The Nucleosome

A nucleosome is a DNA-histone protein complex that represents the fundamental unit of DNA compaction. It consists of a histone octamer core—eight histone proteins—wrapped by 145 to 147 base pairs of DNA. Nucleosomes compress the nearly two-meter-long human DNA molecule into a chromatin thread one-third of its original length, enabling it to fit inside the nucleus.

Histones are small, positively charged proteins that tightly associate with negatively charged DNA. Each histone has a positively charged tail 11 to 27 amino acids long that aids in keeping DNA wound around the histone core. These tails also interact with neighboring core particles, facilitating DNA packaging and maintaining the structural integrity of the nucleosome.

Histone H1 acts as a clamp that keeps linker DNA in place. Linker DNA is a variable-length stretch of DNA that separates each nucleosome core particle, resembling beads on a string. By securing this linker DNA, H1 stabilizes the overall chromatin structure and contributes to higher-order DNA compaction.

Histone proteins are highly conserved because they perform essential DNA compaction functions critical to cell survival. The amino acid sequences of core histone proteins remain remarkably similar even between distantly related species. For example, histone H3 differs by only four amino acids between a calf thymus and a pea plant, reflecting the fundamental importance of these proteins.

The histone octamer core consists of eight histone proteins: two molecules each of histones H2A, H2B, H3, and H4. This protein complex forms the structural foundation of the nucleosome, around which 145 to 147 base pairs of DNA wrap nearly two complete turns. The octamer's positive charge attracts and binds the negatively charged DNA backbone.

Non-histone proteins bind to the nucleosome complex and help maintain DNA compaction while organizing long chromatin loops. These proteins also regulate critical cellular processes including DNA replication and RNA synthesis. Though comprising only a small proportion of the nucleosome complex, non-histone proteins are essential for coordinating gene expression and DNA replication.

Human DNA measures nearly two meters long but must fit inside a nucleus only a few microns in diameter. The nucleosome compresses DNA into a chromatin thread one-third of its original length. Nucleosomes can further coil into higher-order compact structures, achieving the sequential levels of organization necessary to package DNA into chromosomes visible during cell division.