Native histones isolated from calf thymus. A nucleosome is the basic repeating unit of chromatin in which 146 base pairs of DNA wrap twice around a histone octamer consisting of two copies of each of the core histones, H2A, H2B, H3, and H4. The combination of two H2A/H2B dimers and one H3/H4 tetramer create the nucleosome core.2 Histone H3 undergoes many modifications which include acetylation, methylation, and phosphorylation that are important for regulation of gene transcription.