The nucleosome is the smallest part of the chromosome. It consists of two core histones of each class H2A, H2B, H3 and H4 which assemble and are wrapped around 146 base pairs of DNA. Histones are the main constituents of the protein part of chromosomes of eukaryotic cells. They pack the DNA into tight masses of chromatin. Histones are rich in the amino acids arginine and lysine and have been greatly conserved during evolution. Histone tails undergo numerous post-translational modifications, which either directly or indirectly alter chromatin structure to facilitate transcriptional activation or repression or other nuclear processes.