HISTONES
Histones are small basic chromosomal proteins with high content of arginine and lysine. They serve for DNA stabilizing and folding in cell nuclei. There are five kinds of histones: H1 (very rich in lysine), H2a and H2b (rich in lysine), H3 (rich in arginine) and H4 (rich in glycine and arginine). Two molecules of each of four core histones (H2A, H2B, H3 è H4) form an octamer in the following sequence: H2A-H2B-H4-H3-H3-H4-H2BH2A. DNA is wrapped in repeating units around it. Each turn is about 146 nucleotides long. DNA turns, winding around octamers, are called nucleosomes. The linker histone, H1, interacts with DNA between nucleosomes and functions in the compaction of nucleosomes into higher order structures. H1 can be absent, like, for example, in yeasts.
Histone genes are situated in the genome in the form of large clusters. An example of one of the three histone clusters, regulated by the cell cycle, which are placed on the 6th chromosome.