histonene

Histones are small, highly basic nuclear proteins that package and organize DNA in eukaryotic cells. The core histones—H2A, H2B, H3, and H4—assemble into a histone octamer around which about 147 base pairs of DNA are wrapped to form a nucleosome, the fundamental unit of chromatin. Two copies of each core histone constitute the octamer. The linker histone H1 binds to the DNA between nucleosomes and helps compact chromatin into higher-order fibers.

Variants of histones exist. In addition to canonical histones, cells express replacement variants such as H3.3, CENP-A, and H2A.Z, which can influence chromatin function at active genes, centromeres, or sites of DNA damage. H1 also has several variants that affect chromatin structure.

Histone tails protruding from the nucleosome are subject to a range of post-translational modifications, including acetylation, methylation, phosphorylation, and ubiquitination. These modifications, installed and removed by specific enzymes (for example, histone acetyltransferases and deacetylases, methyltransferases and demethylases, kinases, ubiquitin ligases), influence chromatin accessibility and recruitment of effector proteins. The combination of modifications is often described by the histone code hypothesis as a mechanism to regulate transcription, replication, repair, and recombination.

Histones are deposited onto DNA by histone chaperones and remodeled during transcription and replication. Complexes such as ASF1, CAF-1, and FACT assist in histone storage, supply, and assembly. Proper histone regulation is essential for epigenetic inheritance and genome stability.