Histone, nucleosome, and chromatin structure

In organisms ranging from yeast to humans, DNA is packaged inside the nucleus of cells in a polymeric complex called chromatin. The fundamental unit of chromatin is the nucleosome, consisting of a nucleosome core, a linker histone, and a segment of linker DNA. The nucleosome core is constructed from 145 to 147 base pairs of DNA wrapped around an octamer containing two copies of each of the core histone proteins (H2A, H2B, H3, and H4). While its composition has long since been established, more recent X-ray crystallographic investigations of the nucleosome core have painted a high-resolution picture of the histone octamer and its interactions with DNA. The nucleosome core and linker DNA connecting it to an adjacent nucleosome interact with the linker histone (H1/H5). This unit repeats throughout the genome every 160-240 bp, forming long arrays of nucleosomes that progressively condense into higher-order chromatin structures. This chapter details the structure and dynamics of the nucleosome core particle and discusses our current understanding of nucleosome recognition, the linker histone, and the first level of higher-order chromatin structure, the 30nm fiber.