Methylation is a form of alkylation, i.e. the transfer of an alkyl group to another molecule. Methylation is specifically the addition or substition of a methyl group to a molecule. Methyl groups are alkyls made from methane and are carbon atoms attached to 3 hydrogen atoms -CH3. It can be involved in the expression of genes, as well as protein function regulation and the metabolism of RNA. An example of this is the tri-methylation of lysine 36 on the H3 protein (of a histone), which is involved in the response of plants to necrotrophic fungal attack.
Methylation of a gene, addition of a methyl group (-CH3), is catalyzed by the family of enzymes DNA-methyltransferases (DNMTs) and occurs at the 5-carbon on a cytosine ring; the methyl group projects into the major groove of the DNA double helix, causing the DNA-histone complex to become more tightly condensed, which prevents/negatively affects the binding of transcriptional factors to promoter regions or induces deacetylation of the histone proteins which inhibits transcription of the DNA sequence to mRNA.