Question

How did Hershey and Chase prove DNA as the hereditary material?

Answer 1

Hershey and Chase (1952) used bacteriophage T₂, which consists of DNA enclosed in a protein coat and infects E. coli. They tested which viral component enters the bacterium during infection.

Procedure

1. They prepared one batch of phages with protein labelled by radioactive sulphur ³⁵S. A second batch had DNA labelled by radioactive phosphorus ³²P. Sulphur occurs in some proteins but not DNA; phosphorus occurs in DNA.
2. Each batch was allowed to infect separate cultures of non-radioactive E. coli.
3. After infection had begun, blending detached the phage coats from the bacteria. Centrifugation then separated the heavier bacterial cells (pellet) from the lighter coats (supernatant).

Radioactive ³⁵S was found mainly in the supernatant, showing that the protein coats remained outside the bacteria. Radioactive ³²P was found in the bacterial pellet, showing that phage DNA had entered the cells. The infected bacteria subsequently produced new phages.

Therefore, DNA—not protein—enters the bacterial cell and directs the production of new phages, establishing DNA as the hereditary material in the bacteriophage.