Worn away


By Nell Boyce THE ends of Dolly the cloned sheep’s chromosomes are shorter than those of normal sheep of the same age. This suggests that she inherited some of the wear and tear suffered by her six-year-old mother’s cells. Whenever a cell divides, the end of its chromosomes, called telomeres, erode away. When they wear down to a critical length, the cell stops dividing. Older animals have shorter telomeres, and biologists consider telomere shortening a hallmark of ageing. So when a team at the Roslin Institute near Edinburgh turned an adult cell into Dolly, scientists wanted to know about her telomeres. Now the Roslin team and the affiliated company PPL Therapeutics say that Dolly’s telomeres are shorter than those of two sheep cloned from embryonic cells—and all of the clones’ telomeres are shorter than those of normal sheep (Nature, vol 399, p 316). For the time being, Dolly seems healthy. And biologists will need to study many cloned animals before they know whether their shortened telomeres will make them age more quickly. The problem could get worse if cloning is carried out over several generations. But normal reproduction seems to repair the shortened telomeres: Dolly’s first daughter, a lamb called Bonnie conceived in the traditional way, has normal chromosomes. Given the uncertainties, however,
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