The DNA found inside the eukaryotic nucleus is usually associated with special classes of proteins that form a DNA-protein complex termed chromatin. In turn, chromatin clumps into tightly packed units termed nucleosomes. The structure of nucleosomes can restrict the accessibility of DNA and affect gene regulation.

The nuclei of most eukaryotic cells (cells with a nucleus defined by a nuclear membrane) contain nearly equal amounts of DNA and specialized proteins that bind to the DNA to form chromatin. Approximately half of the proteins associated with DNA are a form of very basic protein termed histones. There are five major classes or forms of histone proteins usually associated with DNA. The histones most commonly associated with DNA are designated H1, H2A, H2B, H3 and H4.

The packaging of DNA with histones and non-histone chromosomal proteins allows bonding to occur in the molecules surrounding the actual DNA nucleotides...