What is DNA Methylation?

DNA methylation is the process in which a methyl group is attached to a DNA molecule. This is done to repress that part of the DNA as a form of epigenetics. Methlylation can only occur on Adenine and Cytosine bases.

Even though DNA methylation is epigenetic it, it can be passed down through replication. There are several factors that makes this possible in Mammals. First, methylation almost exclusively occurs at CpG dinucleotide (this is a C followed by a G when reading in the 5' -> 3' direction). Second, is the DNMT1 enzyme. To replicate methylation into a daughter strand, DNA must first replicate using the normal process (with DNA polymerase and such). After the daughter strand has been constructed, DNMT1 passes through both strands looking for a CpG dinucleotide and its complementary GpC dinucleotide on the daughter strand. It then checks if the C in the parent strand is methylated. If it is, then it will methylate the C in the daughter strand.

Finally, there is some interesting side effects of the methylation process. When Cytosine is methylated it is methylated in the 5th position on it's ring. This is actually the same position that Thymine's methyl group is (Thymine's chemical structure naturally has a methyl group here). Thus, a methylated Cytosine is very close to a Thymine. In fact, the addition of water will spontaneously turn the methylated Cytosine into a Thymine. This is normally caught by DNA repair enzymes, but sometimes isn't leading to a permanent mutation.