Fig. 5.

Mechanism of CpG island protection. Preimplantation, the genome is largely unmethylated (white circles) and RNAP II complexes are associated with sites of potential transcription by virtue of TBP as directed by transcription-factor recognition (1). RNAP II then recruits the MLL1 complex that brings about lysine 4 methylation of histone H3 within local nucleosomes (2). De novo methylation at the time of implantation (red circles) is carried out by the DNMT complex that can bind all regions of the DNA, as long as the local nucleosomes are not methylated at the H3 lysine 4 position (3).