These are basic proteins, bristling with positively charged arginine and lysine residues. Chromatin also contains small amounts of a wide variety of nonhistone proteins . H2A H2B H3 and H4 form a core of protein, the nucleosome core. From 50-200 bp of DNA link one nucleosome to the next. The binding of histones to DNA does not depend on particular nucleotide sequences in the DNA but does depend critically on the amino acid sequence of the histone.
Histones are some of the most conserved molecules during the course of evolution.
Histone H4 in the calf differs from H4 in the pea plant at only 2 amino acids residues in the chain of 102. The arrows point to the nucleosomes. The formation of nucleosomes helps somewhat, but not nearly enough, to make the DNA sufficiently compact to fit in the nucleus. Lengths of nucleosomes are coiled together producing a compact fiber 30 nm in diameter. These fibers appear to be folded into a series of tight loops extending from a central protein scaffold .