Chromatin

Chromatin is the complex of DNA and proteins that forms the contents of eukaryotic cell nuclei. It packages long DNA molecules into compact structures and regulates access to genetic information, thereby influencing transcription, replication, and repair.

The basic repeating unit is the nucleosome, in which about 147 base pairs of DNA wrap around a histone octamer (two each of H2A, H2B, H3, and H4). A linker histone H1 binds to the DNA between nucleosomes, aiding higher-order organization. Nucleosomes connect to form chromatin fibers that loop and condense to produce the chromosomes visible during cell division.

Chromatin exists in two main functional states. Euchromatin is relatively decondensed and enriched for active genes, whereas heterochromatin is more condensed and often transcriptionally silent. Constitutive heterochromatin remains repressed at repetitive regions, while facultative heterochromatin can switch between active and repressed states depending on context and development.

Regulation of chromatin involves chemical modifications to histones, DNA methylation, and the action of chromatin-remodeling enzymes. Histone acetylation and methylation, among other marks, influence chromatin accessibility. ATP-dependent remodelers reposition or remove nucleosomes, altering transcriptional potential. Higher-order genome organization also matters, with domains such as lamina-associated regions and topologically associating domains contributing to gene regulation.

Chromatin dynamics are coordinated with the cell cycle; chromatin condenses into chromosomes during mitosis and meiosis and decondenses in interphase. Misregulation of chromatin structure is linked to diseases, notably cancer and developmental disorders. In bacteria, DNA is organized differently, using nucleoid-associated proteins, while some archaea possess histone-like proteins, reflecting diversity in genome packaging across life.