Final answer:
During their experiments, Hershey and Chase discovered radioactivity inside bacterial cells when they were infected with phages labeled with 32P, which specifically marks DNA. This radioactivity indicated that the DNA, rather than the protein, of the phages was the genetic material responsible for infection..
Step-by-step explanation:
Alfred Hershey and Martha Chase conducted experiments to determine the molecular basis of viral infection and heredity. They used the T2 bacteriophage, a virus that infects the bacterium E. coli, to understand whether it was proteins or DNA from the phages that carried the genetic instructions. The critical part of their experiment involved labeling the DNA of phages with radioactive phosphorus (32P) and labeling the proteins of phages with radioactive sulfur (35S). They knew that DNA, but not proteins, contains phosphorous, and that proteins, specifically the amino acids methionine and cysteine, but not DNA, contain sulfur.
When Hershey and Chase allowed the 32P-labeled phage to infect E. coli cells, they found that after they blended the mixtures to detach the phage coats and centrifuged them, the radioactivity from 32P was found only inside the bacterial cells in the pellet. This was evidence that the DNA from the phages had entered the cells. In contrast, when they infected the cells with 35S-labeled phages, they found the radioactivity only in the supernatant, indicating that the protein coats remained outside the cells. Hershey and Chase therefore concluded that the DNA from the phages was responsible for infecting the bacteria and carrying the genetic information, establishing that DNA is the genetic material.