Nuclear Transplantation from Somatic Cells

The second kind of cloning to produce an animal is the one that made headlines with the birth of Dolly which was done quite differently. A genetic copy of an adult animal was made, by passing the usual reproductive process. An egg cell from one sheep was emptied of its nucleus, and then fused with a body cell containing the nucleus of another adult  sheep. This was then implanted in the uterus of yet another ewe. Dolly does not have two genetic parents, rather her genomic DNA is a copy of the DNA in the nucleus of an adult animal. In other words - she is a clone of that adult.

The key to Wilmut's success, where so many others had failed, seems to have been in using a method that made the donor nucleus and the cytoplasm of the recipient oocyte able to work together. He starved of nutrients the cells that would provide the nucleus, to induce the nucleus into a quiescent phase of the cell cycle where many genes are shut down;  the GO phase(71). In the GO phase, cells contain only one complete set of chromosomes, whereas in later cell cycle phases the DNA is replicated in preparation for cell division (mitosis) so that cells have a duplicate copy of each chromosome (one copy will go to each daughter cell). If a nucleus were transferred in this later phase and then instructed by cytoplasmic factors in the egg to duplicate for cell division, the new cell would end up with too much DNA, and chromosomal abnormalities(74). The approach using a GO phase donor cell seemed to facilitate programming of the new DNA by factors in the egg cytoplasm so that cell division and further development occurred normally.