Goodbye, Dolly

By Jeffrey Kluger

Ever since Dolly the sheep was famously cloned more than two years ago, scientists have been troubled by a nagging question: Will an infant animal that is a genetic photocopy of an adult live a full life, or will the advanced years of its "parent" make it older too? Last week the first answers arrived, and the news was not good. Some clones may indeed be growing old before their time, according to a study in the journal Nature.

The problem appears to involve telomeres, cufflike bands at the ends of chromosomes that cap the strands like the plastic sleeves at the tips of shoelaces. As animals age, telomeres shorten, causing chromosomes to fray, cells to wink out and the organism as a whole to become frail.

British researchers have studied the telomeres of Dolly and two other cloned sheep and found that they are shorter than those of similar-age sheep conceived normally. Dolly, cloned from a six-year-old animal, had the smallest telomeres of all--barely 80% of the proper length. The other two sheep, cloned from embryos, were better off but still came up short.

While the age of the donor is probably partly responsible, the research team--which included Ian Wilmut, Dolly's creator--may have discovered another factor. The more time a clone embryo spent in a test tube before transfer to a womb, the shorter the clone's telomeres. "In culture, cells go through 20 divisions," says research director Alan Colman. "That's a significant percentage of the 150 they go through in a lifetime."

For now, the sheep clones have little to worry about. While their truncated telomeres may burn down relatively fast, the animals are likely to die of natural causes before frayed chromosomes claim them. But if scientists ever get around to cloning humans, things could get stickier. You might end up with the worst of both worlds, says Thomas Murray of the Hastings Center, a New York-based think tank, "combining the inexperience of youth with the biology of the aged."

--By Jeffrey Kluger