DP Biology (last assessment 2024)

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Question 22M.2.HL.TZ2.2b.ii

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Date May 2022 Marks available [Maximum mark: 2] Reference code 22M.2.HL.TZ2.2b.ii
Level HL Paper 2 Time zone TZ2
Command term Explain Question number b.ii Adapted from N/A
b.ii.
[Maximum mark: 2]
22M.2.HL.TZ2.2b.ii
(b.ii)

Explain Cairns’s technique to measure the length of the DNA molecule.

[2]

Markscheme
  1. Cairns grew prokaryotes/E. coli in radioactive thymidine/thymine/thymine containing tritium;
  2. contents of cell put on photographic film/surface (for several weeks) / used autoradiography and electron microscopes;
  3. measured the length of the DNA molecule and photographed it / produced image of DNA;
  4. could show the new strands were all labelled with thymidine/thymine;
Examiners report

Asking students to outline Cairn’s technique led to a number of blank answers. That there was a role for autoradiography was better known than how this was used to obtain the length estimate.

Syllabus sections