DNA methylation

Imagine putting a small lock on a particular paragraph of a book so it stays shut. DNA methylation works a bit like that: cells attach a tiny chemical tag, a methyl group, to certain spots on the DNA, and this often keeps the nearby gene quiet.

Chemically, methylation in mammals usually means adding a methyl group to a cytosine base that sits next to a guanine, a pairing written as CpG. Clusters of these sites near a gene's start, called CpG islands, are normally kept unmethylated when the gene is active; methylating them tends to recruit proteins that compact the chromatin and block transcription.

Methylation patterns are copied to daughter cells during DNA replication, which is what makes the silencing heritable. Dedicated enzymes establish new patterns and maintain existing ones, while other enzymes can remove the marks, so the system is dynamic rather than permanent.

Methylation is central to normal processes such as genomic imprinting and X-chromosome inactivation, but abnormal patterns appear in many cancers, where tumor-suppressor genes can be wrongly silenced. It is a powerful regulator, not simply an on-off switch.