A nucleosome is the basic structural unit of DNA packaging in eukaryotes. It consists of a segment of DNA wound around eight histone proteins, resembling thread wrapped around a spool. The nucleosome is the fundamental subunit of chromatin, and each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, known as a histone octamer. Nucleosomes play a key role in the compaction of DNA to fit within the cell nucleus and serve as a scaffold for the formation of higher-order chromatin structure as well as for a layer of regulatory control of gene expression
. Each human cell contains about 30 million nucleosomes, and they are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones
. Nucleosomes were first observed as particles in the electron microscope in 1974, and their existence and structure were proposed by Roger Kornberg
. Nucleosomes are highly dynamic and can undergo spontaneous sliding, "splitting," or even complete dissociation
