histones

Examples of histones in the following topics:

• Epigenetic Control: Regulating Access to Genes within the Chromosome

  • How the histone proteins move is dependent on signals found on both the histone proteins and on the DNA.
  • These signals are tags, or modifications, added to histone proteins and DNA that tell the histones if a chromosomal region should be open or closed.
  • DNA is folded around histone proteins to create (a) nucleosome complexes.
  • Modifications to the histones and DNA affect nucleosome spacing.
  • Histone proteins and DNA nucleotides can be modified chemically.

• Epigenetic Alterations in Cancer

  • Common in cancer cells, silencing genes, which occur through epigenetic mechanisms, include modifications to histone proteins and DNA.
  • Silencing genes through epigenetic mechanisms is very common in cancer cells and include modifications to histone proteins and DNA that are associated with silenced genes.
  • Histone proteins that surround that region lack the acetylation modification (the addition of an acetyl group) that is present when the genes are expressed in normal cells.
  • This combination of DNA methylation and histone deacetylation (epigenetic modifications that lead to gene silencing) is commonly found in cancer.
  • Mechanisms can include modifications to histone proteins and DNA associated with these silencing genes.

• Eukaryotic Chromosomal Structure and Compaction

  • In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome .
  • The DNA-histone complex is called chromatin.
  • The beadlike, histone DNA complex is called a nucleosome.
  • A DNA molecule in this form is about seven times shorter than the double helix without the histones.
  • Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of "beads on a string."

• Transcriptional Enhancers and Repressors

  • The corepressor can repress transcriptional initiation by recruiting histone deacetylase, which catalyzes the removal of acetyl groups from lysine residues.
  • This increases the positive charge on histones, which strengthens the interaction between the histones and DNA, making the DNA less accessible to the process of transcription.

• Elongation and Termination in Eukaryotes

  • The DNA is tightly packaged around charged histone proteins at repeated intervals.
  • These DNA–histone complexes, collectively called nucleosomes, are regularly spaced and include 146 nucleotides of DNA wound twice around the eight histones in a nucleosome like thread around a spool.
  • FACT partially disassembles the nucleosome immediately ahead (upstream) of a transcribing RNA Polymerase II by removing two of the eight histones (a single dimer of H2A and H2B histones is removed.)
  • FACT reassembles the nucleosome behind the RNA Polymerase II by returning the missing histones to it.
  • DNA in eukaryotes is packaged in nucleosomes, which consist of an octomer of 4 different histone proteins.

• The Nucleus and Ribosomes

  • In order to organize the large amount of DNA within the nucleus, proteins called histones are attached to chromosomes; the DNA is wrapped around these histones to form a structure resembling beads on a string.
  • Along the chromatin threads, unwound protein-chromosome complexes, we find DNA wrapped around a set of histone proteins.

• DNA Packaging

  • At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes.
  • The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer.
  • The DNA (which is negatively charged because of the phosphate groups) is wrapped tightly around the histone core.

• Binary Fission

  • Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus, no nucleosomes in prokaryotes.

• Altered Gene Expression in Cancer

  • Therefore, changes in histone acetylation (epigenetic modification that leads to gene silencing), activation of transcription factors by phosphorylation, increased RNA stability, increased translational control, and protein modification can all be detected at some point in various cancer cells.

• Characteristics of Eukaryotic DNA

  • Eukaryotic DNA is packed into bundles of chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones, which wind the DNA into a more compact form.