Nucleosomes

Nucleosomes are the fundamental units of chromatin, the complex that packages eukaryotic DNA. A nucleosome core particle consists of a histone octamer—two copies each of H2A, H2B, H3, and H4—around which about 147 base pairs of DNA are wrapped in roughly 1.65 left-handed turns. This arrangement forms the bead-like unit often described as DNA wrapped around a protein core. The term nucleosome can refer specifically to the core particle, or to the core plus linker DNA and the associated linker histone H1, which helps stabilize higher-order chromatin structure.

In most regions of the genome, linker DNA connects adjacent nucleosomes, producing the “beads-on-a-string” appearance. The presence of histone H1 and additional folding is associated with higher-order chromatin fibers, though the precise in vivo organization is actively studied and varies by cell type and conditions.

Nucleosome positioning and composition influence gene expression and DNA-related processes. Post-translational modifications of histone tails, such as acetylation, methylation, and phosphorylation, alter chromatin accessibility by recruiting effector proteins, thereby regulating transcription, replication, and repair. Nucleosome remodeling complexes (e.g., SWI/SNF, ISWI, CHD) reposition or eject nucleosomes to permit or restrict access to DNA.

New histones are deposited during replication by chaperones such as CAF-1 and ASF1, and histone variants (e.g., H3.3, H2A.Z, CENP-A) provide functional diversity in chromatin. Nucleosome organization is a central aspect of epigenetic regulation and chromatin dynamics in health and disease.