Cloning after Dolly

Dolly was the first cloned mammal and must be the most famous sheep in the world. Two things made her famous: first that she is an artificially induced clone, genetically identical to a previously existing sheep, for Dolly can be thought of as a vertical twin with identical genes, in this case to the parent. Second is the way in which it was done. The scientists took the nucleus from a cell from the udder of an adult sheep and implanted it in an egg cell whose original nucleus had been removed. Very surprisingly, the adult cell DNA was able to give rise to a complete individual, not just more udder cells. This was surprising because, although at the earliest stage of development the DNA is still pluripotent, capable of generating the growth of the whole animal, later in development, specialization sets in, and cells become differentiated into heart cells, liver cells etc. The whole of the genetic material is still there, but most of it is switched off and only the genes relevant to that particular cell are active. It had been thought that this process of differentiation in mammals was irreversible, so that an udder cell could only give rise to more udder cells. But we now know that differentiation can be reversed.