Q25 - The Luria–Delbrück Replica Plating Experiment

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Genetics and Evolution

A test tube contains a suspension of bacteria that were never exposed to the antibiotic kanamycin. The bacteria were spread onto a so-called master plate, which contains nutrients for the bacteria and does not contain kanamycin. Millions of colonies (descendants of a single founder bacterium) developed on the master plate. Replicas (copies) of the master plate colonies were stamped - with a sterile velvet plate - onto three replica plates that contained growth medium supplemented with kanamycin.

Diagram showing a cylindrical velvet-covered stamp being pressed onto a “Master plate” (a petri dish with several colonies marked as dots), then the same stamp transferring an identical pattern of colonies onto three separate “Replica plates” (each also petri dishes containing kanamycin). Fig.1. Legend 1 = Master plate. Legend 2 = Replica plates.

Indicate with an X if each of the following statements are true (T) or false (F).

(Note: the original Q.25.6 was redacted by IBO itself in the official materials and is omitted here.)

Q25.1. A spontaneous frameshift mutation caused the kanamycin resistance in bacteria which previously didn't contain the resistance gene.

Q25.2. The resistance of certain colonies is uncovered on the Replica plates.

Q25.3. A mutation induced by the kanamycin caused the resistance.

Q25.4. The kanamycin resistance occurred first on the 'Replica plates'.

Q25.5. The best explanation is that kanamycin has a mild mutagenic effect on these bacteria.

Q25.7. There were no kanamycin resistant bacteria in the tube originally.

Q25.8. The kanamycin resistance can spread on the Master Plate.

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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key