Matthew Meselson and Franklin Stahl performed the following experiment in 1958. They grew E. coli in a medium containing 15NH4C1 (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that l5N was incorporated into newly synthesised 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 15N is not a radioactive isotope, and it can be separated from 14N only based on densities).