Question

In 1952, Alfred Hershey and Martha Chase, working with a virus called T2, conducted what is now considered a landmark experiment to determine whether genes are made of DNA or protein. When this virus, which is made entirely of DNA and protein, infects E. coli, it injects its genetic material into the bacterial cell, leaving the empty virus head stuck to the cell surface. To determine whether it was DNA or protein that enters the infected bacterial cell, the researchers radioactively labeled one batch of T2 with the isotope 35S, which resulted in radiolabeled viral proteins, and a second batch of T2 with 32P, which resulted in radiolabeled DNA. They then incubated the radioactive viruses with E. coli. After allowing a few minutes for the viruses to transfer their genetic material to the bacterial cells, the researchers used a blender to shear the empty virus heads from the bacterial cell surface. They then used a centrifuge to separate the infected bacteria from the empty virus heads: spinning the sample at high speed caused the heavier, infected bacteria to pellet at the bottom of the centrifuge tube, while the lighter, empty virus heads remained in solution. Using this protocol, what should the researchers have seen in terms of the distribution of radioactivity in the centrifuged sample?

Choose one:
A. 32P in the pellet, 35S in the solution
B. an equal amount of 35S and 32P in the pellet and the solution
C. no radioactivity in the pellet
D. 35S in the pellet, 32P in the solution
E. no radioactivity in the solution

Answer 1

In the Hershey-Chase experiment, radioactive phosphorus 32P, marking DNA, was found in the pellet with the bacteria, while radioactive sulfur 35S, marking proteins, was in the supernatant. This demonstrated that DNA is the genetic material. Hence, the correct distribution of radioactivity was 32P in the pellet, 35S in the solution.

Step-by-step explanation:

Alfred Hershey and Martha Chase designed a pivotal experiment in 1952 to identify the genetic material responsible for heredity within T2 bacteriophages when infecting E. coli bacteria. By utilizing radioactive isotopes, 32P marked DNA and 35S marked proteins due to phosphorous being exclusive to DNA and sulfur being prevalent in proteins. After the bacteriophages infected the E. coli and the mixture underwent centrifugation, Hershey and Chase observed that the 32P was in the pellet with the bacteria, and the 35S was in the supernatant with the empty virus heads.

This indicated that DNA, not protein, was the material injected into the E. coli cells and was therefore the genetic material responsible for heredity. Thus, the correct answer to their experiment's expected outcome is A. 32P in the pellet, 35S in the solution, which is where the researchers would have seen the distribution of radioactivity after centrifugation.