The replicated daughter strands will contain N 14 isotopes because as these daughter cells are synthesized we're going to take the nitrogen to synthesize the nitrogenous bases and sugars and so forth from this median that contains the M 14. So the newly synthesized strands will contain the regular N 14 isotope. But the other ones, the original will contain that labeled N 15 isotope. 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.