Cloning is making a biological copy of another organism with the identical genetic makeup of the founding individual. It is an asexual method of reproduction. Natural examples of cloning include organisms such as bacteria and yeast. Bacteria that result from asexual reproduction are genetically identical. Specifically, animal cloning refers to the creation of a new genetic replica of an original living or dead animal. The only clones produced naturally in mammals are identical twins. These are formed when cells produced by the early divisions of the fertilized egg separate and independently develop into two new individuals. They are therefore genetically identical to each other but not identical to their parents.
Animals can be cloned by three processes: embryo splitting, blastomere dispersal, and nuclear transfer. Our focus will be cloning by nuclear transfer (NT). Nuclear transfer involves the complete removal of genetic material (chromosomes) from an egg to produce an enucleated cell (cytoplast). It is replaced by a nucleus containing a full complement of chromosomes from a suitable donor cell (the karyoplast), which is introduced into the recipient cytoplast by direct microinjection or by fusion of the donor and recipient cells. The egg is then implanted in another adult female for normal gestation and delivery (Fig. 1). Depending upon the source of donor nuclei, cloning by nuclear transfer can be classified into embryonic cell (unspecialized) NT and somatic cell (differentiated) NT. Somatic cell NT is the most important process used for animal cloning and has most potential applications.
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