Suppose that a certain wild-type bacteria can synthesize substance D, but various mutant strains cannot.We know that substance D is synthesized from substance X in a pathway that involves three intermediate substances (A, B, and C), but we do not know the order of the steps in the pathway.The table shows four different mutant strains that have been tested for their ability to grow on the various substances.The "+" means that the strain can grow on that substance.The "‒" means that the strain cannot grow on that substance. If another strain (say, strain 5) grows on substance B but does not grow on substance C, we can infer that it
A) can grow on substance A.
B) cannot grow on substance A.
C) has a defect in the enzyme that converts substance B into substance C.
D) cannot grow on substance D.
E) has a defect in the enzyme that converts substance D into substance B.

Question

Suppose that a certain wild-type bacteria can synthesize substance D, but various mutant strains cannot.We know that substance D is synthesized from substance X in a pathway that involves three intermediate substances (A, B, and C), but we do not know the order of the steps in the pathway.The table shows four different mutant strains that have been tested for their ability to grow on the various substances.The "+" means that the strain can grow on that substance.The "‒" means that the strain cannot grow on that substance.  If another strain (say, strain 5) grows on substance B but does not grow on substance C, we can infer that it
A) can grow on substance A. 
B) cannot grow on substance A. 
C) has a defect in the enzyme that converts substance B into substance C. 
D) cannot grow on substance D. 
E) has a defect in the enzyme that converts substance D into substance B.

Quizplus · Accepted Answer

If strain 5 can grow on substance B but cannot grow on substance C, it indicates that the pathway proceeds as follows:
X -> A -> B -> C -> D
Since strain 5 can grow on substance B, it can synthesize substance C from B. But since it cannot grow on substance C, it cannot synthesize substance D from C. Therefore, it must be able to grow on substance A in order to synthesize substance C from A, as it cannot do so from C. Therefore, option A is the best choice.

Suppose That a Certain Wild-Type Bacteria Can Synthesize Substance D