2

The fundamental repeating unit of the chromatin polymer is the nucleosome

(Fig. 1.1 ). The nucleosome contains a nucleosome core with 145–147 base pairs (bp)

of DNA wrapped around an octamer of histone proteins, constructed from two copies

of each of the core histones, histone H2A, histone H2B, histone H3, and histone H4.

Each nucleosome core is connected to an adjacent nucleosome core through a segment

of linker DNA to form the chromatin polymer with a repeat length ranging from

160 to 240 bp (McGhee and Felsenfeld 1980 ). Approximately 20 bp of this linker

DNA is typically found in association with the linker histone H1 (also H5). The

nucleosome core together with the linker histone is called the chromatosome . Adding

the remaining linker DNA to the chromatosome completes the nucleosome .