Cells begin ordinary cell division by first duplicating their DNA, arranged in chromosomes. When the chromosome number has doubled, the cells can divide, and each daughter cell will have the correct number of chromosomes. At the end of each chromosome is a structure called a telomere, and with each division, the telomeres shorten until they reach a critically short length, and the cells can divide no more. The cells then enter a period of senescence. This period of senescence typically occurs after a specific number of doublings and is thought to represent one component of the aging process.

This raises important issues in cloning. For example, when scientists tested the cells of Dolly, the first cloned sheep, they found that she had shorter telomeres than one would expect for a sheep of Dolly's age. This implied that she might actually be "older" in a cellular sense than her chronological age from birth. If a cloned animal actually reflected the age of the cell nucleus from which it was derived, then possibly the cells, tissues and organs from those animals might wear out relatively quickly and be less useful therapeutically.

A scientific team that successfully cloned several calves has attempted to address this issue. Researchers started with cells from a fetal calf, matured them in cell culture, then inserted them into cow egg cells without nuclei (and thus without their own DNA). Six calved were born, and they all had cells that were "younger" than those of uncloned calves of the same age. This was a remarkable accomplishment, because in the case of the calves, the genetic clock of aging was actually turned back, not merely slowed. These calves might well prove to live much longer lives than their conventionally conceived and calved cousins. Scientists speculate that they may live as much as two extra decades. Time will tell. It is not clear that simply altering the length of telomeres will in fact increase life span.