When complete animals are obtained from somatic cells of an animal, it is termed as animal cloning.
Cloning is routine in plants, but in case of animals only a limited success had been achieved so far. earlier, nuclei from tadpole were transplanted into the cytoplasm of an enucleated fertilized frog egg, and normal frogs were obtained.
But early in 1997, British scientists announced successful cloning of sheep by transferring the nucleus from the udder cell of an adult sheep into the cytoplasm of an eunucleated fertilized egg. The egg was then transplanted into the uterus of a surrogate mother where it developed like a normal zygote into a normal lamb, which grew into a normal adult sheep, called 'Dolly'
Cells from mammary gland of an adult sheep( 6yr old ewe) were first cultured in vitro. The cultured cells were arrested , induced to enter the G0 phase (quiescent phase) by reducing the concentration of serum in the medium from 10% to 0.5% for 5 days. The G0 cells were then fused in vitro with the ennucleated ova of appropriate stage. oocyte were recovered between 28 to 33 hr after injection of gonadotropin releasing hormone and enucleated as soon as possiblecell fusion was induced by electrical pulse, which also activated the oocytes.The fusion product were cultured invitro in ligated oviducts of sheep upto morula or blastula stage befor their transfer into the uteri of the surrogate. the rate of success in obtaining normal embryo development was rather low: a total of 277 oocytes were fused with cultured mammary gland cells: of these 29 reached morula/blastula stage, and were transferred into surrogate mothers leading to 13 pregnancies, but only one live birth.
Cloning in many situations, highly desirable sincethis allows (1) indefinite multiplication of an elite desirable genotype without the risk of segrgation and recombination during meiosis, which must precede sexual reproduction. Obviously (2) the technique holds a great promise in genetic research, and impact of epigenetic changes, such as, imprinting and telomere shortening, etc. In addition, (3) This technique should make it feasible to target transgenes in livestock by nuclear transfer from transgenic cell populations developedinvitro into enucleated oocytes to recover nonchimaeric transgenic animals. Obviously this approach would oviate the generations of breeding necessary for the recovey of transgenic animals from the chimaeric ones that are generated by embryonic stem cell transfer.
It has been suggested that inducing the differentiated nucleus of donor cells to enter G0 Phase causes such changes in chromatin structure that facilitate reprogramming of gene expression. Therefore, If nuclei from G0 cells were transferred into enucleated oocytes, the resulting diploid cell may be expected to develop like a normal animal. The technique needs to be refined & expanded to other animals. however, in most countries, especially in all developed countries, human cloning is illegal. Following the above report American scientist announced successful cloning of rhesus monkey using the embryo splitting technique: this is the closest species to humas where embryo splitting has been successful. however some groups have claimed success in human cloning as well