Now, remember, the normal nitrogen atom contains an atomic mass of 14, and that's because it contains seven protons and seven neutrons in the nucleus. But in the case of this heavy isotopic nitrogen atom, we have eight neutrons and seven protons. And so eight plus seven gives us an atomic mass of 15. 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.