nucleosomen

Nucleosomen, or nucleosomes, are the fundamental repeating units of chromatin in eukaryotic cells. A nucleosome consists of about 147 base pairs of DNA wrapped around a core histone octamer. The octamer contains two copies each of the histones H2A, H2B, H3, and H4. A linker histone, H1, binds to the DNA between nucleosomes and contributes to higher-order chromatin compaction.

The nucleosome core particle forms a structure in which DNA makes approximately 1.65 turns around the histone core. The linker DNA length varies between nucleosomes, and H1 stabilizes the entry and exit of DNA, aiding the folding of chromatin beyond the basic 10 nm “beads-on-a-string” form. In some contexts, chromatin can further compact into a 30 nm fiber, although the existence and relevance of this form in living cells is a topic of ongoing debate.

Nucleosomes regulate DNA accessibility and gene activity. They act as barriers to transcription, replication, and repair and are dynamic, capable of sliding, ejecting, or temporarily restructuring in response to cellular signals. Chromatin remodeling complexes that hydrolyze ATP reposition or remove nucleosomes to expose DNA regions. Post-translational modifications of histone tails extending from the core—such as acetylation, methylation, phosphorylation, and ubiquitination—create an epigenetic layer that influences chromatin structure and gene expression without altering the DNA sequence.

Nucleosome assembly involves histone chaperones during DNA replication and repair. Together, nucleosomes organize the genome in a compact, regulable way that supports DNA packaging, replication, repair, and regulated transcription.