Fig. 2.

The two major phases of genome-wide erasure of DNA methylation in the early embryo and in primordial germ cells (PGCs) of the mouse are shown. Thickness of the outer arrows indicates levels of DNA methylation. Red, maternal genome; blue, paternal genome. After fertilization the paternal genome is more rapidly demethylated than the maternal one. During gametogenesis de novo methylation in spermatogenesis occurs earlier than in oogenesis. The inner circle shows factors or candidate factors that are implicated in de novo methylation, the maintenance of methylation, or demethylation, respectively. Solid arrows in the inner circle show at what developmental time these epigenetic regulators are thought to act.