62. Cytoplasmic extract from the wild type strain of a bacterium has the ability to
convert a colorless substrate (S) to a colored product
(P) via three colorless intermediates
X, Y and Z, in that order.
Each step of the pathway involves a specific enzyme coded by a distinct gene.
Four mutant strains (a−, b−, c−,
d−) were isolated, whose extracts are incapable of producing
the colored product in the presence of S.
In a series of experiments, extracts from the individual mutants were incubated
with X, Y or Z and scored for
color development. The data are summarized below
(Yes: color developed, No: no color developed).
| Mutants |
Compounds |
| X |
Y |
Z |
| a− |
No |
No |
No |
| b− |
No |
Yes |
Yes |
| c− |
Yes |
Yes |
Yes |
| d− |
No |
No |
Yes |
Based on the data, which one of the following is the correct order of enzymes
involved in the pathway?
(A) S → d → X → c → Y → b → Z → a → P
(B) S → a → X → d → Y → b → Z → c → P
(C) S → b → X → a → Y → c → Z → d → P
(D) S → c → X → b → Y → d → Z → a → P
Correct Answer: (B) — S → a → X → d → Y → b → Z → c → P
Introduction (SEO Optimized)
Understanding metabolic pathways using mutant analysis is a fundamental concept in biochemistry and molecular biology, commonly tested in exams such as CSIR-NET, GATE, JAM, and IIT-JAM Biotechnology/Life Sciences.
This question focuses on identifying the correct order of enzymes in a biochemical pathway by interpreting accumulation and conversion of intermediates in mutant bacterial strains.
By carefully analyzing which intermediates lead to color development in each mutant, we can reconstruct the enzyme sequence involved in converting substrate S into product P.
Problem Summary
-
Wild-type bacteria convert:
S → X → Y → Z → P
-
Each step is catalyzed by a different enzyme encoded by genes a, b, c, d
-
Four mutant strains (a⁻, b⁻, c⁻, d⁻) lack one enzyme each
-
Cell extracts from each mutant were incubated with X, Y, Z
-
Color development (Yes) indicates the compound can still be converted to P
-
No color means the pathway is blocked
Experimental Data Interpretation
| Mutant |
X |
Y |
Z |
| a⁻ |
No |
No |
No |
| b⁻ |
No |
Yes |
Yes |
| c⁻ |
Yes |
Yes |
Yes |
| d⁻ |
No |
No |
Yes |
Step-by-Step Reasoning
🔹 Mutant a⁻
🔹 Mutant b⁻
🔹 Mutant c⁻
🔹 Mutant d⁻
Correct Enzyme Order
Putting all deductions together:
✔️ Correct Option: (B)
Explanation of Incorrect Options
(A) S → d → X → c → Y → b → Z → a → P
(C) S → b → X → a → Y → c → Z → d → P
(D) S → c → X → b → Y → d → Z → a → P
🏁 Final Conclusion
By analyzing which intermediates lead to color development in each mutant strain, we can accurately deduce the order of enzymes in the pathway.
Only Option (B) satisfies all experimental conditions.
Correct Answer: (B)
