English: The mouse
sperm genome has 80–90% overall
methylation of its
CpG sites, about 20 million methylated sites. After
fertilization,
demethylation of the paternal chromosomes is almost completed in 6 hours by an active process, before DNA replication (blue line). The mature
oocyte has about 40% methylation of its CpG sites. Demethylation of the maternal chromosomes largely takes place by blockage of the methylating enzymes from acting on maternal-origin DNA and dilution of the methylated maternal DNA during replication (red line). The
morula (at the 16 cell stage), has only a small amount of
DNA methylation (black line). Methylation begins to increase at 3.5 days after fertilization in the
blastocyst and a large wave of methylation then occurs in days 4.5 to 5.5 in the
epiblast, going from from12% to 62% methylation, then reaching maximum level after implantation in the uterus. By day seven after fertilization, the newly formed
primordial germ cells (PGC) in the implanted
embryo segregate from the remaining
somatic cells. At this point the PGCs have the same level of methylation as the somatic cells.
[1][2]