Nucleosomering

Nucleosomering is the process by which DNA is organized into nucleosomes, the fundamental repeating units of chromatin. A nucleosome consists of about 147 base pairs of DNA wrapped around an octamer of histone proteins (two copies each of H2A, H2B, H3, and H4). The DNA-histone core is typically followed by a variable-length stretch of linker DNA, to which histone H1 may bind, helping to stabilize higher-order chromatin structure. Together, nucleosomes package the genome and regulate access to genetic information.

In a typical cell, nucleosomes are not arranged in a uniform, static array. Nucleosome positioning and spacing are influenced by DNA sequence, transcriptional activity, and the action of chromatin remodelers. Assembly of nucleosomes occurs in two main contexts: replication-coupled chromatin assembly, which reconstitutes chromatin on newly synthesized DNA, and replication-independent assembly, which replenishes nucleosomes during transcription or repair. Histone chaperones, such as CAF-1 and ASF1, guide the delivery and deposition of histones H3–H4 and H2A–H2B onto DNA, ensuring proper histone supply and preventing inappropriate interactions.

Chromatin remodelers—ATP-dependent complexes such as SWI/SNF, ISWI, and CHD families—reposition, remove, or replace nucleosomes to alter DNA accessibility. Histone variants and post-translational modifications (for example, acetylation or methylation) further influence nucleosome stability and chromatin states, contributing to active or repressed transcription, replication timing, and DNA repair.

The concept of higher-order chromatin structure, including the existence of a uniform 30 nm fiber, remains contested in vivo; current views emphasize a dynamic, irregular arrangement of nucleosomes that supports regulated gene expression and genome maintenance.