| MadSci Network: Biochemistry |
In fact, DNA does not coil around a single histone, the DNA is wrapped (twice) around a complex of proteins formed by four different histones called H2A, H2B, H3 and H4. Each complex contains 2 of each of these four proteins (a total of eight proteins in the complex) and is called the "nucleosome core". Approximately 150 base pairs of DNA are wrapped two times around the protein complex; you can imagine a spool (the nucleosome core) with thread (the DNA) wrapped around twice. At the site on the "core" where the DNA enters and leaves the protein complex, a fifth histone (H1), binds to the core forming a complete nucleosome. Histones H2A, H2B, H3 and H4 are some of the most highly conserved proteins throughout evolution. The amino acid sequences have changed very little over time and are similar in plants and animals. All of these proteins are rich in the basic amino acids lysine and arginine. This gives these proteins an overall positive charge; in addition, these positively charged amino acids tend to be arranged on the outside surface of the proteins where they can interact with DNA. Remember that the sugar-phosphate backbone of DNA give the molecule a uniformly distributed negative charge. Thus, DNA wraps around the nucleosome core by mainly electrostatic interaction between the individual histone proteins arranged in the "core" and the backbone of DNA molecule.
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