Discuss the significance of the heavy isotope of nitrogen in the Meselson and Stahl experiment.

(N/A) Matthew Meselson and Franklin Stahl performed their landmark experiment in $1958$. They cultured $E. coli$ in a medium containing $^{15}NH_4Cl$ ($^{15}N$ is the heavy isotope of nitrogen) as the sole nitrogen source for many generations. As a result,$^{15}N$ was incorporated into the newly synthesized $DNA$ molecules.
This heavy $DNA$ molecule could be distinguished from normal $DNA$ $(^{14}N)$ by centrifugation in a cesium chloride $(CsCl)$ density gradient. The significance of $^{15}N$ lies in the fact that it is a non-radioactive,stable isotope that allows for the separation of $DNA$ strands based solely on differences in their buoyant density,thereby proving the semi-conservative mode of $DNA$ replication.