function, or whose function has no positive or a net negative effect on the fitness of the genome from which it is transcribed. Despite remaining untranslated, many non-coding RNAs still serve important functions, whereas junk RNAs are truly useless: often they are the product of accidental transcription of a junk DNA sequence, or they may result from post-transcriptional processing of primary transcripts, as with spliced-out introns. Junk RNA is usually quickly degraded by ribonucleases and other cytoplasmic enzymes.

K

karyogram: A karyotype which depicts the entire set of chromosomes in a cell or organism by using photomicrographs of the actual chromosomes as they appear in vivo (usually during metaphase, in their most condensed forms), as opposed to the idealized illustrations of chromosomes used in idiograms. The photomicrographs are often still arranged in pairs and by size for easier identification of particular chromosomes, whereas in the actual nucleus there is seldom any apparent organization.
karyon: See nucleus.
karyotype: The number and appearance of chromosomes within the nucleus of a eukaryotic cell, especially as depicted in an organized karyogram or idiogram (in pairs and arranged by size and by position of the centromere). The term is also used to refer to the complete set of chromosomes in a species or individual organism or to any test that detects this complement or measures the chromosome number.
ketogenesis
ketogenic amino acid: Any amino acid that can be converted directly into acetyl-CoA, which can then be oxidized for energy or used as a precursor for many compounds containing ketone groups. This is in contrast to the glucogenic amino acids, which can be converted into glucose. In humans, seven of the 20 amino acids are ketogenic, though only leucine and lysine are exclusively ketogenic; the other five (phenylalanine, isoleucine, threonine, tryptophan, and tyrosine) are both ketogenic and glucogenic.
ketolysis
kilobase (kb): A unit of nucleic acid length equal to 1,000 bases in single-stranded molecules or 1,000 base pairs in duplex molecules such as double-stranded DNA.
kinase: Any of a class of enzymes which catalyze the transfer of phosphate groups from high-energy, phosphate-donating molecules such as ATP to one or more specific substrates, a process known as phosphorylation. The opposite process, known as dephosphorylation, is catalyzed by phosphatase enzymes.
kinesis: Any movement or change in activity by a cell or a population of cells in response to a stimulus, such that the rate of the movement or activity is dependent on the intensity of the stimulus but not on the direction from which the stimulus occurs. Kinesis is often defined as any non-specific, non-directional response, in contrast to taxis and tropism.
kinetochore: A disc-shaped protein complex which assembles around the centromere of a chromosome during prometaphase of mitosis and meiosis, where it functions as the attachment point for microtubules of the spindle apparatus.
knob: In cytogenetics, an enlarged, heavily staining chromomere that can be used as a visual marker, allowing specific chromosomes to be easily identified in the nucleus.[4]
knockdown: A genetic engineering technique by which the normal rate of expression of one or more of an organism's genes is reduced, either through direct modification of a DNA sequence or through treatment with a reagent such as a short DNA or RNA oligonucleotide with a sequence complementary to either an mRNA transcript or a gene.
knockin
knockout: A genetic engineering technique in which an organism is modified to carry genes that have been made inoperative ("knocked out"), such that their expression is disrupted at some point in the pathway that produces their gene products and the organism is deprived of their normal effects. Contrast knockin.
Kozak consensus sequence: A highly conserved nucleic acid sequence motif which functions as the recognition site for the initiation of translation in most eukaryotic messenger RNAs, generally a sequence of 10 bases immediately surrounding and inclusive of the start codon: GCCRCCAUGG. As the pre-initiation complex scans the transcript, recognition of this sequence (or a close variant) causes the complex to commit to full ribosome assembly and the start of translation. The Kozak sequence is distinct from other recognition sequences relevant to translation such as ribosome binding sites and internal ribosome entry sites.[16]
Krebs cycle: See citric acid cycle.

L

labelling: A laboratory technique involving the chemical attachment of a highly selective substance, known as a label, tag, or probe, to a particular cell, protein, amino acid, or other biomolecule of interest, either in vivo or in vitro. The label is typically a reactive derivative of a naturally fluorescent compound (e.g. green fluorescent protein) or any other substance that makes its target distinguishable in some way; other commonly used labels include dyes, enzymes, antibodies, and radioactive molecules. The labelled targets are thereby rendered distinct from their surroundings, allowing them to be easily detected, identified, quantified, or isolated for further study.
lagging strand: In DNA replication, the nascent strand for which DNA polymerase's direction of synthesis is away from the replication fork, which necessitates a complex and discontinuous process, in contrast to the streamlined, continuous synthesis of the other nascent strand that occurs simultaneously. Because DNA polymerase works only in the 5' to 3' direction, but the overall direction of chain elongation must ultimately be the opposite (i.e. 3' to 5', toward the replication fork), the elongation must occur by an indirect mechanism in which a primase synthesizes short RNA primers complementary to the template DNA, and DNA polymerase then extends the primed segments into short chains of nucleotides known as Okazaki fragments. The RNA primers are then removed and replaced with DNA, and the Okazaki fragments are joined by DNA ligase.
lampbrush chromosome
lateral gene transfer (LGT): See horizontal gene transfer.
leader sequence: See 5' untranslated region.
leading strand: In DNA replication, the nascent strand for which both DNA polymerase's direction of synthesis and that of the overall chain elongation are toward the replication fork, i.e. both occur in the 5' to 3' direction, resulting in a single, continuous elongation process with few or no interruptions. By contrast, the other nascent strand, known as the lagging strand, is assembled in a discontinuous process involving the ligation of short DNA fragments which are synthesized in the opposite direction, away from the replication fork.[4]
left splicing junction: The boundary between the left end (by convention, the 5' end) of an intron and the right (3') end of an adjacent exon in a pre-mRNA transcript.
leptonema: In meiosis, the first of five substages of prophase I, following interphase and preceding zygonema. During leptonema, the replicated chromosomes condense from diffuse chromatin into long, thin strands that are much more visible within the nucleus.
lethal equivalent value: The average number of recessive deleterious genes existing in the heterozygous condition that is carried by a member of a population of diploid organisms, multiplied by the average probability that each such gene will cause premature lethality when homozygous. For example, an organism carrying eight recessive semilethal alleles, each of which produces only a 50% probability of premature death when homozygous, is said to carry a genetic burden of four "lethal equivalents".[4]
lethal mutation: Any mutation that results in the premature death of the organism carrying it. Recessive lethal mutations are fatal only to homozygotes, whereas dominant lethals are fatal even in heterozygotes.[4]
leucine zipper
ligase: A class of enzymes which catalyze the joining of large molecules such as nucleic acids by forming one or more chemical bonds between them, typically C–C, C–O, C–S, or C–N bonds via condensation reactions. An example is DNA ligase, which catalyzes the formation of phosphodiester bonds between adjacent nucleotides on one or both strands of a DNA molecule, a reaction known as ligation.
ligation
linkage: The tendency of DNA sequences which are physically near to each other on the same chromosome to be inherited together during meiosis. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often loci or genetic markers) will be separated on to different chromatids during chromosomal crossover is statistically very low; such loci are then said to be more linked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly unlinked. The standard unit for measuring genetic linkage is the centimorgan (cM).
linkage disequilibrium
linker DNA: 1. A short, synthetic DNA duplex containing the recognition sequence for a particular restriction enzyme.[4] In molecular cloning, linkers are often deliberately included in recombinant molecules in order to make them easily modifiable by permitting cleavage and insertion of foreign sequences at precise locations. A segment of an engineered plasmid containing many such restriction sites is sometimes called a polylinker.; 2. A section of chromosomal DNA connecting adjacent nucleosomes by binding to histone H1.[4]
linking number: The number of times that the two strands of a circular double-helical DNA molecule cross each other, equivalent to the twisting number (which measures the torsion of the double helix) plus the writhing number (which measures the degree of supercoiling). The linking number of a closed molecule cannot be changed without breaking and rejoining the strands. DNA molecules which are identical except for their linking numbers are known as topological isomers.[4]
lncRNA: See long non-coding RNA.
locus: A specific, fixed position on a chromosome where a particular gene or genetic marker resides.
LOD score
long arm: In condensed chromosomes where the positioning of the centromere creates two segments or "arms" of unequal length, the longer of the two arms of a chromatid. Contrast short arm.
long interspersed nuclear element (LINE): Any of a large family of non-LTR retrotransposons which together comprises one of the most widespread mobile genetic elements in eukaryotic genomes. Each LINE insertion is on average about 7,000 base pairs in length.
long non-coding RNA (lncRNA): A class of non-coding RNA consisting of all transcripts of more than 200 nucleotides in length that are not translated. This limit distinguishes lncRNA from the numerous smaller non-coding RNAs such as microRNA. See also long intervening non-coding RNA.
lyonization: See X-inactivation.
lysis: The disruption and decomposition of the plasma membrane surrounding a cell, or more generally of any membrane-bound organelle or vesicle, especially by osmotic, enzymatic, or other chemical or mechanical processes which compromise the membrane's integrity and thereby cause the unobstructed interchange of the contents of intracellular and extracellular spaces. Lysis generally implies the complete and irreversible loss of intracellular organization as a result of the release of the cell's internal components and the dilution of the cytosol, and therefore the death of the cell. Such a cell is said to be lysed, and a fluid containing the contents of lysed cells (often including nucleic acids, proteins, and many other organic molecules) is called a lysate. Lysis may occur both naturally and artificially, and is a normal part of the cellular life cycle.
lysosome

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Lewin, Benjamin (2003). Genes VIII. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0-13-143981-2.
MacLean, Norman (1987). Dictionary of Genetics & Cell Biology. New York: New York University Press. ISBN 0-8147-5438-4.
Nishikawa, S. (2007). "Reprogramming by the numbers". Nature Biotechnology. 25 (8): 877–878. doi:10.1038/nbt0807-877. PMID 17687365. S2CID 39773318.
Connerly, P. L. (2010). "How Do Proteins Move Through the Golgi Apparatus?". Nature Education. 3 (9): 60.
Priness, I.; Maimon, O.; Ben-Gal, I. (2007). "Evaluation of gene-expression clustering via mutual information distance measure". BMC Bioinformatics. 8: 111. doi:10.1186/1471-2105-8-111. PMC 1858704. PMID 17397530.
Rieger, Rigomar (1991). Glossary of Genetics: Classical and Molecular (5th ed.). Berlin: Springer-Verlag. ISBN 3540520546.
"Functional Genomics Data Society – FGED Society".
Edgar, R; Domrachev, M; Lash, AE (1 January 2002). "Gene Expression Omnibus: NCBI gene expression and hybridization array data repository". Nucleic Acids Research. 30 (1): 207–10. doi:10.1093/nar/30.1.207. PMC 99122. PMID 11752295.
Barrett, T; Wilhite, SE; Ledoux, P; Evangelista, C; Kim, IF; Tomashevsky, M; Marshall, KA; Phillippy, KH; Sherman, PM; Holko, M; Yefanov, A; Lee, H; Zhang, N; Robertson, CL; Serova, N; Davis, S; Soboleva, A (January 2013). "NCBI GEO: archive for functional genomics data sets--update". Nucleic Acids Research. 41 (Database issue): D991-5. doi:10.1093/nar/gks1193. PMC 3531084. PMID 23193258.
Yakovchuk P, Protozanova E, Frank-Kamenetskii MD (2006). "Base-stacking and base-pairing contributions into thermal stability of the DNA double helix". Nucleic Acids Res. 34 (2): 564–74. doi:10.1093/nar/gkj454. PMC 1360284. PMID 16449200.
Kozak, M. (February 1989). "The scanning model for translation: an update". The Journal of Cell Biology. 108 (2): 229–241. doi:10.1083/jcb.108.2.229. ISSN 0021-9525. PMC 2115416. PMID 2645293.

External links

National Human Genome Research Institute (NHGRI) Talking Glossary of Genetic Terms

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[1] "Talking Glossary of Genetic Terms". genome.gov. 8 October 2017. Retrieved 8 October 2017.

[CoG-2] Klug, William S.; Cummings, Michael R. (1986). Concepts of Genetics (2nd ed.). Glenview, Ill.: Scott, Foresman and Company. ISBN 0-673-18680-6.

[Ussery-3] Ussery, David W. "DNA Structure: A-, B-, and Z-DNA Helix Families". Lyngby, Denmark: Danish Technical University.

[DoG7-4] King, Robert C.; Stansfield, William D.; Mulligan, Pamela K. (2006). A Dictionary of Genetics (7th ed.). Oxford: Oxford University Press. ISBN 978-0-19-530762-7.

[NCI-5] "NCI Dictionary of Genetics Terms". National Cancer Institute. PDQ® Cancer Genetics Editorial Board.

[Lewin-6] Lewin, Benjamin (2003). Genes VIII. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0-13-143981-2.

[MacLean-7] MacLean, Norman (1987). Dictionary of Genetics & Cell Biology. New York: New York University Press. ISBN 0-8147-5438-4.

[8] Nishikawa, S. (2007). "Reprogramming by the numbers". Nature Biotechnology. 25 (8): 877–878. doi:10.1038/nbt0807-877. PMID 17687365. S2CID 39773318.

[9] Connerly, P. L. (2010). "How Do Proteins Move Through the Golgi Apparatus?". Nature Education. 3 (9): 60.

[Priness-10] Priness, I.; Maimon, O.; Ben-Gal, I. (2007). "Evaluation of gene-expression clustering via mutual information distance measure". BMC Bioinformatics. 8: 111. doi:10.1186/1471-2105-8-111. PMC 1858704. PMID 17397530.

[Rieger-11] Rieger, Rigomar (1991). Glossary of Genetics: Classical and Molecular (5th ed.). Berlin: Springer-Verlag. ISBN 3540520546.

[urlFGED_Homepage-12] "Functional Genomics Data Society – FGED Society".

[13] Edgar, R; Domrachev, M; Lash, AE (1 January 2002). "Gene Expression Omnibus: NCBI gene expression and hybridization array data repository". Nucleic Acids Research. 30 (1): 207–10. doi:10.1093/nar/30.1.207. PMC 99122. PMID 11752295.

[14] Barrett, T; Wilhite, SE; Ledoux, P; Evangelista, C; Kim, IF; Tomashevsky, M; Marshall, KA; Phillippy, KH; Sherman, PM; Holko, M; Yefanov, A; Lee, H; Zhang, N; Robertson, CL; Serova, N; Davis, S; Soboleva, A (January 2013). "NCBI GEO: archive for functional genomics data sets--update". Nucleic Acids Research. 41 (Database issue): D991-5. doi:10.1093/nar/gks1193. PMC 3531084. PMID 23193258.

[Yakovchuk2006-15] Yakovchuk P, Protozanova E, Frank-Kamenetskii MD (2006). "Base-stacking and base-pairing contributions into thermal stability of the DNA double helix". Nucleic Acids Res. 34 (2): 564–74. doi:10.1093/nar/gkj454. PMC 1360284. PMID 16449200.

[Kozak-16] Kozak, M. (February 1989). "The scanning model for translation: an update". The Journal of Cell Biology. 108 (2): 229–241. doi:10.1083/jcb.108.2.229. ISSN 0021-9525. PMC 2115416. PMID 2645293.

Glossary of genetics (0–L)

0–9

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C

D

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H

I

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See also

References

Further reading

External links