histoonide

Histones are small, highly basic proteins that package and organize DNA in eukaryotic chromatin. The core histones H2A, H2B, H3, and H4 form an octamer around which about 146 base pairs of DNA wrap to create a nucleosome. The linker histone H1 binds the DNA between nucleosomes and contributes to higher-order chromatin structure.

The N-terminal tails of histones extend from the nucleosome and are subject to post-translational modifications, including acetylation, methylation, phosphorylation and ubiquitination. These chemical marks influence chromatin compaction and gene expression by altering histone–DNA interactions and by recruiting effector proteins. Enzymes such as histone acetyltransferases, deacetylases, methyltransferases and demethylases implement these marks; reader domains such as bromodomains and chromodomains interpret them.

Histone variants, such as H2A.Z and H3.3, can replace canonical histones in specific regions, affecting nucleosome stability and transcriptional outcomes. Histone chaperones, including CAF-1, ASF1 and FACT, assist assembly and disassembly of nucleosomes during replication, transcription and DNA repair. Replication-coupled deposition of H3/H4 contrasts with transcription-associated exchange of histones.

Because histones govern access to genetic information, their modification and turnover are central to epigenetic regulation, development and disease. Alterations in histone-modifying enzymes or variant distribution are linked to cancer and other disorders, making histone biology a focus of extensive research.