Histone Complexes, Nucleosomes, Chromatin and Cell-Cycle Dependent Modification of Histones

The concept of a repeating chromatin subunit introduced by Hewish and Burgoyne (1), Olins and Olins (2) and Woodcock (3) is now well established. This subunit, the nucleosome (4), has a variable DNA repeat depending on the tissue of origin, although for most somatic tissues the DNA repeat is 195 ± 5 base pairs; in addition, the nucleosome contains two each of the histones H2A, H2B, H3 and H4 (5), the so-called ‘core histones’, and probably one molecule of H1. Irrespective of their origin, further digestion of nucleosomes with micrococcal nuclease leads to an apparently invariant “core’ particle containing 140 base pairs of DNA and the eight histone molecules (6,7). This particle is not found to contain H1, and so it is generally assumed that the H1 is associated with the linker DNA which joins the core particles, although it may well have additional sites of interaction in whole chromatin.