Telomeres are bits of DNA that sit at the end of normal chromosomes. With each cell reproductive cycle, telomeres shorten until eventually they are so short that the cells can no longer reproduce. Telomerase is an enzyme that lengthens telomeres after they have been shortened, permitting extra reproductions. Telomerase is found in only a few healthy cell lines, but is found in most cancer cell lines, and is part of the mechanism that makes cancer cells able to reproduce indefinitely.

Scientists studying cloned animals have been curious to know how long these animals' telomeres would be. Would they be the length of the telomeres in the nucleus of the donor cell, or would they be the age (and length) of the host cell?

In examining the chromosomes of Dolly, the cloned sheep, when she was three years old, researchers discovered that her telomeres were shorter than would have been expected for a sheep of her chronological age. In fact, her telomeres were found to be the average length of those expected in a six-year-old sheep-the age of the ewe from whose cells Dolly was cloned.

Other scientists engaged in experiments to clone other animals began studying their telomeres as well. Researchers at Advanced Cell Technology in Massachusetts reported in the April 28, 2000, issue of Science that they had successfully cloned six healthy calves. The donor cells from which these calves were cloned were senescent; that is, they had doubled as often as they were able until their telomeres were too short to allow further doubling. The cloned animals, however, were found to have young, rather than senescent cells, capable of as many as 90 reproductive cycles. Interestingly, their telomeres were found to be longer than conventionally born calves, both newborn and of the same age.

Other researchers at the Ontario Veterinary College looked at the role of telomerase in the cells of cloned cattle. In their work, the telomeres of the cloned calves were not significantly longer than those of conventionally conceived calves of the same age. Some of the cells in the clones in the embryonic stages of development did in fact have significantly longer telomeres than anticipated, and the researchers determined that this was due to greatly increased activity of telomerase, the enzyme capable of increasing telomere length. They concluded that their cloned calves had inherited changes in their donor cells that were subsequently reversed during development. This provocative finding will likely be elucidated by further studies.