Fifteen bacterial colonies growing on a complete medium are transferred to a minimal medium. Twelve of the colonies grow on minimal medium.

The serine biosynthetic pathway is a three-step pathway in which each step is catalyzed by the enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.

Mutant 1 grows only on  In addition to growth on , mutant 2 also grows on  and . Mutant 3 grows on  and . Identify the step of the serine biosynthesis pathway at which each mutant is defective.

Verified step by step guidance

1. Identify the compounds that each mutant can grow on, as these will help determine which step in the pathway is defective.
2. For Mutant 1, since it grows only on a specific compound, it indicates a defect in the first step catalyzed by Enzyme A, as it cannot convert 3-Phosphoglycerate to 3-Phospho-hydroxypyruvate.
3. For Mutant 2, since it grows on two compounds, it suggests a defect in the second step catalyzed by Enzyme B, as it cannot convert 3-Phospho-hydroxypyruvate to 3-Phosphoserine.
4. For Mutant 3, since it grows on two compounds, it indicates a defect in the third step catalyzed by Enzyme C, as it cannot convert 3-Phosphoserine to Serine.
5. Match each mutant to the corresponding enzyme defect based on the growth patterns on the specific compounds.