How did Hershey and Chase prove that DNA is the hereditary material? Explain their experiment with suitable diagrams.
Experiments with diagrams:
1. Hershey and Chase (1952) worked with viruses that infect bacteria called bacteriophages.
2. Bacteriophage attaches to the bacteria and create a pore to enter its genetic material (DNA) into the bacterial cell. Virus controls bacterial DNA and instructs to make its own replica i.e. virus particles.
3. They wanted to discover whether it was protein or DNA from the viruses that entered the bacteria.
4. They grew some viruses on a medium containing radioactive phosphorus (32P) and some other on a medium containing radioactive sulphur(35S).
5. Radioactive sulphur was incorporated in the protein part of virus as sulphur is present in the amino acids of protein but not in DNA.
6. Radioactive phosphorus was incorporated in the DNA part of virus as phosphorus is present in the DNA not in protein.
7. These two phage preparations were allowed to infect the bacterial cells separately.
8. Before lysis of bacteria both the cultures were gently agitated in a blender to separate the viral coats from bacterial cells.
9. The culture was then centrifuged to separate viral particles from the bacterial cells.
10. The heavier bacteria settled down in the form of pellets. The supernatant contains lighter viral coats which do not enter in the bacterial cells.
11. Bacteria which were infected with radioactive 35S virus, was detected in supernatant not in bacterial cells.
12. Bacteria which were infected with radioactive 32P virus was detected in bacterial cells not in supernatant.
13. This indicates protein did not enter the bacteria from viruses. DNA is therefore the genetic material that is passed from virus to bacteria.