Discussion
In the interrupted mating experiment, the order in which genes enter the recipient indicates
Answer: D
Correct answer: their order along the donor chromosome
In an interrupted mating experiment an Hfr donor is mated with an F− recipient and the pairs are separated at intervals - classically in a kitchen blender. Testing which markers have arrived at each time reveals the order of the genes along the donor chromosome. The method works because Hfr transfer is sequential: it begins at the point where the F plasmid integrated and proceeds linearly, so a gene nearer that origin enters earlier. The resulting map is measured in minutes rather than in recombination frequency, and the whole E. coli chromosome takes about 100 minutes to transfer - which is why the standard genetic map of E. coli is a circle of 100 minutes. Different Hfr strains have the plasmid integrated at different sites and in either orientation, so each transfers a different starting point and direction. Combining several such maps was what established that the chromosome is circular. The technique is due to Wollman and Jacob in the 1950s.
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