This is the technology which resulted in Dolly the sheep – the most famous clone of them all. Cloning carrots from a single adult carrot cell has been done for years. Even cloning frogs from adult frog cells is not new technology. The big breakthrough with Dolly is the ability to clone a mammal.

Adult cells are differentiated – each one carries out a particular function in the body, and although they contain the DNA needed to form an entire new organism, most of the genes have been inactivated or switched off. In adult cell cloning those genes are reactivated – switched on again – because this form of cloning involves taking a cell from an adult animal and producing a new, identical individual.

The process is very complex, involving three different females and several different stages. When Dolly was produced she was the only success from hundreds of attempts. The technique is still difficult and unreliable, but one hope for the future is that animals which have been genetically engineered to produce therapeutic proteins in their milk may then be cloned. This is seen as the best way of producing large numbers of cloned, medically useful animals.

The nucleus is removed from a normal body cell of an adult animal (in the case of Dolly it was an udder cell). At the same time the nucleus is removed from the mature ovum of another animal of the same species. The nucleus from the body cell is put into the empty ovum and exposed to a mild electric shock. This stimulates the cell to begin dividing to form an early embryo. This embryo is then transferred into the uterus of yet another animal where it develops and is born. The cloned animal which results is genetically identical to the original adult animal from which a cell was taken. Click on the image to open a larger version of it in a new page

The development of this type of cloning has raised many concerns and issues, because in theory if it is possible to clone a sheep, a cat and a monkey, it would also be possible to clone a human being.