In E. coli, the lac operon gets switched on when

Options

A. lactose is present and it binds to the repressor
B. repressor binds to operator
C. RNA polymerase binds to the operator
D. lactose is present and it binds to RNA polymerase

Explanation

To answer the question "In E. coli, the lac operon gets switched on when," we will analyze the components and functioning of the lac operon step by step.

### Step-by-Step Solution:

1. **Understanding the Lac Operon**:
The lac operon in E. coli is a classic example of gene regulation. It consists of structural genes (Z, Y, A), a promoter, an operator, and a regulator gene. The structural genes code for enzymes that are involved in the metabolism of lactose.

2. **Role of the Repressor**:
The regulator gene produces a repressor protein that normally binds to the operator region of the operon. When the repressor is bound to the operator, RNA polymerase cannot transcribe the structural genes, keeping the operon "switched off."

3. **Inducer Function**:
The lac operon is an inducible operon, meaning it can be turned on in response to certain signals. The presence of lactose serves as an inducer. When lactose is available, it binds to the repressor protein.

4. **Formation of Inducer-Repressor Complex**:
When lactose binds to the repressor, it causes a conformational change in the repressor protein. This change prevents the repressor from binding to the operator, effectively removing the block for RNA polymerase.

5. **Activation of Transcription**:
With the repressor no longer bound to the operator, RNA polymerase can now bind to the promoter and transcribe the structural genes (Z, Y, A). This leads to the production of enzymes that metabolize lactose.

6. **Conclusion**:
Therefore, the lac operon gets switched on when lactose is present and it binds to the repressor, allowing transcription to occur.

### Final Answer:
The lac operon in E. coli gets switched on when lactose is present and it binds to the repressor.

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