Fig. 1: DNA methylation and hydroxymethylation.
DNA methylation occurs through the addition of a methyl (CH3) group to the cytosine of DNA by DNA methyltransferases (DNMTs) to produce 5-methylcytosine (5-mC). DNA methylation leads to densely packed heterochromatin that is consistent with gene inactivation. 5-mC can subsequently be converted to 5-hydroxymethylcytosine (5-hmC) by the ten-eleven translocation (TET) dioxygenases. Hydroxymethylation loosens chromatin to promote gene activation.