How did Hershey and Chase establish that DNA is transferred from virus to bacteria ? (5 marks)
1. Hershey and Chase (1952) worked with viruses that infect bacteria called bacteriophages.
2. Bacteriophage attaches to the bacteria and create to enter its genetic material (DNA) into the
bacterial cell. Virus controls bacterial DNA and instructs to make its own replica i.e. virus
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 and some other
on a medium containing radioactive sulphur .
5. Radioactive sulphur was incorporated in the protein part of virus as sulphur in present in the
amino acids of proteins 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 bacteria cells.
11. Bacteria which were infected with radioactive virus, was detected in supernatant not in
12. Bacteria which were infected with radioactive virus was detected in bacterial cells not in
13. This indicates protein did not enter the bacteria from viruses. DNA is therefore the genetic
material that is passed from virus to bacteria.