The Nucleosome

A nucleosome is a structure formed when DNA wraps around a histone octamer, consisting of two copies each of H2A, H2B, H3, and H4 proteins. Approximately 145-147 base pairs of DNA wind nearly twice around this protein core. Nucleosomes function as the fundamental unit of DNA compaction, reducing a long DNA molecule to about one-third its original length and enabling the nearly 2-meter-long human genome to fit within the nucleus.

Histones are small, positively charged proteins that bind tightly to negatively charged DNA through electrostatic interactions. Each histone in the nucleosome core particle has a positively charged tail consisting of 11-27 amino acids that extends outward and aids in maintaining the association between DNA and histones. These tails also interact with neighboring core particles, facilitating DNA packaging and chromatin structure formation.

The nucleosome core particle consists solely of the histone octamer and the 145-147 base pairs of DNA wrapped around it. A nucleosome, however, encompasses the core particle plus the adjacent linker DNA that separates each core particle. Linker DNA varies in length from about 30 to 40 base pairs between cell types, making the complete nucleosome a larger functional unit than the core particle alone.

H1 is a fifth type of histone that binds to DNA at the point where it enters and exits the histone octamer, functioning as a clamp to keep DNA in place. Unlike the core histones, H1 is not part of the nucleosome core particle itself. When DNA is isolated under physiological salt conditions, H1 binding helps stabilize the fiber-like 30-nanometer chromatin structure, preventing DNA from slipping away from the nucleosome.

Nonhistone proteins are a small proportion of proteins that bind to the nucleosome complex and help maintain DNA compaction and organize long chromatin loops. These proteins play regulatory roles in DNA replication and RNA synthesis. While histones are the primary structural components of nucleosomes, nonhistone proteins contribute to the dynamic regulation and higher-order organization of chromatin beyond the basic nucleosome unit.

When DNA is extracted from cells under low salt conditions and examined under a microscope, it resembles beads on a string. The bead-like structures represent individual nucleosomes, while the string represents the free linker DNA connecting them. This appearance reflects the fundamental repeating unit structure of chromatin and demonstrates how nucleosomes organize DNA into a more compact form while maintaining accessibility for cellular processes.

Histone proteins are highly conserved because they perform essential structural functions in DNA packaging that are critical for cell survival. The amino acid sequences of core histone proteins show remarkable similarity across distantly related species; for example, H3 histone between calf thymus and pea plant differs by only four amino acids. This conservation reflects the fundamental importance of nucleosome structure in organizing and protecting DNA across all eukaryotic organisms.