Histonernes

Histones are a class of highly conserved, basic nuclear proteins that package and organize DNA in eukaryotic chromosomes. The core histones—H2A, H2B, H3, and H4—assemble into an octamer around which about 146 base pairs of DNA wrap to form the nucleosome, the fundamental unit of chromatin. The linker histone H1 associates with the DNA between nucleosomes, aiding higher-order folding of the chromatin fiber.

Histones have N-terminal tails that protrude from the nucleosome core and are subject to a variety of post-translational modifications, including acetylation, methylation, phosphorylation, ubiquitination, and SUMOylation. These chemical marks, together with the recruitment of specific reader proteins, constitute an epigenetic code that influences chromatin accessibility, transcription, replication, and genome stability.

Histones are deposited and exchanged with the help of histone chaperones during replication and transcription. In addition to canonical histones, variant histones such as H3.3, H2A.Z, and CENP-A replace standard forms in particular genomic contexts, affecting gene expression, centromere function, and chromatin structure.

Histones are conserved across eukaryotes. Alterations in histone expression, modification patterns, or deposition can affect development and are associated with diseases, including cancer. The study of histone biology is central to the field of epigenetics.