Chromatin is the macromolecular state in which nuclear DNA is packaged within the cell. Nucleosomes are the structural units of chromatin. Each nucleosome is composed of ~147 bp double stranded DNA and a histone octamer which contains two copies of each core histone (H2A, H2B, H3 and H4). Histone H1, a linker histone, is involved in binding between nucleosomes to further compact the chromatin. The striking features of these nucleosomal histones are their amino terminus tails which are often modified by various histone modifying enzymes. The predominate modifications on histone tails include but are not limited to methylation, acetylation and phosphorylation. Histone modifications are key to transcription, replication and DNA repair functions (1, 2).
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