Telomere
A telomere is a "genetic clock" in DNA. Every time a  divides, the telomere grows shorter, allowing a person's age to be determined at the time the cell sample was taken.

In 2154, Doctor Phlox studied a cell sample of T'Pau that was being used to implicate her in a bombing of the United Earth Embassy. By examining the telomeres, Phlox found that the DNA on the bomb had been taken from T'Pau when she was an infant, and that she was therefore not responsible for the bombing. ()

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de:Telomer
Biology