DNA methylation is a chemical modification in which methyl groups are added to DNA, usually at cytosine bases in CpG dinucleotides. It does not change the DNA sequence but strongly influences how genes are used. In general, heavy methylation in gene promoters is associated with reduced gene activity, whereas low methylation permits transcription. This mechanism is central to normal development, cell differentiation and maintenance of cell identity.

Research shows that methylation patterns are not fixed at birth. They change with age, environmental exposures, diet, smoking and stress. Twin studies reveal that identical twins start life with highly similar methylation profiles, which diverge over time as their life experiences differ. This has led to the concept of the epigenome as a dynamic interface between genes and environment.

Altered methylation is a hallmark of many diseases. In cancer, regions that normally silence growth promoting genes can become demethylated, activating oncogenes, while tumor suppressor genes are often hypermethylated and switched off. These changes occur early in tumor development and are being explored as biomarkers for early detection using blood based tests.

Researchers have created “epigenetic clocks” by combining methylation at specific CpG sites to estimate biological age. These clocks often predict health outcomes and mortality better than chronological age. There is growing interest in whether lifestyle or interventions can slow or reverse epigenetic aging, although causal links are still under investigation. Overall, DNA methylation research is reshaping understanding of how genes, environment and time interact to influence health and disease.