The Histone Octamer Assembly & the Nucleosome

Two molecules of each of the four core histone proteins form the histone octamer via formation of one tetramer of H3 and H4 and two dimers of H2A and H2B. Each of these entities are held together by a so called hand-shake motif of protein structure. Note that the nucleosome containing the two turns of DNA has the N-terminal tails of the eight histone protein sticking out from the nucleosome like the legs of a spider. The structure of the portion of these N-terminal tails outside of the DNA is not known, and more importantly, nor is the 30 nm chromatin fiber.