increasing number of copies. you start with one after one round of pcr, you end up with two, then four, then eight and so on. the big advantage of pcr for forensic work is that it is so sensitive. very small amount of dna can yield a very large amount of information. it s a fortunate thing that they didn t try to test it at the time, because as you test biological material it s destroyed. had they, at the time of helena s autopsy, tested that material, they would have found nothing and the material would have been destroyed. when the biological material from helena s fingernails was tested using pcr, scientists made an important finding. on some of the fingernail clippings, they found only dna that was consistent with dr. greenwood s. and on one of the clippings, they found dna that was essentially pure, not from dr. greenwood at all. that s when they got very interested.