DNA methylation

DNA methylation n the enzymatically controlled methylation of a nucleotide base (as cytosine in eukaryotes) in a molecule of DNA that plays a role in suppressing gene expression

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the postsynthetic addition of methyl groups to specific sites on DNA molecules; the reaction is catalyzed by DNA methyltransferases specific for nucleotide and position of methylation, and methylation of sites is passed down to daughter strands at mitosis. In eukaryotes, the C5 position of cytosine is methylated; in animal cells, at 5′-CG-3′ doublets. Methylation is involved in gene expression, and plays a role in a variety of epigenetic mechanisms, including development, X chromosome inactivation, genomic imprinting, mutability of DNA, and uncontrolled cell growth in cancer. In prokaryotes, the N6 position of adenosine is also a site of methylation, and methylation of bacterial DNA protects it from cleavage by the restriction enzymes that inactivate bacteriophage DNA and plays a role in the DNA repair process.