Wiktionary
n. 1 (context chemistry English) The addition of a methyl group to a molecule. 2 (context genetics English) The addition of a methyl group to cytosine and adenine residues in DNA that leads to the epigenetic modification of DNA and the reduction of gene expression and protein production.
Wikipedia
In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group, rather than a larger carbon chain, replacing a hydrogen atom. These terms are commonly used in chemistry, biochemistry, soil science, and the biological sciences.
In biological systems, methylation is catalyzed by enzymes; such methylation can be involved in modification of heavy metals, regulation of gene expression, regulation of protein function, and RNA processing. (Methylation of heavy metals can also occur outside biological systems.) Chemical methylation of tissue samples is also one method for reducing certain histological staining artifacts. The counterpart of methylation is demethylation.
Usage examples of "methylation".
Synthetically it may be prepared by the methylation of silver theobromine and silver theophyllin or by boiling heteroxanthine with methyl iodide and potash.
But maintenance methylase would do exactly the same thing to it, duplicating cytosine methylation on both sides, if originally present on one side.