Question

Give an account of the Hershey-Chase experiment proving that DNA and not the protein coat of the virus is the infecting agent.

Answer 1

The Hershey-Chase experiment (1952) used T2 bacteriophages to definitively prove that DNA, not protein, is the genetic material. The experiment was conducted in three main stages:

• Radioactive Labelling: Two batches of phages were prepared. One batch was grown in radioactive Phosphorus (³²P) to label the DNA, and the other in radioactive Sulphur (³⁵S) to label the protein coat. This was possible because phosphorus is unique to DNA, while sulphur is unique to proteins.
• Infection and Blending: The labelled phages were allowed to infect E. coli bacteria. After infection, the mixture was agitated in a blender to dislodge the viral protein coats (ghosts) from the surface of the bacterial cells.
• Centrifugation and Analysis: The mixture was centrifuged to separate the heavy bacteria (forming a pellet at the bottom) from the light viral coats (remaining in the supernatant at the top).

The Result: Radioactivity from ³⁵S was found only in the supernatant, whereas radioactivity from ³²P was found inside the bacterial pellet. This proved that only DNA entered the bacteria to direct the production of new viruses, confirming it as the hereditary infecting agent.