DNA methylation occurs predominantly on cytosines followed by guanine residues (CpG).
CpG site on one DNA strand versus C-G base pair on the complementary strand. (13)
Hypermethylation of CpG islands is normally associated with compacted or closed chromatin structure resulting in transcriptional silencing of the gene. DNA methylation can occur at cytosine bases when a methyl group is added at the 5' position on the pyrimidine ring by the enzyme DNA methyltransferase (DNMT). This is called 5-methylcytosine (5mC).
In addition, de novo DNMT methylates previous nonmethylated cytosines, whereas maintenance DNMT methylates hemimethylated DNA at the complementary strand. Hemimethylated DNA occurs when the methylation is only on one of the two DNA strands.
