Histone

In biology, histones are the chief proteins of chromatin. They act as spools around which DNA winds and they play a role in gene regulation. Histones are found in the nuclei of eukaryotic cells. Bacteria do not have histones, but histones are found in certain Archaea, namely Euryarchaea. These archaeal histones may well resemble the evolutionary precursors to the eukaryotic histones.

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Six histone classes are known:

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• H1 (sometimes called the linker histone or H5.)
• H2A
• H2B
• H3
• H4
• Archaeal histones

Two each of the class H2A, H2B, H3 and H4 assemble to form one octameric nucleosome core by wrapping 146 base pairs of DNA around the protein spool in nearly two turns. The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order stuctures of DNA topology. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA spaced between each nucleosome. Higher order stuctures include the 30 nm fiber and 100 nm fiber, until finally through the combination of nucleosome interactions with other proteins, the chromosome is assembled.

Histone: Classes ►



 

Protein: A protein (in Greek πρωτεϊνη = first thread) is a complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds. Proteins are essential to the structure and function of all living cells and viruses....

Chromatin: Chromatin is the substance of a chromosome and consists of a complex of DNA and protein in eukaryotic cells. It can be made visible by staining, hence its name, which literally means coloured material. The nucleic acids are generally in the form of double-stranded DNA. The major proteins involve...

DNA: :For other uses, see DNA (disambiguation)....