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.

• 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.

• 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.

• Bacterial Chromosomes in the Nucleoid

  • Proteins helping to maintain the supercoiled structure of the nucleic acid are known as nucleoid proteins or nucleoid-associated proteins, and are distinct from histones of eukaryotic nuclei.
  • In contrast to histones, the DNA-binding proteins of the nucleoid do not form nucleosomes, in which DNA is wrapped around a protein core.

• Psychrophilic Crenarchaeota

  • However, other physiological features, such as lack of histones have supported this division, although some crenarchaea were found to have histones.

• Chromosomes and Genes

  • Chromosomes are structures in the nucleus of a cell containing DNA, histone protein, and other structural proteins.
  • Each chromosome is made up of a single DNA molecule coiled around histone proteins.