Histones

Histones are a family of small, highly basic nuclear proteins that package and organize DNA in eukaryotic cells. The core histones, H2A, H2B, H3, and H4, assemble into a histone octamer consisting of two copies of each protein. About 147 base pairs of DNA wrap around this octamer to form a left-handed nucleosome core particle, and a linker histone H1 binds the DNA between nucleosomes to promote higher-order compaction. Histones are highly conserved across eukaryotes and are rich in lysine and arginine residues, which give them a positive charge that facilitates interaction with the negatively charged DNA backbone.

In addition to the canonical core histones, cells express histone variants such as H2A.Z, H3.3, and CENP-A that can replace standard histones in specific chromatin contexts, providing specialized functions at promoters, active genes, centromeres, or other regions.

Histones undergo a diverse repertoire of post-translational modifications, including acetylation, methylation, phosphorylation, and ubiquitination, primarily on their N-terminal tails. These chemical marks influence chromatin structure and gene expression and comprise an epigenetic layer of regulation. Chromatin remodeling complexes can reposition, eject, or replace nucleosomes, while histone chaperones assist in assembling and disassembling nucleosomes during processes such as transcription, replication, and repair.

Genes encoding histones are often clustered, and replication-dependent histones are typically expressed during S phase, whereas replication-independent variants are produced throughout the cell cycle. The proper function of histones is essential for DNA packaging, genome stability, development, and epigenetic inheritance.