Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
DNA: Structure of Chromosomes and Genes
Histones are small basic proteins

Histones are found in the Chromatin of all somatic Eukaryotic Cells, but have never been detected in prokaryotes. Their molecular weights range from 11,000 to 21,000. Histones are remarkably rich in the basic Amino Acids Arginine and Lysine, which account for up to 25% of the protein's amino acid residues. Because the side (R) groups of arginine and lysine residues are protonated at pH 7 and thus carry a positive charge, histones bind to the negatively charged double-stranded DNA to form a DNA-histone complex stabilized by electrostatic attractive forces.

The five main classes of histones found in all eukaryotic cells differ in molecular weight and Amino Acid Composition (Table 27-6). Histone H1 is a lysine-rich protein (29% lysine), histones H2A and H2B contain high amounts of both lysine and arginine, with lysine predominating; histones H3 and H4 contain slightly more arginine than lysine and are classified as arginine-rich Proteins. The Amino acid sequences of histones H3 and H4 are nearly identical across all eukaryotes, implying that both histones perform the same fundamental function in all eukaryotic cells. The Structure/154.html">Sequence Homology among histones H1, H2A, and H2B from different eukaryotic species is less pronounced.

Class="center">Table 27-6. Histones

Fig. 27-22. Stages of mitosis. Note that during interphase (i.e., between divisions), chromatin is dispersed throughout The Nucleus. As The Cell prepares for division, the chromatin condenses into distinct, well-defined Chromosomes. Subsequently, in anaphase, the paired daughter chromosomes separate. At the late telophase stage, immediately prior to the division of the daughter cells, the chromatin disperses once again.

Each of the histones can exist in various forms because the R groups of certain constituent Amino acids can be modified enzymatically—namely, methylated, phosphorylated, or acetylated. Such modifications of histone R groups can alter their net electrical charge and other properties. For instance, the Acetylation of the ε-amino groups of lysine residues neutralizes their positive charges.