Fig. 2

DNA methylation profiles during early preimplantation development and in clones. The paternal (black dashed line) and maternal (black solid line) genome DNA methylation profiles through preimplantation development are shown for mouse (top panel) and cow (bottom panel). Very soon after fertilization the paternal genome is demethylated in a replication-independent genome-wide manner. In contrast, demethylation of the maternal genome occurs later. In cloned embryos (red line), moderate demethylation occurs by the blastocyst stage, after nuclear transfer and before de novo methylation, with hypermethylation of the TE. In cattle there is active and passive demethylation followed by de novo methylation at the 8- to 16-cell stage. In cloned embryos, de novo methylation occurs at the 4-cell stage and TE is hypermethylated. ICM inner cell mass—cells that reside within the trophoblast and consist of pluripotent cells that give rise to the embryo. TE trophoblastic cells—cells that surround the blastocyst cavity and which give rise to the placenta (adapted from [83])