In the mid-1990s, scientists in Edinburgh, Scotland announced that they had cloned Dolly, an adult sheep. This was the first time in history an adult mammal had been successfully cloned. Dolly was created using a process called somatic cell nuclear transfer. In this technique, the nucleus of a body cell from the animal being cloned is transplanted into an egg harvested from a different organism of the same species.

First, the nucleus is removed from a somatic cell of the animal being cloned. All cells in the body, with the exception of egg and sperm cells, are somatic cells. In the case of Dolly, the nucleus was extracted from a mammary cell, a cell from the animal’s udders. In the next step of the process, an unfertilized egg cell is taken from another animal of the same species. This cell is known as the donor egg cell. The nucleus of the donor egg cell is removed and replaced with the nucleus of the somatic cell from the animal being cloned. An electric shock is then administered, which stimulates the egg cell to divide and form an embryo. The embryo cells contain the exact same genetic information as the adult cell taken from the organism being cloned. So it is genetically identical to the original animal. When the embryo has grown into a ball of cells, it is transferred into the uterus of a surrogate mother, where it continues to develop.

The cloning of Dolly was considered highly successful. In her six years of life, Dolly had three pregnancies achieved through traditional sexual fertilization, which was one indication that she was a healthy, viable organism. Although Dolly died prematurely before her life expectancy of around 11 years, it is unclear whether her early death was a result of cloning. Since Dolly, scientists have cloned many other mammals including mice, monkeys, pigs, and cows. Despite these successes, the practice of cloning remains controversial among experts and the general public.