[Campbell Biology P.387] The "WHAT IF?" section presents a hypothetical scenario: "Suppose t... | Practice Question

The "WHAT IF?" section presents a hypothetical scenario: "Suppose the experiment had shown that class I mutants could grow only in MM supplemented by ornithine or arginine and that class II mutants could grow in MM supplemented by citrulline, ornithine, or arginine." Based on this hypothetical result, what revised conclusion would the researchers draw regarding the biochemical pathway and the defect in class I and class II mutants?

  • A: The defect in Class II mutants is in Enzyme A, and for Class I, ornithine can be converted to arginine via an alternative pathway that bypasses citrulline.
  • B: Both Class I and Class II mutants have a defect in Enzyme A, which implies the experiment contradicts the "one gene-one enzyme" hypothesis.
  • C: The defect in Class I mutants is in Enzyme B, and the defect in Class II mutants is in Enzyme C.
  • D: Citrulline is no longer considered an intermediate in the arginine biosynthesis pathway for any of the Neurospora mutants.

Explanation

In the hypothetical scenario, Class II mutants grow on MM supplemented by citrulline, ornithine, or arginine. This means the block must be before ornithine, making Enzyme A defective as ornithine is the first intermediate that allows growth . For Class I mutants, the key is they grow on ornithine and arginine, but not citrulline. If they can utilize ornithine, their defect is at Enzyme A. However, their inability to grow on citrulline while still growing on arginine implies that citrulline is not a direct or essential intermediate in the pathway from ornithine to arginine for these specific mutants, or that ornithine can be converted to arginine via an alternative pathway that bypasses citrulline.