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The mutagen 5-bromouracil (5BU) was added to a rapidly dividing culture of wild-type E. coli cells growing in a liquid medium containing a rich variety of nutrients, including arginine. After one cell division, the cells were washed free of the mutagen, resuspended in sterile water, and plated onto master plates containing minimal medium supplemented only with arginine. Plates were obtained having well-separated colonies, so that each colony derived from just one progenitor cell. The colonies were then replica-plated from the master plates onto plates containing minimal medium. One colony that grew in the presence of arginine but failed to grow on minimal medium was selected from the master plate. The cells of this colony were suspended in sterile water, and each of 20 tubes containing minimal medium supplemented with arginine was inoculated with a few cells from this suspension. After the 20 cultures grew to a density of 108 cells/mL, 0.1 mL from each was plated on plates containing minimal medium. The following table shows the number of bacterial colonies that grew on each plate.a. In which stage(s) of this process did mutations occur? What is the evidence that a mutational event occurred?

b. At each stage where mutations occurred, were the mutations induced or spontaneous? Were they forward or reverse mutations?

c. At each stage where mutations were recovered, how were they selected for?

d. Though all of the 20 cultures started from a single colony that failed to grow on minimal medium were treated identically, they produced different numbers of bacterial colonies when they were plated. Why did this occur?

e. Suppose that 5BU had been added to the medium in the 20 tubes. Would plating the 20 cultures have given the same results? If not, how would they have differed?

f. Supposing that methylmethane sulfonate (MMS) rather than 5BU had been added to the medium in the 20 tubes, answer the questions given above in part (e).

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