DNA methylation

DNA methylation is a chemical modification in which an enzyme attaches a methyl group, one carbon atom bonded to three hydrogens, to a base in DNA. In mammals the target is mostly cytosine where it sits next to guanine, a pair called a CpG site. Dense clusters of these sites, known as CpG islands, are common in promoters, the control regions at the start of genes.

When a promoter is heavily methylated, transcription factors struggle to bind and the DNA tends to pack into closed chromatin, so the gene is usually silenced; removing the methyl groups can switch it back on. Because the pattern is copied when cells divide, methylation helps cells keep their identity and is a central mechanism of epigenetics. It underlies genomic imprinting, where a gene's activity depends on which parent passed it on, and helps keep one X chromosome inactive in females. Patterns also drift with age and respond to exposures such as smoking.

Cancer cells often show extra methylation at tumor suppressor genes but less across the genome overall. Bacteria methylate their own DNA too, which shields it from their restriction enzymes.