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IB Biology SL 1.1.2 Semi-conservative replication Question Bank

Practise IB Biology SL 1.1.2 by explaining semi-conservative DNA replication and interpreting isotope evidence.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam points

  • Distinguish semi-conservative replication from conservative replication using strand composition.
  • Predict the DNA density pattern after one or more replication rounds in isotope experiments.
  • Explain how complementary base pairing conserves the sequence during replication.

D1.1.2—Semi-conservative replication question 1

[Maximum number: 1]

Meselson and Stahl grew bacteria in a medium with 15 N{ }^{15} \mathrm{~N} (heavy nitrogen) so that all bacteria in the parental culture contain heavy DNA. They then transferred some of the bacteria to a medium with 14 N{ }^{14} \mathrm{~N} (light nitrogen).

Samples were taken at 0,20 and 40 minutes and centrifuged with the results as shown.

Figure for Question D1.1.2—Semi-conservative replication question 1 — IB Biology SL

State the conclusion Meselson and Stahl drew from their experiment.

Up to one additional mark is available for the construction of your answer.

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