Now, why did we want to use N 15 and N 14? Well, because they have a difference in mass the DNA molecules, the old ones and the new ones will also different mass and we can separate them via the process of centrifugation which separates bimass by size and by density. So the DNA with different densities can be separated by centrifugation. So what we basically do is we allow these cells, the equalized cells, to replicate, to divide and replicate and we extract the DNA. Then we centrifuge that DNA and we analyze our DNA. So what they obtained was the following three photographs.