The URA3 gene is required to make uracil in yeast. A geneticist obtained a yeast strain from another lab for an experiment and discovered it cannot grow on medium lacking uracil due to a mutation in the ura3 gene. She sequenced the URA3 and ura3 alleles and discovered a single base pair substitution in the ura3 allele (shown below),
URA3: 5'...AATGATGGA...3' ura3: 5'...AATGCTGGA...3 3'...TTACTACCT...5' 3'...TTACGACCT...5'
To use the strain, she needs it to be able to make its own uracil, so she decided to revert the ura3 allele back to URA3. To increase the frequency of reversion, she treated the ura3 cells with a mutagen, and then plated the mutagenized cells onto medium lacking uracil to select for colonies that could grow. Assume that only one base pair substitution occurs in each revertant. The mutagens she had on hand were:
Type of mutagen Mode of action
hydroxylamine Modifies C so that it pairs with A
X-rays Makes double strand breaks
Nitrous acid Modifies A so that it pairs with C
Oxidizer Modifies G so that it pairs with A
Based on this information select the statement below that is TRUE.
A. The mutagen MOST LIKELY to revert the ura3 mutant to URA3 is hydroxylamine.
B. The mutagen MOST LIKELY to revert the ura3 mutant to URA3 are X-rays.
C. The mutagen MOST LIKELY to revert the ura3 mutant to URA3 is nitrous acid.
D. The mutagen MOST LIKELY to revert the ura3 mutant to URA3 is the oxidizer.