42.8:
Cloning of Dolly the Sheep
Reproductive cloning is a technique of producing a genetically identical copy — a clone — of a multicellular organism.
The first-ever mammal produced using reproductive cloning was a sheep.
In this experiment, mammary gland cells isolated from an adult Finn Dorset sheep were cultured on media.
After a few cycles of cell division, the cultured cells were deprived of serum nutrients to arrest them in the G0 phase of the cell cycle.
Meanwhile, an egg from a Scottish blackface sheep was enucleated.
Next, the cultured mammary gland cells were fused with the enucleated eggs using brief electric pulses.
The electric pulses also stimulated cell division in the fused cell, generating a blastocyst — an early-stage embryo.
The blastocyst was then implanted into a surrogate Scottish blackface sheep, where it developed into a fetus.
Five months later, a healthy lamb named Dolly was born that shared genetic and morphological features with a Finn Dorset sheep.
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.
Suggested Reading
- Scientific and Medical Aspects of Human Reproductive Cloning. Washington (DC): National Academies Press (US); 2002. 2, Cloning: Definitions And Applications.
- Rahbaran, Mohaddeseh, Ehsan Razeghian, Marwah Suliman Maashi, Abduladheem Turki Jalil, Gunawan Widjaja, Lakshmi Thangavelu, Mariya Yurievna Kuznetsova et al. "Cloning and embryo splitting in mammalians: brief history, methods, and achievements." Stem Cells International 2021 (2021).