Meselson and Stahl proved the semi-conservative nature of DNA replication using the isotope ___.

Step-by-step solution

Meselson and Stahl performed an experiment in 1958 to prove the semi-conservative nature of DNA replication. They grew E. coli in a medium containing ¹⁵ NH ₄ Cl ( ¹⁵ N is the heavy isotope of nitrogen) for many generations. The result was that ¹⁵ N 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. Therefore, the isotope used was ¹⁵ N . Answer: