42.8: Cloning of Dolly the Sheep
The first successfully cloned mammal was Dolly, a sheep, born on 5th July 1996 at Roslin Institute, Scotland. The cloned sheep was named after the American singer Dolly Parton. Dolly lived for seven years and died of respiratory complications, which is speculated to be due to the actual age of her DNA. Because the DNA in cloned cells belongs to an older individual, the cloned individual’s life expectancy may be affected. Indeed, analysis of Dolly’s DNA revealed shorter telomeres than other sheep of the same age. Despite the low success rate and common abnormalities of the face, limb, and heart, Dolly’s cloning paved the way for cloning in other animals, including horses, bulls, and goats.
Cloning versus Sexual Reproduction
Sexual reproduction requires two haploid cells, the egg and the sperm, which fuse to produce a diploid zygote. Whereas the zygote nucleus contains the genetic information to produce a new individual, early embryonic development also requires the cytoplasmic material in the egg cell. The idea that the cytoplasm of an egg can induce a somatic nucleus to develop into an embryo forms the basis for reproductive cloning.
Reproductive cloning is the process of producing genetically identical individuals or clones. The clones share the same set of genetic material as the donor individual and thus share similar phenotypes as the donor. While Dolly was cloned by the somatic cell nuclear transfer method, the embryo splitting method can also be used to produce clones.
Cloning by Embryo Splitting
Cloning by embryo splitting is a recent method in reproductive biotechnology that produces clones using in vitro fertilization (IVF). The process begins with the in vitro fertilization of sperm and egg to produce a zygote. Following zygotic division, the cells are separated at the 4-cell stage. Each cell is then allowed to develop as an individual embryo. The identical blastocysts are then implanted into the uterus for gestation.
