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← Theoretical 1

Q5 — Meselson–Stahl Experiment — DNA Replication Modes

Theoretical 1 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
DNA Structure & Replication

To find out how DNA replicates, Matthew Meselson and Franklin Stahl performed the following classical experiment. Bacteria were grown in a medium with heavy nitrogen (¹⁵N) until all the DNA was labeled. Then they were grown in a normal nitrogen-containing culture medium (¹⁴N) for 5 generations. The percentage of heavy nitrogen remaining in the DNA was tracked across these 5 generations, under four candidate curves (A–D) representing different hypothetical replication modes.

Percentage of 15N in DNA vs. generation, for four candidate curves A–D. Percentage of ¹⁵N in DNA vs. generation. Curve A rises from 0% toward ~83%. Curve B stays flat at 50% throughout. Curve C starts at 100% and halves each generation, reaching ~3% by generation 5. Curve D starts at 100% and drops to ~0% by generation 3.

Using the information and figure, determine whether the following statements are true or false:

A. Curve A corresponds to semi-conservative DNA replication.
B. Curve B corresponds to dispersive DNA replication.
C. Curve C corresponds to semi-conservative DNA replication.
D. Curve D corresponds to conservative DNA replication.
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Question reproduced from IBO 2022, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)