What was the nitrogen source used in the experiments performed by Meselson and Stahl?

Step-by-step solution

(i) They grew E. coli in a medium containing 15NH₄Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that 15N was incorporated into newly synthesized DNA (as well as other nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCl) density gradient (Please note that 15N is not a radioactive isotope, and it can be separated from 14N only based on densities). (ii) Then they tran