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an Auxotrophic E

question 24

Multiple Choice

Use the following to answer questions
An auxotrophic E. coli strain requires adenine to grow because of a mutation in a gene for an adenine synthesis enzyme. The following shows part of the wild-type and mutant alleles of the gene, including the start codon. The bottom strand is the template for transcription. $$\begin{array} { c c } \text { ade } 1 ^ { + } \text {wild-type allele } & \text { ade1 } ^ { - } \text {mutant allele } \\\text { 5'...TTATGGGCAAGATCCCA...3' } & \text { 5'....TTA TGGGCTAGATCCCA...3' } \\\text { 3'...AATACCCGTTCTAGGGT...5' } & \text { 3'...AATATGGGCTAGATCCCA...3' }\end{array}$$
-E. coli strains that have both the original ade1^- mutant allele shown above and a mutant allele for a gene that encodes a lysine tRNA are able to make some normal ade1 enzyme. The mutant lysine tRNA allele makes a tRNA that has one base that is different from the wild-type tRNA. Why might the mutant lysine tRNA allele affect overall cell growth?

• A) The mutant tRNA^lysine will occasionally cause translation to continue beyond the normal stop codon on mRNAs that normally use UAG as a stop codon.
• B) The premature stop codon from the ade1^- allele will always cause shortened proteins.
• C) The mutant tRNA will prevent the translational machinery from recognizing the normal stop codon on the mRNA and so will result in incorrect proteins.
• D) The mutant tRNA^lysine will recognize the stop codon on the mutant ade1^- allele and cause the protein to be terminated.
• E) The mutant tRNA^lysine will incorporate a lysine at every UAG stop codon.

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