Monday, August 08, 2005

DNA Cloning and Somatic Cell Nuclear Transfer – a.k.a. SCNT 101

It might be surprising to some to learn that there is more than one kind of cloning. That is why it is important to differentiate between them by referring to them by their actual names rather than the umbrella term “cloning.”

DNA cloning, recombinant DNA, gene cloning – these all refer the “the production of many identical copies of a specific DNA fragment.”

However, when the word “clone” in any of its forms is used these days, what is being referred to is another kind of cloning called somatic cell nuclear transfer, or SCNT.

All of the cells in the body, with the exception of one type of cell, contain two strands of DNA – one from each parent – they are thus known as diploid (they are also known as somatic.) The exceptions are gametes, also known as germ cells, i.e., egg and sperm cells. They are haploid, i.e., they only have one strand of DNA, so that one day, when the egg and sperm merge, they will form a single cell, known as a zygote, with precisely the desired number of strands of DNA - two.

With SCNT, there is no sperm involved. Instead, the haploid nucleus of an egg is removed and replaced with a diploid nucleus taken from any other cell in the body, resulting in an unfertilized egg that nonetheless has diploid DNA. If, then, a little bit of electricity is applied to the unfertilized, diploid egg, that somehow jumpstarts the same chemical reactions that are triggered by the completion of the union of egg and sperm, and the egg begins to divide like an embryo would.

Barring any unforeseen difficulties, this entity will continue to divide like an embryo, and on the fourth or fifth day, it will be possible to extract stem cells from it, as would also be possible with an embryo. At this point in time, it is not possible to perform the extraction without destroying the entity or embryo. Alternatively, as with an embryo, in the absence of any interference, and with the provision of a suitable environment, the entity could go on dividing and potentially grow into a fully formed adult. SCNT has been used to produce many animals, including Dolly the sheep and Snuppy the dog. However, at this time, the failure rate on producing animals via SCNT is extremely high – for example, Snuppy was the only success out of 1,000 implantations.

That is a summary of SCNT in my words. NIH says it much more succinctly:

Somatic cell nuclear transfer
—The transfer of a cell nucleus from a somatic cell into an egg from which the nucleus has been removed.

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