Histones Are the Most Abundant Chromatin Proteins

The histones are a small family of closely related basic proteins. H1 histones are the ones least tightly bound to chromatin Figure 36-1) and are, therefore, easily removed with a salt solution, after which chromatin becomes soluble. The organizational unit of this soluble chromatin is the nucleosome. Nucleosomes contain four classes of histones: H2A, H2B, H3, and H4. The structures of all four histones—H2A, H2B, H3, and H4, the so-called core histones forming the nucleo-some—have been highly conserved between species. This extreme conservation implies that the function of histones is identical in all eukaryotes and that the entire molecule is involved quite specifically in carrying out this function. The carboxyl terminal two-thirds of the molecules have a typical random amino acid composition, while their amino terminal thirds are particularly rich in basic amino acids. These four core histones are subject to at least five types of covalent modification: acetylation, methylation, phosphorylation, ADP-ribosylation, and covalent linkage (H2A only) to ubiq-uitin. These histone modifications probably play an important role in chromatin structure and function as illustrated in Table 36-1.

The histones interact with each other in very specific ways. H3 and H4 form a tetramer containing two mol-

Figure 36-1. Electron micrograph of nucleosomes attached by strands of nucleic acid. (The bar represents 2.5 |im.) (Reproduced, with permission, from Oudet P, Gross-Bellard M, Chambon P: Electron microscopic and biochemical evidence that chromatin structure is a repeating unit. Cell 1975;4:281.)

ecules of each (H3/H4)2, while H2A and H2B form dimers (H2A-H2B). Under physiologic conditions, these histone oligomers associate to form the histone oc-tamer of the composition (H3/H4)2-(H2A-H2B)2.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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