The lac operon consists of a regulation gene and three structural gene. The lactose acts as inducer. In the presence of an Inducer such as lactose, the repressor is in inactivated during the interaction. This allows RNA polymerase access to the promoter and transcription proceeds. The repressor is synthesized which in turn binds with the operator region of the operon and prevents RNA polymerase from transcribing the operon.
When the process of Lac operon is blocked by a repressor it represents:

Options

A. Positive regulation
B. Negative regulation
C. sometimes positive sometimes negative
D. both positive and negative regulation

Explanation

To answer the question regarding the lac operon and its regulation by a repressor, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding the Lac Operon Structure**:
- The lac operon consists of a regulatory gene (which produces the repressor) and three structural genes (lacZ, lacY, and lacA) that encode proteins necessary for lactose metabolism.

2. **Role of Lactose as an Inducer**:
- Lactose acts as an inducer in the lac operon system. When lactose is present, it binds to the repressor protein, causing a conformational change that inactivates the repressor.

3. **Repressor Inactivation**:
- In the presence of lactose, the repressor is inactivated. This means it cannot bind to the operator region of the operon, allowing RNA polymerase to access the promoter region.

4. **Transcription Activation**:
- With the repressor inactivated, RNA polymerase can bind to the promoter and initiate transcription of the structural genes (lacZ, lacY, and lacA), leading to the production of enzymes that help in the metabolism of lactose.

5. **Repressor Binding and Blocking**:
- When lactose is absent, the repressor is active and binds to the operator region. This binding blocks RNA polymerase from transcribing the operon, effectively preventing the synthesis of the enzymes needed for lactose metabolism.

6. **Conclusion on Regulation**:
- When the lac operon is blocked by the repressor (due to the absence of lactose), it represents negative regulation of the operon. Thus, the correct answer to the question is that the blocking of the lac operon by the repressor indicates negative regulation.

### Final Answer:
When the process of the lac operon is blocked by a repressor, it represents **negative regulation**.

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