Telomere
From Memory Alpha, the free Star Trek reference.
A telomere is a "genetic clock" in the DNA. Every time a cell divided it'll grow shorter. They tell the age of an individual after a cell sample was studied.
In 2154, Doctor Phlox studied the cell sample of T'Pau and found the evidence that the found DNA on the bomb was not her DNA, because the telomeres told him that the cell sample was taken when she was an infant. The false DNA was placed on the bomb. (ENT: "The Forge")
