DNA condensation in eukaryotes (chromatin packing)

The total length of the DNA molecules in a single human nucleus is about two metres, while the diameter of that nucleus does not exceed 2–3 microns. To fit, the DNA must be shortened by a factor of 100,000 through precisely ordered stages:

1. Coiling around histone proteins — nucleosomes. Histones are rich in the two basic amino acids arginine and lysine and therefore carry positive (+) charges, while the phosphate groups of the sugar–phosphate backbone carry negative (−) charges. The charges attract, so the DNA strand winds around a disc of eight histone molecules forming repeating "beads-on-a-string" loops called nucleosomes — this shortens the DNA by only 10 times.
2. Coiling by non-histone structural proteins. The nucleosome threads coil tightly around one another with the help of non-histone proteins to form bands of condensed chromatin, then arrange into large loops attached to a cytoskeleton of non-histone structural proteins, shortening the molecule by 100,000 times until the chromosome appears in its condensed form at metaphase.

Functional note: regulatory non-histone proteins determine whether the code of a DNA molecule will be used to build enzymes and proteins or not.