Which is true for repressible operon :-

Options

A. Off `underset("Inducer")to` on
B. Inactive repressor + Co-repressor = active repressor
C. Active repressor + Inducer = inactive repressor
D. On `overset("Inducer")to` off

Answer

### Step-by-Step Solution:

1. **Definition of Repressible Operon**: A repressible operon is a type of operon that is typically in the "off" state. This means that the genes within the operon are not being expressed or transcribed into mRNA.

2. **Role of the Regulator Gene**: The I gene (regulator gene) produces a repressor protein. This repressor is crucial for controlling the operon’s activity.

3. **Binding of the Repressor**: The repressor protein binds to the operator region of the operon. When the repressor is bound to the operator, it prevents RNA polymerase from transcribing the downstream genes.

4. **Induction by Lactose**: When lactose is present in the environment, it acts as an inducer. The lactose binds to the repressor protein, causing a conformational change that prevents the repressor from binding to the operator.

5. **Activation of the Operon**: Once the repressor is no longer bound to the operator, RNA polymerase can access the promoter and transcribe the genes Z, Y, and A. These genes are responsible for the metabolism of lactose.

6. **Function of the Genes**:
- Gene Z codes for beta-galactosidase, which breaks down lactose into glucose and galactose.
- Gene Y codes for lac permease, which facilitates the transport of lactose into the cell.
- Gene A codes for transacetylase, which is involved in the metabolism of lactose derivatives.

7. **Return to Off State**: When lactose is consumed and no longer present, the repressor can again bind to the operator, turning the operon back to the "off" state.

### Conclusion:
The true statement regarding a repressible operon is that it is typically in the "off" position until an inducer (like lactose) is present, which allows for the expression of the operon.

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