Eight histone proteins make up the nucleosome core, forming a complex called a histone octamer.
The octamer contains two copies each of four core histones: H2A, H2B, H3, and H4. At its center is an H3-H4 tetramer, made from two H3 proteins and two H4 proteins. Two H2A-H2B dimers then attach to that tetramer, producing the eight-protein core.
DNA wraps around this positively charged histone assembly because DNA’s phosphate backbone is negatively charged. The nucleosome core particle contains roughly 146 base pairs of DNA wound around the octamer. A separate linker histone, usually called H1, can associate with DNA entering and leaving the nucleosome, but H1 is not one of the eight proteins in the core octamer.
This distinction explains a common mix-up: histone is the name of a protein family, while octamer describes the eight-protein arrangement at the nucleosome’s center. Histone tails also carry chemical modifications that influence chromatin compaction and gene accessibility.