You try to generate E. coli mutants by inducing pyrimidine dimers through UV exposure to bacteria on Petri plates. You confirm t
he formation of pyrimidine dimers because some cells are unable to replicate or undergo transcription. You find, however, when the plates containing the newly exposed bacteria are incubated in your new warm room (which is a big walk-in incubator), very few mutations are created. You are puzzled by this until you look closely at the large sunny window in your new walk-in incubator and you conclude that: A. an increase in SOS repair has occurred, which has reduced the number of mutations created.
B. the mismatch repair system has been repressed, and this has caused most of the cells to not survive and not form colonies on the plates.
C. base-excision repair has removed all the pyrimidine dimers from the bacteria so that mutations were not created.
D. nucleotide-excision repair has been repressed, which has allowed enhanced mismatch repair to remove most of the mutations from the DNA.
E. direct repair using photolyase has removed many of the pyrimidine dimers from the DNA in an error-free manner, so there are fewer lesions for SOS repair to process.
The correct answer is option E. "direct repair using photolyase has removed many of the pyrimidine dimers from the DNA in an error-free manner, so there are fewer lesions for SOS repair to process".
Explanation:
Photolyase is a type of enzyme that catalyses DNA repair from damage caused by exposure to ultraviolet light. Photolyase requires activation by exposure visible light, particularly from the violet end of the visible spectrum. If the Petri plates of the bacteria exposed to UV are located in an incubator that is closely at a large sunny windows, it is very likely that the bacteria is performing direct DNA repair using its photolyase enzymes.
