Epigenetics

Nearly every cell in a person carries the same DNA, yet a skin cell and a nerve cell use very different sets of genes. Epigenetics studies the marks that help make this possible: chemical tags added to DNA itself or to the histone proteins that DNA wraps around. These tags change how tightly DNA is packed and whether the transcription machinery can reach a gene, without altering a single base. The full set of marks in a cell is its epigenome.

The best-known marks are DNA methylation, which usually silences genes, and histone modifications such as acetylation, which tends to loosen chromatin so genes can be read. Many marks are copied when a cell divides, keeping cell identity stable through development, and some may pass to offspring. Unlike mutations, they can be added and removed, and they shift with age and with environmental exposures such as diet and smoking.

Epigenetic control explains X-chromosome inactivation in females and genomic imprinting, and abnormal methylation is common in cancer. The British biologist Conrad Waddington coined the term in the 1940s.