When operator of operon is off

Options

A. The structural genes cannot function
B. The inducer cannot function
C. The repressor protein cannot function
D. The repressor gene cannot function

Answer

### Step-by-Step Solution:

1. **Understanding the Operon Concept**: An operon is a cluster of genes under the control of a single promoter and operator. It includes structural genes, a promoter, and an operator. The operator is a regulatory sequence where a repressor protein can bind.

2. **Role of the Operator**: The operator acts as a switch that can turn the transcription of the structural genes on or off. When the operator is "off," it means that the repressor protein is bound to the operator, preventing RNA polymerase from binding to the promoter.

3. **Impact of the Repressor Binding**: When the repressor is bound to the operator, it physically blocks RNA polymerase from accessing the promoter. This means that the structural genes cannot be transcribed into mRNA, and therefore, cannot be translated into proteins.

4. **Evaluating the Options**:
- **Option A**: "The structural genes cannot function." - This is correct because if the operator is off (repressor bound), the structural genes are not transcribed.
- **Option B**: "The inducer cannot function." - This is not directly relevant to the operator being off; the inducer's role is to deactivate the repressor.
- **Option C**: "The repressor protein cannot function." - This is incorrect; the repressor is functioning by binding to the operator.
- **Option D**: "The repressor gene cannot function." - This is also incorrect; the repressor gene is separate and is not affected by the operator's state.

5. **Conclusion**: The correct answer is **Option A**: "The structural genes cannot function."