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. Author manuscript; available in PMC: 2011 Oct 1.
Published in final edited form as: Trends Cell Biol. 2010 Aug 31;20(10):609–617. doi: 10.1016/j.tcb.2010.08.003

Figure 2. DNA methylation and demethylation.

Figure 2

De novo DNA methylation patterns are established by catalytically active methyltransferases DNMT3A and DNMT3B. This process is enhanced in the presence of the catalytically inactive DNMT3L. During replication, the original DNA methylation pattern is maintained largely by the activity of DNMT1, with some participation by DNMT3A and DNMT3B. DNA methylation patterns can be erased by DNA demethylation, through active or passive processes. Active demethylation can occur by enzymatic replacement of a methylated cytosine with an unmethylated residue without the requirement of cell division. Several enzymes, such as AID and Tet1, are proposed to play roles as DNA demethylases. On the other hand, passive demethylation can occur during successive replications, when the activity of maintenance DNA methylation is abrogated. Black circles represent methylated CpG sites and open circles represent unmethylated CpG sites.