Calf clones produced by a new method have stunned scientists by appearing genetically younger than their actual age. The oldest is about a year old but has genetic material that would be more typical of a new-born.

Researchers at Advanced Cell Technologies of Worcester, Massachusetts, who produced the clones, have not discovered a fountain of youth; nor have they shown that these calves will live any longer than their uncloned peers. Their findings, published in the current issue of the journal Science, have caused quite a bit of excitement, nonetheless, because of the possibilities they raise.

The key to the research relates to a fragment of genetic material called a telomere, located at the end of each chromosome. Telomeres can be likened to the bit of plastic that keeps the end of a shoelace from fraying.

Each time a cell divides it must produce an exact copy of the genetic material at its core, and researchers believe that telomeres help this process, ensuring a faithful replica of all the chromosomes. As we age, however, the telomeres gradually shrink and the copying process becomes less reliable, leading to genetic errors and possible disease.

The six healthy cow clones, however, show no sign of this ageing process and in fact have telomeres more typical of a new-born calf than of a 12-month-old. "It's really remarkable. Telomeres from all of the cows, including one who is celebrating her first birthday this week, look like those of a new-born," said Dr Robert Lanza of the Advanced Cell Technologies team which made the discovery.

This immediately raises the possibility that the clones might in turn live longer, given their "youthful" telomeres. It will take years to prove that this is so, however, and the experience with Dolly the cloned sheep seems to limit any claim that a method to reverse ageing has been discovered.

Dolly, created by the Roslin Institute near Edinburgh, was produced using adult mammary cells that had been starved and sent into a resting state.

Once born, Dolly was shown to have telomeres typical of a six-year-old animal, the age of the animal that provided the donor cells for Dolly. Since then, however, Dolly has gone on to produce several lambs and shows no apparent sign of more rapid ageing.

The donor cells used in the calf clones, however, were taken from an adult cow's connective tissue. For an as yet unknown reason, when these cells were cloned and the calves produced, they had telomeres that were longer than usual, not shorter.

"Previous studies have indicated that there may be variation in how different cell types repair telomeres, which could make the choice of donor cell significant," said Dr Lanza.

It will take years to establish whether these new clones, because of their "younger" telomeres, will live longer or avoid signs of ageing better than non-cloned calves. Scientists are excited about the finding, however, because this could be an implication of the discovery.

They will want to learn whether the change in the telomeres arises because of the type of cell used for cloning or whether some other factor is at work. It will open up many new avenues of research into this emerging technology.