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. Author manuscript; available in PMC: 2017 Dec 1.
Published in final edited form as: Crit Rev Biochem Mol Biol. 2016 Feb 25;51(3):185–194. doi: 10.3109/10409238.2016.1150958

Figure 2.

Figure 2

DNA methylation and maintenance. Top. Cytosine can be methylated at the 5-position by DNMTs, using SAM (S-adenosyl methionine) as an activated methyl source, to form 5-methylcytosine (mC). The methyl group is stable in vivo until oxidized by Tet dioxygenases to generate hydroxymethylcytosine (hmC), which is the first step in the de-methylation pathway. Bottom. De novo DNMTs methylate unmodified DNA to form a hemi-methylated CpG. Hemi-methylation recruits maintenance DNMTs that methylate and encode the mark on the complementary strand. Maintenance DNMT activity provides the mnemogenic nature of the DNA methylation epigenetic mark, which are stable until oxidized by Tet enzymes.