Reprogramming DNA methylation in the preimplantation stage: peeping with Dolly's eyes

Oocyte cytoplasmic factors can reprogramme the sperm genome during fertilisation or the somatic cell genome during cloning. Diverse reprogramming machinery acts sequentially and interdependently on the imported genome to drive it to totipotency, but their three-dimensional interactions in the cytoplasm remain unknown. Aberrant epigenetic phenomena in early cloned embryos indicate that parts of the somatic cell genome are unyielding to reprogramming forces, owing to their 'knotty' epigenetic features. This fastidious nature of the donor genome might prevent completion of epigenetic reprogramming. It might also help to explain the chronic developmental defects seen in many cloned embryos.