The graph shows results of an experiment by Hershey and Chase in 1952 in which bacteria were infected with a mixture of virus particles labelled with either ³²P or ³⁵S. A suspension of the infected bacteria was agitated with a blender, and samples collected from the suspension were centrifuged to record the percentage of isotope remaining on the outside of the cells.

[Source: Republished with permission of ROCKEFELLER UNIVERSITY PRESS, from Independent functions of protein and nucleic acid in growth of bacteriophage. Hershey, A.D. and Chase, M., 1952. (Journal of General Physiology, 36(1), p.47). Society of General Physiologists, Rockefeller Institute for Medical Research, Rockefeller Institute; permission conveyed through Copyright Clearance Center, Inc.]

What do curves X and Y represent?

B

The statistics show that the candidates found this question difficult, and it did not discriminate well. The graph in the stem of the question was taken from the original Hersey and Chase 1952 publication. The G2 forms suggest two reasons why the candidates found this question difficult.

First, they may not have remembered what is meant by supernatant and sediment. However, these terms are important in understanding the Hershey and Chase experiment and should have been known.

Secondly, 'the percentage of isotope remaining on the outside of the cells' was possibly misleading. This may have led candidates to think that some of the isotopes were still attached to the outside of the bacterial cell when the intended meaning was the amount of isotope apart from the bacterial cells which would therefore be in the supernatant.

As the question is only considering the percentage of the isotope in the supernatant the 35S from the protein shell of the bacteriophage would be found in the highest percentage as it does not enter the bacterial cell as shown by line X. As some of the 32P from the phage genetic material would have entered the bacterial cell, a lower percentage would be found in the supernatant which corresponds to line Y. This gives correct answer B.