|Courtesy of Richard Roberts, New England Biolabs
Model methylation reaction: Cytosine nucleotide (red) is flipped out of the DNA double helix by a methyltransferase (white), so it can be methylated. The end product after the methyl group has been transferred to the DNA is pictured in green.
The term methylation refers to the addition of a methyl group to the cyclic carbon 5 of a cytosine nucleotide. A family of conserved DNA methyltransferases catalyzes this reaction. Basically, the methyl group tags a gene so it is turned off, and an unnecessary protein product is not produced in a particular cell. For instance, one of the two X chromosomes in female mammals is inactivated by methylation.