The conventional wisdom that repeated cloning, like the repeated use of an office copier, leads to inferior copies has been turned upside down. Researchers at the University of Hawaii and Rockefeller University in New York have found evidence that, in one case at least, cloning seems to reverse the wear and tear experienced by chromosomes as they age.
Over the past two years, a team led by biologist Teruhiko Wakayama at the University of Hawaii has produced six generations of cloned mice— clones of clones of clones of clones of clones of clones. The researchers paid particular attention to the animals' telomeres, the caps at the tips of their chromosomes. These genetic caps are associated with aging. They grow shorter over time and are thought to contribute to cell death as they wear away. In conventional reproduction, telomeres are restored during the formation of sperm and egg cells, but cloning bypasses that process. "We might therefore expect telomeres to become shorter and shorter in successive generations of clones," says Tony Perry of Rockefeller University, one of Wakayama's collaborators. "Bafflingly, they seem to get longer."
An earlier study found that Dolly, the cloned sheep, appears to have shortened telomeres, as expected. Perry does not know why his mice are different, nor can he tell whether the extra DNA hanging at the end of their telomeres has a beneficial effect. Even if it does, the results might be difficult to replicate. It took 3,920 cloning attempts to make 35 mice; perhaps the mice with long telomeres were simply the only ones that survived. Unfortunately, Wakayama's cloned mice won't be able to resolve these questions: The experiment came to an abrupt end when the sixth and final generation, consisting of just one individual, was recently eaten by her surrogate mother.