Eukaryotic DNA has a group of proteins associated with it. These small, basic proteins are called histones. They are basic because approximately 25 percent of the amino acid residues present are either arginine or lysine. These are tightly bound to the DNA and total approximately half of the mass of a chromosome. A complex of the cell’s DNA and associated protein is a chromatin, and there are five important histones present in chromatin: H1 — and four that associate with each other: H2A, H2B, H3, and H4. A chromatin apparently consists of repeat units consisting of two copies each of H2A, H2B, H3, and H4, with a strand of DNA consisting of about 200 base pairs tightly wrapped around this histone octomer. Each of these repeating units is a nucleosome. The wrapping of the DNA to form a nucleosome yields a significant compaction of the DNA. Research indicates that about 145 of the 200 base pairs are actually associated with the histone octomer, and the remaining base pairs are linker DNA that link one histone octomer to the next. Histone H1 usually binds to linker DNA. The eight histones in a histone octomer are arranged into a tetramer with the composition (H3)2(H4)2 and two dimers each with the composition (H2A)(H2B). All the histone proteins have long tails rich in arginine and lysine residues that extend out of the coreModification of these tails is important for gene regulation. The structure of chromatin is a factor in eukaryotic gene regulation. For a gene to be available for transcription, the tightly packed chromatin structure must open.
