Matthew Meselson and Franklin Stahl performed the following experiment in $1958$ . They grew E. coli in a medium containing ${ }^{15} \mathrm{NH} 4 \mathrm{C} 1\left({ }^{15} \mathrm{~N}\right.$ is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that ${ }^{15} \mathrm{~N}$ 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 $(\mathrm{CsCl})$ density gradient ${ }^{15} \mathrm{~N}$ is not a radioactive isotope, and it can be separateDfrom ${ }^{14}  \mathrm{~N}$ only based on densities).