homologous structure

(noun)

the traits of organisms that result from sharing a common ancestor; such traits often have similar embryological origins and development

Related Terms

Examples of homologous structure in the following topics:

  • Homologous Structures

    • Homology is the relationship between structures or DNA derived from the most recent common ancestor.
    • If we go all the way back to the beginning of life, all structures are homologous!
    • As a result, hybrid or mosaic structures can evolve that exhibit partial homologies.
    • The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately (rather than being present in the last common ancestor).
    • Describe the connection between evolution and the appearance of homologous structures

  • Distinguishing between Similar Traits

    • Similar traits can be either homologous structures that share an embryonic origin or analogous structures that share a function.
    • For example, the bones in the wings of bats and birds have homologous structures .
    • Homologous structures share a similar embryonic origin; analogous organs have a similar function.
    • Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous.
    • Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary past.

  • Vestigial Structures

    • These unused structures without function are called vestigial structures.
    • Vestigial structures are often homologous to structures that function normally in other species.
    • In some cases the structure becomes detrimental to the organism.
    • The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure.
    • Homologous structures indicate common ancestry with those organisms that have a functional version of the structure.

  • Convergent Evolution

    • However, the wings of bats and insects have evolved from very different original structures.
    • Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye.
    • Traits arising through convergent evolution are analogous structures, in contrast to homologous structures, which have a common origin, but not necessarily similar function.
    • The British anatomist Richard Owen was the first scientist to recognize the fundamental difference between analogies and homologies.
    • Bat and pterosaur wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions.

  • Homologs, Orthologs, and Paralogs

    • A homologous trait is often called a homolog (also spelled homologue).
    • In genetics, the term "homolog" is used both to refer to a homologous protein and to the gene (DNA sequence) encoding it.
    • As with anatomical structures, homology between protein or DNA sequences is defined in terms of shared ancestry.
    • As with anatomical structures, high sequence similarity might occur because of convergent evolution, or, as with shorter sequences, because of chance.
    • Human angiogenin diverged from ribonuclease, for example, and while the two paralogs remain similar in tertiary structure, their functions within the cell are now quite different.

  • Gene rearrangement within genomes

    • Most recombination is naturally occurring and the rearrangement of DNA can occur by several different mechanisms and result in structural variation.
    • It is known that this pairing and interaction between homologous chromosomes, known as synapsis, does more than simply organize the homologs for migration to separate daughter cells.
    • When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms .
    • In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA.
    • Recombination can occur between DNA sequences that contain no sequence homology.

  • Chromosomal Structural Rearrangements

    • Structural rearrangements of chromosomes include both inversions and translocations, which may have detrimental effects on an organism.
    • Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common.
    • To maintain point-for-point synapsis during meiosis, one homolog must form a loop, and the other homolog must mold around it.
    • Not all structural rearrangements of chromosomes produce nonviable, impaired, or infertile individuals.
    • Describe the various types of structural rearrangements of chromosomes and how they can impact an organism

  • Meiosis I

    • The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate.
    • The tight pairing of the homologous chromosomes is called synapsis.
    • The homologous pairs orient themselves randomly at the equator.
    • Crossover occurs between non-sister chromatids of homologous chromosomes.
    • Early in prophase I, homologous chromosomes come together to form a synapse.

  • Generalized Recombination and RecA

    • After strand invasion, one or two cross-shaped structures called Holliday junctions connect the two DNA molecules.
    • Homologous recombination is conserved across all three domains of life as well as viruses.
    • Homologous recombination is a major DNA repair process in bacteria.
    • Homologous recombination has been most studied and is best understood for Escherichia coli.
    • If the D-loop is cut, another swapping of strands forms a cross-shaped structure called a Holliday junction.Resolution of the Holliday junction by some combination of RuvABC or RecG can produce two recombinant DNA molecules with reciprocal genetic types, if the two interacting DNA molecules differ genetically.

  • Genomic DNA and Chromosomes

    • Before discussing the steps a cell must undertake to replicate, a deeper understanding of the structure and function of a cell's genetic information is necessary.
    • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
    • Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the genes themselves are not identical.
    • The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.
    • There are 23 pairs of homologous chromosomes in a female human somatic cell.