DYNAMICS OF DNA METHYLATION DURING DEVELOPMENT

DNA methylation plays a role in the repression of gene expression in a nimal cells. In the mouse preimplantation embryo, most genes are unmet hylated but a wave of de novo methylation prior to gastrulation genera tes a bimodal pattern characterized by unmethylated CpG island-contain ing housekeeping genes and fully modified tissue-specific genes. Demet hylaton of individual genes then takes place during cell type specific differentiation, and this demodification may be a required step in th e process of transcriptional activation. DNA modification is also invo lved in the maintenance of gene repression on the inactive X chromosom e in female somatic cells and the marking of parental alleles at genom ically imprinted gene loci.