The nucleosome core particle is the fundamental structural unit of the eukaryotic genome. It consists of a histone octamer composed of two H2A-H2B dimers and a H3-H4 tetramer wrapped by ~146 base pairs of DNA. A linker histone (i.e., H1) associates with the nucleosomal dyad as well as with linker DNA on either side of the nucleosome, resulting in the formation of the chromatosome. All the core and linker histones are posttranslationally decorated, with at least 160 total modifications described to date including acetylation, methylation, phosphorylation, propionylation, citrullination, formylation, proline isomerization, butyrylation, ADP ribosylation, ubiquitylation, sumoylation, and the more recently identified glycosylation and crotonylation.
These modifications are thought to impact chromatin functions by either altering chromatin packaging or through the recruitment/inhibition of specific chromatin binding factors. Thus, the combined signal from a particular collection of histone marks constitutes a “histone code” that affects gene expression or other chromatin-based functions.