A strain of Neurospora is able to grow on minimal medium.When substance A is added to the medium, however, the strain cannot grow.Supplying a small amount of substance B has no effect on the strain, but it cannot reverse the effect of substance A.Supplying a small amount of substance C, by contrast, enables the strain to grow with substance A.Which of the following statements best explains these facts?
A) Substance A is a toxin that the strain cannot break down, and substance B is an enzyme that breaks down substance A.
B) Substance A is a toxin that the strain cannot break down, and substance C is an enzyme that breaks down substance A.
C) The strain has the capacity to produce substance C.
D) Substance A is a mutagen.
E) Substance C is a mutagen.

Question

Quizplus · Accepted Answer

Substance A is a toxin that the strain cannot break down, and substance C is an enzyme that breaks down substance A. Substance B has no effect on the strain and cannot reverse the effect of substance A, indicating that it is not an enzyme that breaks down substance A. Therefore, option B is the best choice. The strain's ability to grow with substance A in the presence of substance C suggests that substance C is an enzyme that can break down substance A. Option A is incorrect because substance B is not an enzyme that breaks down substance A. Option C may be plausible, but it is not supported by the information given in the question. Options D and E are incorrect because the question does not provide any indication that either substance A or C is a mutagen.

A Strain of Neurospora Is Able to Grow on Minimal