ABSTRACT
Epigenetic control of gene expression may often seem to have evolved as a semi-reversible mechanism that responds to the changing requirements of the cell. Control of gene expression needs to be highly adaptable and capable of rapid alteration in response to environmental triggers. The greater success rate of serial nuclear transfer suggests that multiple exposure to some stage of the fertilization process or the very early embryonic development process increases the ability of the somatic genome to contribute to the formation of a new organism. This is most likely to be an epigenetic reprogramming step, but it seems that the factors present in the oocyte responsible for this activity are not always able to reprogram a somatic genome effectively. DNA methylation does not influence transcription in isolation; rather, it exerts its influence through interactions with other epigenetic features of chromatin. It is therefore unsurprising that histone modification patterns undergo similar changes immediately after fertilization.
