Histones are proteins that aid in the organization of DNA into chromosomes. There are four types of histones found in chromatin: H2A, H2B, H3, and H4. Two of each histone comprise a protein core, a nucleosome, which DNA wraps around twice. What results is a "bead on a string" model in which the nucleosomes are the beads and the
linker DNA binds them together. Another characteristic of histones are the protein tails that extend outward from the "bead." The tails and the degree of their acetylation is an important factor in gene expression.
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