nucleosomesduring

Nucleosomes are the basic units of chromatin, consisting of approximately 147 base pairs of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, and H4). A linker histone, H1, binds the DNA between nucleosomes and helps compact chromatin into higher-order structures. The position and composition of nucleosomes influence DNA accessibility and gene regulation.

During transcription, nucleosomes are reorganized to permit RNA polymerase II access. Chromatin remodelers slide, evict, or restructure nucleosomes, and histone chaperones such as FACT, ASF1, and Spt6 assist in histone removal and reassembly. Post-translational modifications on histone tails, including acetylation and methylation, correlate with active or repressed transcription, and histone variants like H3.3 may replace canonical H3 at active genes.

During DNA replication, the replication fork disassembles nucleosomes ahead of the fork; parental histones are redistributed onto daughter strands while new histones are deposited behind the fork by chaperones such as CAF-1 and ASF1. This process preserves epigenetic information while enabling rapid nucleosome reassembly on newly synthesized DNA.

During DNA repair, chromatin must be opened to allow lesion recognition and processing. Remodelers reposition nucleosomes, and histone modifications such as γ-H2AX, ubiquitination of H2A/H2B, and acetylation help recruit repair factors. Histone chaperones participate in reassembling nucleosomes after repair to restore chromatin integrity.

In meiosis and development, nucleosome dynamics contribute to chromosome pairing, recombination, and cell fate. Variant histones and marks define specialized chromatin states; centromeric CENP-A nucleosomes mark kinetochores, while H2A.Z and H3.3 are associated with genome regulation and stability.