In the lac operon, the structural genes are switched off when

Options

A. Repressor binds to operator
B. Repressor binds to promoter
C. Repressor binds to regulator
D. Repressor binds to allolactose

Answer

To solve the question regarding when the structural genes in the lac operon are switched off, we can analyze the options provided and understand the mechanism of the lac operon.

### Step-by-Step Solution:

1. **Understanding the Lac Operon**: The lac operon is a set of genes responsible for the metabolism of lactose in bacteria, primarily E. coli. It consists of structural genes (lacZ, lacY, and lacA), a promoter, an operator, and regulatory genes.

2. **Role of the Repressor**: The lac operon is regulated by a repressor protein. This repressor can bind to the operator region of the operon, which is located between the promoter and the structural genes.

3. **Analyzing the Options**:
- **Option A: Repressor binds to operator**: When the repressor binds to the operator, it physically blocks the RNA polymerase from transcribing the structural genes. This prevents the expression of the genes responsible for lactose metabolism, effectively switching them off.
- **Option B: Repressor binds to promoter**: This option is incorrect because the repressor does not bind to the promoter; instead, it binds to the operator to inhibit transcription.
- **Option C: Repressor binds to regulator**: This option is also incorrect. The regulator genes produce the repressor, but binding here does not switch off the structural genes.
- **Option D: Repressor binds to allolactose**: Allolactose is an inducer that binds to the repressor, causing it to change shape and release from the operator, allowing transcription to occur. Therefore, this option is incorrect as it promotes transcription rather than switching it off.

4. **Conclusion**: The correct answer is **Option A: Repressor binds to operator**. This binding prevents RNA polymerase from accessing the promoter and transcribing the structural genes, effectively switching them off.