Cloning, or somatic cell nuclear transfer (SCNT), is the technique used to produce Dolly the sheep, the first animal to be produced as a genetic copy of another adult.
In this procedure, the nucleus of an egg cell is removed and replaced by the nucleus of a cell from another adult. In Dolly’s case, the cell came from the mammary gland of an adult ewe. This nucleus contained that ewe’s DNA. After being inserted into the egg, the adult cell nucleus is reprogrammed by the host cell. The egg is artificially stimulated to divide and behave in a similar way to an embryo fertilised by sperm. After many divisions in culture, this single cell forms a blastocyst (an early stage embryo with about 100 cells) with almost identical DNA to the original donor who provided the adult cell – a genetic clone.
At this stage, cloning can go one of two ways: reproductive or therapeutic cloning
Reproductive cloning
To produce Dolly, the cloned blastocyst was transferred into the womb of a recipient ewe, where it developed and when born quickly became the world’s most famous lamb. When the cloning process is used in this way, to produce a living duplicate of an existing animal, it is commonly called reproductive cloning. This form of cloning has been successful in sheep, goats, cows, mice, pigs, cats, rabbits, gaur and dogs.
This form of cloning is unrelated to stem cell research. In most countries, it is illegal to attempt reproductive cloning in humans.
Therapeutic cloning
In therapeutic cloning, the blastocyst is not transferred to a womb. Instead, embryonic stem cells are isolated from the cloned blastocyst. These stem cells are genetically matched to the donor organism, holding promise for studying genetic disease. For example, stem cells could be generated using the nuclear transfer process described above, with the donor adult cell coming from a patient with diabetes or Alzheimer’s. The stem cells could be studied in the laboratory to help researchers understand what goes wrong in diseases like these.
Another long-term hope for therapeutic cloning is that it could be used to generate cells that are genetically identical to a patient. A patient transplanted with these cells would not suffer the problems associated with rejection.
To date, no human embryonic stem cell lines have been derived using therapeutic cloning, so both these possibilities remain very much in the future.