What fraction of the population was killed per second without photoreactivation? Show your plot of survivors per mL vs UV dose and indicate how you determined this number. 2. What fraction of the population was killed per second with photoreactivation? Did photoreactivation affect the UV killing? Was your survival curve linear? Would you expect it to be linear? 3. Can you determine from your graph, how long it would take to reduce the population of Serratia marcescens to zero? If not, how could you determine a practical time for sterilization of the surface of a plate under the conditions we used? 4. What is the major cause of DNA damage by UV light? Describe three mechanisms for repair of DNA damage by UV light 5. How could you vary the UV dose without changing the time of irradiation? 6. What properties would you expect for a mutant deficient in photoreactivation compared to a mutant deficient in all of the UV repair systems? (Hint: What is unique about photoreactivation?) 7. Serratia marcescens makes “brick” red colored colonies when incubated at room temperature, but sometimes rare colonies appear white colored.

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