Rethinking how DNA methylation patterns are maintained

DNA methylation is a somatically heritable epigenetic mark that must be faithfully maintained through cell division. This re-examination of the evidence relating to how DNA methylation is maintained leads the authors to propose a revised model for this important biological process. DNA methylation patterns are set up early in mammalian development and are then copied during the division of somatic cells. A long-established model for the maintenance of these patterns explains some, but not all, of the data that are now available. We propose a new model that suggests that the maintenance of DNA methylation relies not only on the recognition of hemimethylated DNA by DNA methyltransferase 1 (DNMT1) but also on the localization of the DNMT3A and DNMT3B enzymes to specific chromatin regions that contain methylated DNA.