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Question 3

Biology HL Paper 1B (May 2025, TZ2)

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  1. 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 32P or 35S isotopes. 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 directly outside of the cells.
A line graph showing the percentage of 35S and 32P isotopes remaining outside bacterial cells as a function of running time in a blender. The 35S line (solid, filled circles) shows a higher percentage remaining outside cells compared to the 32P line (dashed, open circles).
A line graph showing the percentage of 35S and 32P isotopes remaining outside bacterial cells as a function of running time in a blender. The 35S line (solid, filled circles) shows a higher percentage remaining outside cells compared to the 32P line (dashed, open circles).

(a) Calculate the difference between the percentages of 35S and 32P remaining outside bacterial cells at 2 minutes. [1]

(b) Explain how this data provides evidence that DNA is the genetic material of cells. [3]

(c) (i) Draw a labelled diagram to show the structure of a DNA nucleotide. [3]

(ii) List the DNA nucleotides that are purines and pyrimidines. [1]
Purines:
Pyrimidines: