Problems from Dean & Professor Stanley Maloy, SDSU.Additional problems on mutation rate:

What is the difference between mutation rate and mutant frequency? [Give a general description of what experiments you would need to do to determine the two values.] ANSWER: The mutation rate is the number of mutations per cell division. Because the cell population is so large, the number of cell divisions is approximately equal to the number of cells in the population (N). The value of h can be determined by a fluctuation test. The mutant frequency is simply the ratio of mutants / total cells in the population. This can be determined by simply plating out aliquots of a culture and counting the number of mutants that arise and the number of cells plated. The mutant frequency is much easier to measure but may show large fluctuations depending upon when the first mutation appeared in the population.

Briefly explain the expected results for the Luria-Delbruck fluctuation test if mutants arise: a. spontaneously in a population of cells before exposure to a selective agent;b. due to adaption after exposure to the selective agent. ANSWER: If the mutations arise spontaneously (as in "a") there will be wide fluctuation in mutants from cells grown in different tubes (i.e., independent samples) relative to cells grown in the same tube. That is, there will be great variation in the number of mutants in each individual tube relative to the mean number of mutants calculated from the sum of all of the tubes. In contrast, if the mutations arise by adaption (as in "b") there will be relatively little fluctuation in mutants from cells grown in different tubes (i.e., independent samples) relative to cells grown in the same tube. That is, in this case the number of mutants on each plate approximately equals the mean number of mutants.

Wild-type Salmonella typhimurium cells are killed by phage F0. A Luria-Delbruck fluctuation test was done to determine the rate of mutation to resistance to phage F0. Twenty tubes of medium were inoculated with S. typhimurium cells and the cultures were grown to 108 cells/ml. A 0.1 ml sample of each culture was then spread on plates with phage F0. The results are shown below. Using the equation for the Poisson distribution, determine the rate of mutation to F0R.