Cloning Organisms

Cloned animals are not 100 percent identical to their "parents." Whenever nuclear transplantation is used to produce cloned organisms, the offspring display some differences from the organism that donated the nuclei. The egg donor contributes mitochondria, the energy producers of eukaryotic cells, and these mitochondria have their own small amount of DNA-containing genes used for energy metabolism. Since mitochondria are inherited only with egg cytoplasm, they will not match the mitochondria of the animal from which the nucleus was taken. In addition, maternally derived gene products, both mRNA (messenger RNA) and protein, which serve to begin embryonic development, will differ from that of the nuclear donor, as will the uterine environment and the external environment. Thus, for example, clones produced by nuclear transplantation will be significantly less identical than will clones produced by twinning.