IB Biology SL D1.1.2 Semi Conservative Replication Topic Practice

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

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

State the conclusion Meselson and Stahl drew from their experiment.

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