Histone

Histones act as spools around which DNA winds. This packs in the large genomes of eukaryotes to fit inside cell nuclei. The compacted molecule is 40,000 times shorter than an unpacked molecule.

Histones undergo changes which alter their interaction with DNA and nuclear proteins. Long-term changes in histone/DNA interaction cause epigenetic effects. Combinations of modifications are thought to constitute a code, the so-called histone code.^[4][5] Histone modifications act in diverse biological processes such as gene regulation, DNA repair and chromosome condensation (mitosis).

Examples of histone modifications in transcription regulation include:

Histones were discovered in 1884 by Albrecht Kossel. The word "histone" dates from the late 19th century and is from the German "Histon", of uncertain origin: perhaps from Greek histanai or from histos. Until the early 1990s, histones were dismissed as merely packing material for nuclear DNA. During the early 1990s, the regulatory functions of histones were discovered.^[11]

The discovery of the H5 histone appears to date back to 1970's.^[12][13]

Histones are found in the nuclei of eukaryotic cells, and in certain Archaea, namely Euryarchaea, but not in bacteria. Histone proteins are among the most highly conserved proteins in eukaryotes,^[14] which suggests they are vital to the biology of the nucleus.^[2]: 939 In contrast, mature sperm cells largely use protamines to package their genomic DNA, most likely to achieve an even higher packaging ratio.^[15]

Core histones are highly conserved proteins, that is, there are very few differences among the amino acid sequences of the histone proteins of different species. Linker histone usually has more than one form within a species and is also less conserved than the core histones.