The compaction of genome to accommodate it inside the nucleus of 20-80 microns is accomplished by spooling the DNA thread over nucleosomes to form chromatin. Thus chromatin contributes to compact the genomic DNA. The multiple interactions between histones and DNA make the nucleosome one of the most stable protein-DNA complexes under physiological conditions. The regulatory role of chromatin is increasingly being recognized and is in the center stage in current literature. Extensive analysis of structure of nucleosome over the years has recently yielded high resolution crystal structure which details the various contacts between the histone core complex and DNA. The nucleosome core is composed of 147 bp of DNA wrapped 1.65 turns around the histone octamer (two copies each of the core histones H3,H4, H2A, H2B). The core histone octamer is assembled into two nearly symmetrical halves forming an H3-H4 tetramer and two pairs of the H2A and H2B histones. Each end of the core histones possesses an N-terminal and a C-terminal tail that protrudes from the nucleosome core. The nucleosome core particle directly interacts with DNA at 116 direct sites and via 358 indirect site(Davey et al., 2002). The tail domains also contribute to several of these interactions, mainly via the DNA minor grove. The crystal structure of thenucleosome indicates that histone tails also make specific contacts with adjacent nucleosomes. For example the histone H4 N-terminal tail interacts with histone H2A/H2B on the adjacent nucleosome. These N-terminal tails are the primary targets for post translational histone modifications.
