<P> Using an electron paramagnetic resonance spin - labeling technique, British researchers measured the distances between the spools around which eukaryotic cells wind their DNA . They determined the spacings range from 59 to 70 Å . </P> <P> In all, histones make five types of interactions with DNA: </P> <Ul> <Li> Helix - dipoles form alpha - helixes in H2B, H3, and H4 cause a net positive charge to accumulate at the point of interaction with negatively charged phosphate groups on DNA </Li> <Li> Hydrogen bonds between the DNA backbone and the amide group on the main chain of histone proteins </Li> <Li> Nonpolar interactions between the histone and deoxyribose sugars on DNA </Li> <Li> Salt bridges and hydrogen bonds between side chains of basic amino acids (especially lysine and arginine) and phosphate oxygens on DNA </Li> <Li> Non-specific minor groove insertions of the H3 and H2B N - terminal tails into two minor grooves each on the DNA molecule </Li> </Ul> <Li> Helix - dipoles form alpha - helixes in H2B, H3, and H4 cause a net positive charge to accumulate at the point of interaction with negatively charged phosphate groups on DNA </Li>

Histones are rich in what type of amino acid
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