(N/A) The proof that $DNA$ is the genetic material came from the experiments of Alfred Hershey and Martha Chase $(1952)$. They worked on viruses that infect bacteria,called bacteriophages.
Bacteriophages attach to bacteria and inject their genetic material into the host cell. The bacteria then treat the viral genetic material as if it were its own and subsequently manufacture more virus particles.
Hershey and Chase conducted experiments to determine whether it was $DNA$ or protein that entered the bacteria from the virus.
They grew some viruses in a medium containing radioactive phosphorus $(^{32}P)$ and others in a medium containing radioactive sulfur $(^{35}S)$. Viruses grown in the presence of radioactive phosphorus contained radioactive $DNA$ but not radioactive protein because $DNA$ contains phosphorus,while proteins do not.
Similarly,viruses grown in radioactive sulfur contained radioactive protein but not radioactive $DNA$ because $DNA$ does not contain sulfur.
These radioactive bacteriophages were then allowed to infect $E. coli$ bacteria. As the infection proceeded,the viral coats were removed from the bacteria by agitating them in a blender. The virus particles were then separated from the bacteria by spinning them in a centrifuge. The bacteria that were infected with viruses that had radioactive $DNA$ were radioactive,indicating that $DNA$ was the material that passed from the virus to the bacteria. Bacteria that were infected with viruses that had radioactive proteins were not radioactive,indicating that protein did not enter the bacteria.