→ The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages.
→ The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell.
→ The bacterial cell treats the viral genetic material as if it was its own and subsequently manufactures more virus particles.
→ Hershey and Chase worked to discover whether it was protein or DNA from the viruses that entered the bacteria.
→ They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that contained radioactive sulfur.
→ Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein because DNA contains phosphorus but protein does not.
→ Similarly, viruses grown on radioactive sulfur contained radioactive protein but not radioactive DNA because DNA does not contain sulfur.
→ Radioactive phages were allowed to attach to E.coli bacteria.
→ Then, as the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender.
→ The virus particles were separated from the bacteria by spinning them in a centrifuge.
→ Bacteria which was 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.
→ This indicates that proteins did not enter the bacteria from the viruses.
→ DNA is therefore the genetic material that is passed from virus to bacteria.
