Figure 1. Model of octameric histone core based on a 3.1 Å resolution structure determined by X-ray crystalography. The histone core contains two copies each of H2A (light blue), H2B (dark blue), H3 (green), and H4 (white). The spheres represent amino acid residues, not atoms. The positively charged arginine and lysine residues are red. The amino termini of the histone proteins are not visualized by X-ray crystallography, but they are thought to extend outward from the top and bottom of this view of the histone octameric core. (from Lodish, H. et al., (1995) Molecular Cell Biology. 3rd edition. Scientific American Books).

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