Histones are a small family of closely related basic proteins. H1 histones are the ones least tightly bound to chromatin (Figures 1, 2, and 3) and are, therefore, easily removed with a salt solution, after which chromatin becomes more soluble. The organizational unit of this soluble chromatin is the nucleosome. Nucleosomes contain four major types of histones: H2A, H2B, H3, and H4. The sequence and structures of all four histones, H2A, H2B, H3, and H4, the so-called core histones that form the nucleosome, have been highly conserved between species, although variants of these four histones exist in various organisms, and are used for specialized purposes. The extreme conservation of core his tone sequences implies that the overall functions of histones are identical in all eukaryotes, and that the entire molecule is involved quite specifically in carrying out these functions. The carboxyl terminal two-thirds of the histone molecules are hydrophobic, while their amino terminal thirds are particularly rich in basic amino acids. These four core histones are subject to at least six major, or frequent, types of covalent modification or posttranslational modifications (PTMs): acetylation, methylation, phosphorylation, ADP-ribosylation, monoubiquitylation, and sumoylation. These histone modifications play important roles in chromatin structure and function, as illustrated in Table 1. Chapters 38 and 42 provide a more detailed discussion of the role of histone PTMs in cellular biology.

Fig1. Electron micrograph of chromatin showing individual nucleosomes (white, ball-shaped) attached to strands of DNA (thin, gray line); see also Figure 2. (Reproduced with permission from Shao Z. Probing Nanometer Structures with Atomic Force Microscopy. News Physiol Sci. 1999;14:142-149.)

Fig2. Model for the structure of the nucleosome. DNA is wrapped around the surface of a protein cylinder consisting of two each of histones H2A, H2B, H3, and H4 that form the histone octamer. The ~145 bp of DNA, consisting of 1.75 superhelical turns, are in contact with the histone octamer. The position of histone H1, when it is present, is indicated by the dashed outline at the bottom of the figure. Note that histone H1 interacts with DNA as it enters and exits the nucleosome.

Fig3. Extent of DNA packaging in metaphase chromosomes (top) to noted duplex DNA (bottom). Chromosomal DNA is packaged and organized at several levels as shown (see Table 2). Each phase of condensation or compaction and organization (bottom to top) decreases overall DNA accessibility to an extent that the DNA sequences in metaphase chromosomes are likely almost totally transcription ally inert. In toto, these five levels of DNA compaction result in nearly a 104-fold linear decrease in end-to-end DNA length. Complete condensation and decondensation of the linear DNA in chromosomes occur in the space of just a few hours during the normal replicative cell cycle.

Table1. Possible Roles of Posttranslationally Modified Histones

The histones interact with each other in very specific ways. H3 and H4 form a tetramer containing two molecules of each (H3–H4)₂ , whileH2A and H2B form dimers(H2A–H2B). Under physiologic conditions, these histone oligomers associate to form thehistone octamerof the composition (H3–H4)2–(H2A–H2B)₂ .

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