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

**Step-by-Step Solution:**

1. **Understanding the lac Operon**: The lac operon in E. coli is a set of genes that are involved in the metabolism of lactose. 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, which is continuously synthesized from the I gene (the regulatory gene). This repressor binds to the operator region of the operon, preventing RNA polymerase from transcribing the structural genes necessary for lactose metabolism.

3. **Inducer Presence**: The operon gets switched on in the presence of an inducer. The primary inducer in this case is lactose, or a derivative called allolactose. When lactose is present, it binds to the repressor protein.

4. **Inactivation of the Repressor**: The binding of lactose (or allolactose) to the repressor causes a conformational change in the repressor, inactivating it. This inactivation prevents the repressor from binding to the operator region.

5. **Transcription Activation**: With the repressor inactivated, RNA polymerase can now access the promoter region of the operon. This allows transcription of the structural genes (lacZ, lacY, and lacA) to proceed, leading to the production of enzymes necessary for lactose metabolism.

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

**Final Answer**: In E. coli, the lac operon gets switched on when lactose is present and binds to the repressor.

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