is located because this determines the accessibility of the DNA to regulatory proteins.

Nucleosomes were first observed as particles in the electron microscope by Don and Ada Olins in 1974, and their existence and structure (as histone octamers surrounded by approximately 200 base pairs of DNA) were proposed by Roger Kornberg. The role of the nucleosome as a regulator of transcription was demonstrated by Lorch et al. in vitro in 1987 and by Han and Grunstein