Chromatin is composed of both DNA and histone proteins, HI, H2A, H2B, H3, and H4. The repeating unit of chromatin is the nucleosome, defined as two of each of the core histones, H2A, H2B, H3, and H4 wrapped almost twice by approximately 146 base pairs of DNA.The nucleosome establishes the first level of chromatin organization. Nucleosomes themselves can be stacked and coiled to form higher order structures; one example of which is the 30 nm chromatin fiber. Greater levels of compaction result in the looped and organized fibers of the metaphase chromosome. Necessarily, wrapping of DNA as nucleosomes and compaction into higher order structures inhibits DNA-dependent nuclear processes. Accessing DNA, which is structured as chromatin, is a major challenge for transcription, replication and repair in all cells. Several mechanisms have evolved to allow access including chromatin remodeling by ATP-dependent protein complexes and enzymes that modify histone N-terminal tails (discussed below and in the next chapter).

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