The Nucleosome

Histones are small, positively charged proteins that tightly associate with negatively charged DNA, forming structures called nucleosomes.

The nucleosome core particle contains a histone octamer composed of two molecules, each of histones H2A, H2B, H3, and H4. This core particle is wrapped around by a 145 to 147 base-pair section of DNA nearly two times.

Each histone has a small, positively charged tail 11 to 27 amino acids long that aids in keeping the negatively charged DNA wound around the histones. Additionally, the tails of neighboring core particles interact with each other facilitating DNA packaging. 

Adjacent to the core particle, a variable length stretch of linker DNA separates each core particle like beads on a string. Another histone, H1, acts as a clamp to keep the linker DNA in place.

Together, the nucleosome compresses a long DNA molecule into a chromatin thread one-third of its original length.