Cloning is making genetically identical copies of living things. Scientists have been doing it since the early 1970s with antibodies, cells and genes but, until Dolly's birth, whole-animal cloning proved elusive.
That's because you can stimulate body cells, such as kidney, eye and nerve cells, to divide and make copies of themselves by growing them in the right conditions (although, you'll only get clumps of cells, not whole organs). But to make a whole animal, convention has it that you need to fertilise an egg with a sperm. This allows the genes of the two cells to mix so that the offspring has genes from both parents and is genetically identical to neither.
In contrast, a clone needs to inherit all its genes from one parent and none from the other. Therefore to clone an animal you need to ensure that only one set of gene-carrying chromosomes is passed on.
Half a century of research has shown that the best way to do this is to remove the chromosomes from the egg by taking out its nucleus and inserting a nucleus with a complete set of genes from the individual you want to clone.
Not surprisingly, it has proved difficult to get the enucleated egg to accept this situation and start to divide as if it had been properly fertilised. The answer, it seems, is to shock the egg and its cargo into submission.
Dolly was created by taking a cell from a sheep's udder, inserting it through the thin outer membrane of a sheep egg from which the nucleus had been removed, and passing an electric shock through the two cells. This made them fuse and start to divide into a tiny embryo, ready to be transferred along with 30 or 40 other embryos into a temporary surrogate ewe and eventually a permanent surrogate to complete the pregnancy. Dolly was the only one of 277 embryos constructed in this way to turn into a living lamb.
2Since July 1995, animal cloners in Europe, Australia and Japan have used the original Dolly method or modified the technique to clone mice, cattle, goats and pigs.
Some researchers are happy to continue fusing whole cells with enucleated eggs, others prefer to use only the chromosome-containing nucleus from the donor cell to avoid any unwanted material getting into the new clone.
3Another option proving popular with some researchers is to do a 'double transfer'.
4In this way, the fused egg and cell are transferred into a second enucleated egg and 'jump-started' again. Although more elaborate, the technique may produce more successful pregnancies.
But at present only about 1-2 per cent of cloned embryos develop into living animals.