In eukaryotes, DNA is wrapped around histone octamers to form the basic unit of chromatin structure. The octamer is composed of histones H2A, H2B, H3 and H4, and it associates with approximately 200 base pairs of DNA to form the nucleosome. The association of DNA with histones results in dense packing of chromatin, which restricts proteins involved in gene transcription from binding to DNA. Histone H3, the core protein of the nucleosome, becomes phosphorylated at the end of prophase. The two major sites of phosphorylation are the mitosis-specific site Ser10, and Ser28, both of which are extensively phosphorylated in DNA-bound forms of histone H3 and in nucleosomal histone H3. The nucleosome structure of histone H3 promotes N-terminal phosphorylation in vitro.
