Structure
In eukaryotes the chromosomes are found packaged within a nuclear membrane, unlike the case in prokaryotes. This membrane consists of a DNA double helix bound to an octamer of core histones (2 dimers of H2A and H2B, and an H3/H4 tetramer). Together, the DNA bound around this histone core forms what is known as the nucleosome. About 147 base pairs of DNA coil around 1 octamer, and ~20 base pairs are sequestered by the addition of the linker histone (H1), and various length of "linker" DNA (~0-100 bp) separate the nucleosomes.
Packaging of DNA is facilitated by the electrostatic charge distribution: phosphate groups cause DNA to have a negative charge, whilst the histones are positively charged. Most eukaryotic cells contain histones (with a few exceptions) as well as the kingdom Archaea, a protist group. Histones are positively charged molecules as they contain lysine and arginine in larger quantities and DNA is neatively charged. So they make a strong ionic bond in between them to form nucleosome.
Read more about this topic: Eukaryotic Chromosome Structure
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