Figure 2.

Overview of the general methylation and transcriptional status of the oocyte, zygote and further developmental stages. From fertilisation on, the paternal DNA is actively demethylated. Demethylation of the maternal DNA occurs more passively, being not replaced during initial cell divisions. From the blastocyst stage on, expression of DNMT1 increases, which goes together with an increase of methylation of the embryonic DNA. Transcripts originating from the oocyte are very stable and constitute most of the mRNA during initial stages. However, from the 4–8 cell stage on, embryonic transcripts take over. SC, synaptonemal complex.