The role of DNA demethylation in development

The pattern of DNA methylation plays a role in the global regulation of gene expression in mammals. This pattern is established anew in each generation by a series of developmental steps involving de novo methylation and demethylation. CpG islands associated with housekeeping genes undergo demodification at the time of implantation while tissue specific genes become demethylated during cell differentiation. In both cases this process is driven by interactions between cis acting sequences and trans acting factors. The. demethylase enzyme itself appears to contain RNase sensitive components and works by removing and then replacing the methylated nucleotides in a reaction which does not require additional sources of energy. Although this enzyme is present ubiquitously in many different cell types, it is directed to selected sites on the genome by interactions with specific proteins.