Nucleosomeiden

Nucleosomeiden are the basic structural units of chromatin in eukaryotic cells. A nucleosome consists of a histone octamer, formed from two copies each of the core histones H2A, H2B, H3 and H4, around which about 147 base pairs of DNA are wrapped. The linker histone H1 binds to the DNA between nucleosomes and promotes higher-order packaging. Nucleosomeiden are connected in a repeating array by linker DNA, forming the chromatin fiber. The assembly and positioning of nucleosomeiden are facilitated by histone chaperones and chromatin remodelers that slide, evict or exchange histone components during replication, transcription and DNA repair.

The N-tail regions of histones protrude from the nucleosome core and are subject to post-translational modifications, including acetylation, methylation and phosphorylation. These modifications, along with DNA methylation, influence chromatin compaction and gene accessibility, constituting an epigenetic layer of regulation. While once the 30-nm fiber model implied a uniform higher-order structure, current evidence points to a more dynamic and accessible organization with localized compaction.

Functional significance: Nucleosomeiden regulate transcription factor binding and nucleosome turnover; active genes tend to have more open chromatin with nucleosome-depleted regions near promoters, whereas silent regions are more densely packed. Nucleosome positioning can change during development and in response to signaling. Abnormal nucleosome dynamics are associated with diseases and developmental disorders. Experimental approaches such as MNase sequencing and ATAC-seq map nucleosome positions genome-wide.