So initially, E. Coli bacterial cells were grown in a medium rich in heavy nitrogen and 15, as shown in the following diagram. And this was done to basically produce E. Coli cells that have incorporated this isotopic N 15 into their DNA. And now the DNA is radioactively labeled and contains the heavier nitrogen atoms. Now once we form these E. Coli cells that contain the heavy DNA molecules, these ecology cells are then transferred into a beaker into a flask that contains the regular nitrogen N 14 isotope. And so now when these E. Coli cells are going to reproduce via mitosis, they're going to incorporate the N 14 isotope into the newly synthesized DNA molecules. And what we want to answer is what will be the distribution of the N 14 and N 15 isotopes following several cycles of replication.