Figure 24. DNA methylation status is heritable but requires maintenance.
(A) The Cs within CG dinucleotides (blue) represent potential methylation sites. In this piece of DNA sites 1 and 2 are fully methylated (i.e. on both strands of the DNA), but site 3 is not methylated. (B) Following replication the new strands of DNA (green) are unmethylated; further rounds of replication of this hemimethylated DNA would lead to some unmethylated DNA. However, hemimethylated DNA is a substrate for the maintenance methylase, DNMT1. (C) The daughter DNA molecules are now fully methylated at the originally methylated positions (1 and 2), but remain unmethylated at position 3, which was unmethylated in the original DNA template. Abbreviation: m, methyl (CH3) group.