Yes, DNA methyltransferases affect DNA. DNA methyltransferases (DNMTs) are a family of enzymes that are made up of four members - DNMT1, DNMT3A, DNMT3B and DNMT3L. These DNMTs are responsible for catalyzing the addition of methyl groups onto DNA. DNA methyltransferases transfer a methyl group from the universal methyl donor, S-adenosyl methionine to the 5-position of cytosine residues in the target DNA.