Cloning of Dolly the Sheep

Reproductive cloning produces a genetically identical copy of a multicellular organism using a somatic cell nucleus. Unlike sexual reproduction, which requires two haploid cells (egg and sperm) to create a diploid zygote, cloning uses the cytoplasm of an egg to reprogram an adult cell nucleus to develop into an embryo. Clones share identical genetic material and phenotypes with the donor organism.

Mammary gland cells from an adult Finn Dorset sheep were cultured and arrested in the G0 phase by depriving them of serum nutrients. An enucleated egg from a Scottish blackface sheep was then fused with these cultured cells using electric pulses, which also stimulated cell division to form a blastocyst. This embryo was implanted into a surrogate sheep, resulting in Dolly's birth five months later.

The egg cytoplasm contains essential materials that induce a somatic nucleus to develop into an embryo. This cytoplasmic induction is fundamental to reproductive cloning, allowing an adult cell nucleus to be reprogrammed to generate a complete organism. The cytoplasm provides the developmental signals necessary for early embryonic development beyond what the nucleus alone can direct.

Dolly's DNA contained shorter telomeres than sheep of the same age, suggesting her cells were genetically older. Because the DNA in cloned cells originates from an older individual, the cloned organism's life expectancy may be reduced. Dolly lived seven years and died of respiratory complications, potentially linked to this accelerated cellular aging.

Embryo splitting begins with in vitro fertilization of sperm and egg to create a zygote. After zygotic division, cells are separated at the 4-cell stage, and each cell develops independently as an individual embryo. The resulting identical blastocysts are implanted into the uterus for gestation, producing genetically identical clones through a different method than somatic cell nuclear transfer.

Somatic cell nuclear transfer, used to create Dolly, transfers an adult cell nucleus into an enucleated egg and requires introduction to nuclear reprogramming. Embryo splitting uses in vitro fertilization and divides early embryonic cells to produce clones. While somatic cell nuclear transfer involves reprogramming an adult nucleus, embryo splitting simply separates naturally developing embryonic cells.

Dolly lived for seven years before dying of respiratory complications. Her case revealed that cloning had a low success rate and often produced abnormalities affecting the face, limbs, and heart. Despite these challenges, Dolly's successful birth demonstrated that reproductive cloning was possible and paved the way for cloning other mammals including horses, bulls, and goats.