Q&A: Can humans be cloned?

Controversial doctor Panos Zavos has claimed he has implanted a cloned embryo into a woman's womb.

BBC News online looks at his claims and the whole issue of human cloning.

What is cloning?

Cloning is the process of making a genetically identical organism, without normal sexual means.

Practically, it means the creation of cells or even whole plants or animals using DNA from a single "parent" - bypassing the normal reproductive process.

The best-known procedure is "nuclear transfer", which produced Dolly the sheep in 1997.

This takes DNA from one adult cell and puts it into a "hollowed out" egg.

A trigger such as electricity is then applied to make the DNA and the egg fuse, and the embryo cells to start dividing as normal.

The resulting embryo has almost identical DNA to that of the donor, but would not be a complete copy because there are some important genes that would come from the egg. These genes reside outside the nucleus.

What does Dr Zavos claim to have done?

Dr Zavos says he has not only created a cloned human embryo, but he has implanted it into the womb of a 35-year-old woman.

He says he got the embryo by using the nuclear transfer process - using the woman's own immature egg, and DNA from a skin cell from her husband.

He then implanted it into the woman's womb.

This does not mean she will definitely be pregnant. Dr Zavos estimates there is a 30% chance implantation will have been successful.

Is he the first person to have apparently achieved this?

Dr Severino Antinori in Italy said a cloned child would be born last year and the Raelian cult insists five babies have already been born.

As yet no proof has emerged of any of these.

Has he any proof?

No. He said proof would be made available if the procedure was successful. He promised to allow independent testing of the embryo's DNA and that of the mother and father.

If the baby was a clone, most of its DNA would be a copy of its father but a small amount of mitochondrial DNA would be inherited from its mother.

What do other scientists think?

There is a lot of scepticism and unease about the claims made by Dr Zavos.

First of all, many do not believe his claim to have obtained a cloned embryo.

They want to see evidence and detailed scientific research in peer-reviewed journals.

Secondly, many say that even if true, the experiment is "irresponsible and unethical".

Ethical considerations aside, they say the procedure is incredibly risky and may cause harm to both the mother and the embryo.

Why are scientists trying to achieve cloning?

Dr Zavos and others say reproductive human cloning gives infertile couples the chance of having a baby.

Other scientists have been looking at animal and plant cloning for a variety of different reasons - such as recreating an animal with a resistance to disease that natural breeding may not produce.

Therapeutic cloning - which creates cells, not entire beings - can create perfect-match tissue which could be used to treate patients with diseases such as leukaemia.

Why are other scientists so uneasy?

They say that the quest to create a human clone will cause a trail of failures and human suffering.

The vast majority of experiments in animal clones have gone very badly.

In most of them, the clone has died and in almost all of them the lives of the mother and clone have been put at risk.

Dolly the sheep, the first mammal clone, was the one success in 247 pregnancies.

The problem has often been that the clone grows abnormally large, often threatening to tear the womb that can also become swollen with fluid. Almost all clone pregnancies spontaneously abort.

Of the small number (a matter of just a few percent) of animal clones that make it to term, most have severe abnormalities: malfunctioning livers, abnormal blood vessels and heart problems, underdeveloped lungs, diabetes, immune system deficiencies and possibly hidden genetic defects. Several cow clones had head deformities - none survived very long.

Recent attempts in America to clone monkeys failed, leading some scientists to suggest that the biological make-up of primates, including humans, makes cloning them almost impossible.

We know very little about the long-term health of clones. There is some evidence that they may not live as long as conventional humans and may have health problems. Studies are ongoing.

Isn't all of this illegal anyway?

In most countries that carry out advanced biomedical research it is. In less developed countries, including some that offer test-tube baby programmes, there are no laws against it.

Just taking the UK as an example, there is now a specific law to ban the placing of any embryo in a woman's womb that is not created by a fertilisation process.

All embryo research in Britain requires a licence and you simply would not get one if you said you wanted to make human clones.