Figure 1. Cytosine and 5-methylcytosine. The enzymatic process of DNA methylation converts cytosine to 5-methylcytosine. DNA demethylation converts 5-methylcytosine to cytosine.

Figure 2. The top half of the figure depicts a methylated DNA molecule undergoing DNA replication followed by maintenance methylation. This process ensures faithful propagation of DNA methylation on both strands throughout multiple rounds of replication. If maintenance methylation does not occur following replication, methylation is present on only one strand as depicted in the lower half of the figure. Further rounds of DNA replication in the absence of maintenance methylation lead to an accumulation of double-stranded DNA molecules without cytosine methylation.

Figure 3. A CpG dinucleotide is depicted along with graphical representations of the potential enzymatic mechanisms leading to DNA demethylation: direct excision of the methyl group by an active demethylase, base excision by a DNA glycosylase and nucleotide excision by an endonuclease are depicted.