33. Three E. coli mutants were isolated which require compound ‘A’ for their growth. The compounds B, C and D are known to be involved in biosynthetic pathway to A. In order to determine pathway, the mutants were grown in a minimal medium supplemented with ONE OF THE COMPOUNDS, A TO D. The results obtained are summarized below:
‘+’ = Growth on medium
‘O’ = No growth
Which of the following equation represents the biosynthetic pathway of A?
(1) BàCàDàA (2) CàDàBàA
(3) BàDàCàA (4) AàCàDàB
The correct biosynthetic pathway is B → C → D → A (Option 1). This order best explains which mutant grows on which supplemented compound.
Question restatement
Three E. coli auxotrophic mutants (1, 2, 3) all require end‑product A for growth. Compounds B, C, and D are intermediates in the biosynthetic pathway leading to A. Each mutant is grown on minimal medium supplemented with only one compound (A, B, C, or D). Growth pattern:
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Mutant 1: grows only on A
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Mutant 2: grows on A and D
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Mutant 3: grows on A, C, and D
‘+’ = growth; ‘0’ = no growth.
Step‑by‑step pathway deduction
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Interpretation of non‑growth
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If a mutant does not grow on a compound, its block lies after that compound in the pathway because it cannot convert that compound into the downstream products.
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Use Mutant 1 (grows only on A)
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No growth on B, C, or D means this mutant is blocked after the earliest of B, C, D, i.e., at the last step just before A.
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Therefore mutant 1 has a defect in the enzyme converting the immediate precursor of A into A.
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Use Mutant 2 (grows on D, not on B or C)
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Growth on D means its block is after D.
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No growth on B or C means the block is downstream of both, so B and C must come before D in the pathway.
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Use Mutant 3 (grows on C and D, not on B)
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Growth on C shows its block is after C.
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Growth on D shows its block is after D as well.
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No growth on B means the block is downstream of B, so B must be before C (and thus before D).
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Combine constraints
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From mutant 2: B and C are before D.
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From mutant 3: B is before C, and C before D.
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From mutant 1: D is just before A.
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Final order that fits all data: B → C → D → A.
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Explanation of each option
Option (1) B → C → D → A ✅ Correct
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Mutant 1: blocked at D → A step
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B, C, D all lie before the block, so none can rescue; only A rescues.
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Mutant 2: blocked at some step after D
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D (and A) rescue; B and C are upstream and cannot bypass the downstream block.
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Mutant 3: blocked between B and C
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C and D (and A) rescue because they are downstream of the block; B does not.
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This matches exactly the given growth pattern, so Option 1 is correct.
Option (2) C → D → B → A ❌ Incorrect
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If C is first, any block after C should still allow growth on C, which is not true for mutant 1.
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Mutant 3 grows on C and D but not B; in this order, B is just before A, so a block after B would prevent growth on D, which contradicts the data.
Option (3) B → D → C → A ❌ Incorrect
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Here D comes directly after B.
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Mutant 2 grows on D but not on C; if C were after D, a block after D should not be rescued by D alone.
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Mutant 3, which grows on C and D but not B, cannot be explained because placing C last would mean growth on C but not D for a block after C.
Option (4) A → C → D → B ❌ Incorrect
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This pathway wrongly places A at the beginning, though A is known to be the end product required for growth.
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Any mutant should then grow on A and also potentially be rescued by downstream compounds, which contradicts the clear behavior of all three mutants.
Introduction
Understanding how to deduce a biosynthetic pathway to compound A using E. coli mutants and intermediates B, C and D is a common and high‑yield problem in CSIR NET life sciences and other genetic analysis exams. By analysing which mutant grows on which supplemented compound, the correct order B → C → D → A can be derived logically, and each alternative pathway can be systematically rejected.
