Histonas

Histones are a family of small, highly basic nuclear proteins that package and organize DNA in eukaryotic chromosomes. They are rich in lysine and arginine and are among the most evolutionarily conserved proteins in the cell. The core histones H2A, H2B, H3, and H4 assemble into an octamer composed of two copies of each, around which about 146 base pairs of DNA are wound to form a nucleosome, the fundamental unit of chromatin. The linker histone H1 binds the DNA between nucleosomes and contributes to higher-order chromatin structure.

Histone tails protrude from the nucleosome and are subject to post-translational modifications such as acetylation, methylation, phosphorylation, and ubiquitination. These chemical marks, collectively known as the histone code, influence chromatin compaction and recruit effector proteins that regulate transcription, DNA replication, recombination, and repair.

Histone variants, such as H3.3, H2A.Z, macroH2A, and centromere-specific CENP-A, replace standard histones in some nucleosomes and can alter nucleosome stability and function. Histone assembly and disassembly during replication and repair are assisted by histone chaperones (for example CAF-1, HIRA, ASF1).

Dysregulation of histone modifications is linked to diseases including cancer; histone deacetylase inhibitors and other epigenetic drugs are used therapeutically in some contexts. Through its central role in packaging and regulating access to genetic information, histones are essential for gene expression, development, and epigenetic inheritance.