Dolly the cloned sheep continues to make scientific news. Dr Paul Shiels of PPL Therapeutics, Roslin, and a member of the group which produced the first clone, writes in Nature that Dolly and two other cloned sheep may be slightly different on a genetic level to sheep born in the usual way.
Mice, cows and goats have all been successfully cloned since Dolly made her appearance. Researchers involved in these projects are interested to identify any differences, no matter how slight, between cloned and naturally delivered animals.
In particular, they want to know whether the cloned animal is like any other newborn or carries the characteristic signs of age if cloned from cells taken from an adult. The Roslin team believes it has identified these age-related signs in the DNA of clones but also says that in all other respects the clones appear healthy and just the same as their non-cloned peers.
The Roslin researchers were looking at telomeres, the part of the DNA sequence located at the ends of chromosomes. Telomeres are like caps, protecting the ends of the chromosome from fraying.
The telomere length is gradually reduced with age, however, due to copying errors as the cell goes though division after division. If this theory is correct, then the older the animal the shorter its telomeres, with newborns having the longest telomeres.
Dolly was delivered using mammary gland tissue from a six-year-old sheep, and the other two sheep under study were cloned from cells taken from a nine-day-old embryo and a 25-day-old foetus. All three were found to have shorter telomeres than in other age-matched, naturally delivered sheep and Dolly's were shorter again than the other two clones.
Dolly's donor was also tested to ensure she was typical of her age group and indeed she was. The researchers also found that the period of time that donor material was kept in culture was another likely influence on telomere length, with shorter telomeres linked to a longer time in culture.
None of this, however, seems to be having a negative effect on Dolly or her fellow clones. The Roslin group points out in its report that recent veterinary examination confirmed them to be healthy and typical for sheep of their age and breed. Dolly has actually undergone two normal pregnancies and has successfully delivered healthy lambs, with telomeres typical of any newborn.
The next question relates to whether the clones, with their shorter telomeres, can expect to die sooner than their non-cloned peers because they have telomeres typical of an older animal. Other research cited by the authors suggests, however, that the clones' telomeres will not become dangerously short within any of their natural lifetimes.