In which of the following genotypes of lac operon the enzyme synthesis would be constitutive?
A. $I^+ O^+ Z^+ Y^+ / I^+ O^+ Z^+ Y^+$
B. $I^+ O^c Z^+ Y^+ / I^+ O^c Z^+ Y^+$
C. $I^- O^+ Z^+ Y^+ / I^+ O^+ Z^+ Y^+$
D. $I^- O^+ Z^+ Y^+ / I^- O^+ Z^+ Y^+$
A. $I^+ O^+ Z^+ Y^+ / I^+ O^+ Z^+ Y^+$
B. $I^+ O^c Z^+ Y^+ / I^+ O^c Z^+ Y^+$
C. $I^- O^+ Z^+ Y^+ / I^+ O^+ Z^+ Y^+$
D. $I^- O^+ Z^+ Y^+ / I^- O^+ Z^+ Y^+$
Show Hint
- A and B Only
- B and C Only
- B and D Only
- C and D Only
The Correct Option is C
Solution and Explanation
Step 1: Concept
Constitutive expression means the operon is always "on," regardless of whether the inducer (lactose) is present.
Step 2: Meaning
This occurs when the repressor cannot bind (mutant Operator, $O^c$) or the repressor is not made/functional (mutant Gene, $I^-$).
Step 3: Analysis
- Genotype B ($O^c$): The operator is constitutive ($O^c$). The repressor cannot bind, so expression is always on. - Genotype D ($I^-$): Both alleles have mutant repressors ($I^-$). With no functional repressor present to block the operator, expression is constitutive. In case C, the functional $I^+$ from one strand can produce repressor that works "in trans" to regulate both strands, so it is not constitutive.
Step 4: Conclusion
Genotypes B and D result in permanent loss of repression. Final Answer: (C)
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