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As a US scientist presses ahead with plans to set up the world's first human cloning clinic, we answer the vital questions: n Should he be allowed to go ahead? n Is it safe? n Is it unnatural? n Are the scientists playing God? n Is it morally acceptable to create a new human being merely as a means to an end - even a life-saving one? n Would a cloned child grow old before its time?

Q Cloning is utterly unnatural and repulsive, isn't it?

A Nature does it all the time. Identical twins have identical DNA, because they start as a single cell - a fertilised egg - which then divides into two.

Q Why are people upset at the idea of human cloning then?

A They don't like the idea of scientists "playing God", nor the idea that rich people might get copies of themselves made. And you just know that some lunatic will be trying to find some of Hitler's DNA right now.

Q So why do it?

A We are far from being able to do it safely and reliably (see below), but there could be benefits from trying. Research currently is permitted on human embryos up to 14 days and there is general agreement that this has brought benefits in terms of understanding infertility and in other ways. But embryos are delicate and 14 days is a short time. Cloning them would mean more genetically identical copies were available, extending the research possibilities, for example, into the biology of DNA damage and repair, the process of parental imprinting and its role in early development

Q Is that the only reason to do it?

A No. The experiments at the Roslin Institute in Edinburgh, home to Dolly the sheep, have already demonstrated the potential human benefits. Polly, another sheep cloned from an embryo at the laboratory, has had a human gene inserted which means it is now producing in its milk human Factor IX, a blood- clotting agent needed by men with haemophilia B. The cloning of human cells in the lab could similarly yield tissue, including blood cells and skin, for surgical or genetic repair or for grafting.

Q Is that the reason to do it?

A No. There could be potential benefits from cloning full human beings. Ruth Deech, chairwoman of the Human Fertilisation and Embryology Authority, spelt these out before the Commons science and technology committee last year. Before doing so, she firmly ruled out the production of human beings as "banks" to provide organs or bone marrow for transplant, or as "consolation" for bereaved parents who wished to reproduce a beloved child. But she said it was essential to retain flexibility in the law to accommodate future scientific developments.

Q Hang on, why should these applications be disallowed? What is wrong with cloning a baby brother or sister for a child with leukaemia to act as a bone-marrow donor when no other match can be found? Parents of children with leukaemia have babies deliberately, hoping they will be a match for their siblings. Cloning would remove the lottery from this process.

A Here we get to the essence of the moral objection to cloning: to create another person as a means to an end is morally unacceptable. It undermines the autonomy of the individual. When Dr Richard Seed, the US scientist who wants to set up the first human clone clinic in Chicago, said he wished he had met Mother Teresa and obtained a sample of her blood before she died from which to clone a replica, he showed why he is not a fit person to run such a clinic. What sort of life would such a baby have? The chances are it would not be treated as an individual because people would be expecting it to grow up into the next Mother Teresa.

Q So under what circumstances would cloning a full human being be acceptable?

A A possible application, cited by Ruth Deech, is in the treatment of sufferers from a rare inherited disorder of the mitochondria - the "power- plant" of the cell - which surround the nuclei of cells. This problem can cause blindness and epilepsy. By removing the nucleus - minus the defective mitochondria - from an embryo created by in vitro fertilisation in the normal way and placing it in a donated egg stripped of its own nucleus, a cloned baby could be created that would be the genetic offspring of its parents, but without the disorder.

Q What's the difference between cloning and IVF (in vitro fertilisation)?

A In IVF, the egg is fertilised by sperm in a test tube; that egg (usually many, to improve the chances of success) is then implanted in the uterus, where itshould grow normally to term. In cloning, the nucleus of the egg cell (containing the DNA that tells it how to grow) is removed in a test tube, and the nucleus from the organism to be cloned is inserted in its place. This means that the egg cell now has the full DNA to create an organism. It is implanted in the uterus to grow like other embryos.

Q Are clone embryos like IVF and normal pregnancies?

A Not so far. The scientists at the Roslin Institute, who pioneered this work, have repeatedly found that clone foetuses grow much larger than normal ones and there is a far higher chance of the pregnancy failing, of stillbirth, or forced Caesarean sections. Dolly was the one successful pregnancy of more than 277 embryos.

Q So, if I'm cloned, the result will have exactly the same DNA as me?

A Only if you're a woman and use one of your own eggs. A key bit of DNA actually resides outside the nucleus, and isn't removed in the "DNA transplant". This is the mitochondrial DNA referred to by Ruth Deech (usually called the mtDNA), which is passed down through the maternal line - everyone inherits their mother's mtDNA. So men can't be cloned exactly, unless they get an egg from an immediate female relative.

Q So, what are the worries about mitochondrial DNA?

A Scientists think it plays a key role in ageing. mtDNA contains the instructions for making the mitochondria. Some theories of ageing hold that when the mitochondria run down, the cell dies.

Q But the mtDNA in an egg are always new, so there's nothing to worry about?

A Possibly. But the genetic DNA in the nucleus also "ages": by the time you're an adult, it has been damaged and repaired in every cell many, many times. Cell DNA damage often leads to cancer - so human clones might be liable to severe illness. Dolly, who was cloned from the udder cells of a six-year-old ewe, seems healthy, so far.

Q What's the difference between cloning from an embryo cell and from an adult cell?

A Embryo cells are called "totipotent" - that is, each has the potential to become an embryo in its own right. But at some stage in its growth, while still in the womb, the cells lose that ability because some genetic switches are flipped off or on. They become any one of the multitude of specialised body cells, and won't revert. That's convenient - it means when you cut yourself, you don't get a baby growing over the cut, you get skin.

Q How do you make an adult cell totipotent again?

A That's what the Roslin team seem to have worked out. It consists of "starving" the isolated cell, so that it stops trying to grow. This state is known as "G0". Then you take the DNA from the nucleus and implant it in an egg.

Q What do the experts think?

A"I think you are always going to run the risk of having ageing DNA," says Professor Lord Robert Winston, an IVF pioneer. "I would hate to think of a child of mine being cloned because I think it would be very likely he would have an accelerated ageing process."

Dr Jamie Grifo, director of the division of reproductive endocrinology at New York University, says: "Cloning is no better than any of the other treatments out there. A biological child is the husband's sperm, the wife's egg. A clone is not a biological child."

Dr David Stevens, of the Christian Medical and Dental Society, asks: "Are we willing to sacrifice hundreds of embryos - developing human beings - to make one baby who may suffer monstrous consequences?"

Q I'd like to read more about cloning. Any suggestions?

A Clone - The Road to Dolly and the Path Ahead by Gina Kolata (Penguin, pounds 15.99) is an excellent read, with the inside story from the Roslin Institute, a history of the subject up to Dolly's birth, and some of the subsequent ethical debate. Remaking Eden - Cloning and Beyond in a Brave New World by Lee M Silver (Weidenfeld & Nicolson, pounds 20) explores the ethical issues and possibilities in a wider context.

Q Was Dolly the sheep the first artificial clone?

A No - she is the first mammal cloned from an adult cell. Previously, frogs have been cloned, and sheep from embryo cells.

Q Why did they use a sheep?

A Two reasons: it's a mammal, and so more like humans than most other animals; and in Scotland, sheep are "very, very, very cheap".