Alan Coleman explains the theoretical link between telomere length and ageing

The possibility that the sheep may die early has been raised after a study of her genetics by some of the same scientists who created the clone in 1996.

Telomeres protect the ends of chromosomes

A team from the biotechnology firm PPL Therapeutics in Scotland examined structures in Dolly's cells called telomeres.

They report in the journal Nature that the structures are slightly shorter than would be expected in a sheep of her age born normally.

Dr Ian Wilmott, project leader: Dolly's clock hasn't been reset

There is some evidence that telomere length is linked to ageing. However, the researchers say it is currently impossible to predict precisely how long the world's most famous sheep will live.

"All we're saying is that yes, as we and others expected, this animal started off with shorter telomeres," Dr Alan Coleman, research director at PPL Therapeutics told the BBC.

Dolly and her triplets

"It doesn't necessarily follow that she will age prematurely because this link between the telomeres and ageing is not proven - it's just an interesting theory.

"In fact, if sheep like Dolly and other mammals made by this [cloning] technique are continually shown to be perfectly healthy and not age prematurely, that would just prove that the whole link between the telomeres and ageing is an unreliable one and not worth worrying about."

Older than they look?

Cell suicide

Telomeres are nubs of protein and nucleic acid that cap the ends of chromosomes, the structures in cells that bundle up all the DNA.

The telomeres are produced during embryonic development but start to crumble away as cells mature and divide. When the erosion is complete, so the theory goes, the cell commits suicide.

A computer simulation of the erosion of a telomere

In recent US research, laboratory mice bred to have shorter telomeres went prematurely grey, got cancer more often and died early.

Dolly's shortened telomeres would seem to stem from two factors:

• She was made using genetic material taken from a six-year-old ewe so some of the telomere shortening could have been passed on.
• The cloning technology requires cells to be cultured in the laboratory for a period of time, which further reduces the length of the telomeres.

Dr Coleman says the research will need to be followed up on many more cloned animals. But from what they know already, he says he would not recommend cloning clones because this would only amplify any problems that might exist.

Exciting technologies

This research is helping us to understand the ageing in cells

The research also has important implications for the new stem cell technologies, now exciting huge interest. Stem cells are the "master" cells in the body which can become any type of tissue.

Scientists believe they can ally cloning and stem cells to create perfect-match transplant tissue.

They envisage, for example, leukaemia victims being able to provide their own bone marrow transplants.

This has important implications for the new stem cell technologies, says Alan Coleman

But the cloning process involved in creating these replacement cells would have an effect on their telomeres.

"Undoubtedly they would go down dramatically," says Dr Alan Coleman. "If you put these cells back into a human, you would be asking yourself how long they would last. How would they behave if they have been having all this erosion taking place at the end of their chromosomes?"

He believes one thing is certain - Dolly and all the other clones now being created around the world will tell us a lot more about the lifecycle of cells and the ageing process.