Figure 1.

Inefficient maintenance of DNA methylation patterns during aging. During DNA replication, methylation patterns are copied from the parental strand to the newly replicated strand (methylated and unmethylated CpG dinucleotides are shown as black and white lollipop symbols, respectively). Most of the corresponding enzymatic activity is provided by the DNMT1 methyltransferase and its cofactor Np95. However, additional factors, such as the DNMT3A methyltransferase and the chromatin remodeling factors ATRX and LSH, are also involved. Aging is characterized by complex methylation changes that can result in decreased methylation of intergenic regions, CpG-poor promoters and gene bodies (red bars). In addition, increases in age-related methylation have been shown for CpG-rich promoters (green bar).