Methylation

<B>1.</B> In chemistry, the addition of a methyl group to a molecule.

<B>2.</B> On a protein level, the addition of a methyl group or groups to the amino acid arginine or lysine in a protein.

<B>3.</B> On a DNA level, the addition of a methyl group to a cytosine residue to convert it to 5-methylcytosine.

Methylation of DNA occurs at CpG sites, where cytosine (C) lies next to guanine (G).

The CpG sites are in regions near the promoters of a genes.

These regions are known as CpG islands.

The state of methylation of CpG islands is critical to both gene activity and gene expression