A Neurospora nonsense mutation known to be UAG is treated with two compounds,hydroxylamine and 5-bromouracil.Hydroxylamine (HA)causes G·C → A·T transitions,while 5-bromouracil causes T·A → C·G transitions.Which of these agents do you expect to induce revertants?
A)Both agents are expected to induce revertants.
B)Both agents are expected to induce revertants,but hydroxylamine will be more effective.
C)5-bromouracil only is expected to induce revertants.
D)Hydroxylamine only is expected to induce revertants.
E)Neither hydroxylamine nor 5-bromouracil agents are expected to induce revertants.

Question

Quizplus · Accepted Answer

The given nonsense mutation UAG can be reverted to a sense mutation by changing it to a codon that specifies an amino acid. Since UAG is a stop codon, any nucleotide substitution that changes it to a sense codon is expected to induce revertants. Among the two compounds given, 5-bromouracil (5BU) is expected to induce revertants as it causes T·A → C·G transitions. This means that a UAG nonsense codon can be changed to CAG, which codes for glutamine (Q). On the other hand, hydroxylamine causes G·C → A·T transitions, which can change a GAG codon (coding for glutamic acid, E) to AAG, but it cannot change the UAG nonsense codon into a sense codon. Therefore, choice C (5-bromouracil only is expected to induce revertants) is correct.

A Neurospora Nonsense Mutation Known to Be UAG Is Treated