Cloning – from the Greek klon, meaning a cutting such as is used to propagate plants – is essentially a form of asexual reproduction. The initial stages were ﬁrst successfully achieved in rabbits. In essence the technique consists of destroying the nucleus of the egg and replacing it with the nucleus from a body cell of the same species – either a male or a female. This provides the egg with a full complement of CHROMOSOMES and it starts to divide and grow just as it would if it had retained its nucleus and been fertilised with a spermatozoon. The vital diﬀerence is that the embryo resulting from this cloning process owes nothing genetically to the female egg. It is identical in every respect with the animal from which the introduced nucleus was obtained.
In 1997 the ﬁrst mammal to be cloned from the tissue of an adult animal was born. A technique that scientists have been trying to perfect for decades, the success of the Roslin Institute, near Edinburgh, in producing ‘Dolly’, a cloned sheep, has profound implications. Already some scientists are talking of cloning humans, although this has great medical, legal and ethical consequences. The key to the scientists’ success in producing Dolly was the ability to coordinate the fusion of a donor cell (from an adult) containing all its DNA with a recipient egg from which DNA had been removed. The diﬃculty of the technique is shown by the fact that, out of 277 fused pairs of cells where the donor cell was from adult tissue, Dolly was the only survivor and she has developed premature arthritis. Research suggests that cloning may be accompanied by a higher than normal incidence of congenital defects.
Since Dolly was born, other animal clones have been produced and American researchers have cloned the ﬁrst human embryo – which grew to six cells – with the aim of providing stem cells for therapeutic use. As a result the UK government passed emergency legislation to outlaw human cloning for reproductive purposes.