Figure 2.

DNA methylation dynamics throughout the human life span. During gametogenesis, the DNA methylation is erased in the primordial germ cells (PGCs) and then acquires new methylation profiles that are in large part sex-dependent, including the methylation present at imprinted genes. At fertilization, the parental pronuclei are erased of nearly all methylation (imprinted genes and “escapees” resist this demethylation—see text). Around the time of implantation, new DNA methylation information is established on the diploid chromosomes in a manner that will aid differentiation of cells to become trophoblast versus embryonic tissues, formation of the three germ layers and then differentiation into the somatic tissues. Many scientists believe that the highly dynamic nature of the genome-wide methylation profiles during these reprogramming and rapid growth periods of development represent windows of vulnerability where an environmental exposure could cause detrimental shifts in methylation by disrupting the fidelity of these reprogramming processes.