Analysis of Dolly's genetic material showed the so-called telomeres, sequences of DNA which cap the ends of chromosomes, were shorter in Dolly than in a normal sheep of the same age.
Telomeres protect chromosomes from fraying and are believed to be involved in the ageing process.
This new discovery means that at genetic level Dolly may be at least six years older than she should be, scientists said. The latest results were published Thursday in the science journal Nature by PPL Therapeutics and the Roslin Institute in Edinburgh, Scotland. The group includes members of the original team involved in producing Dolly nearly three years ago. After studying both Dolly and two other sheep clones produced from embryos, the researchers found that all the three cloned sheep had shortened telomeres. They also stressed that it was too early to say whether Dolly's telomeres would shorten to the critical point where damage is caused during her lifetime, because other research suggested they would not.