Repressible enzymes is

Options

A. Present all the time
B. Functional almost all the time
C. Repressed in presence of a specific chemical
D. All the above.

Answer

**Step-by-Step Solution:**

1. **Understanding Repressible Enzymes**: Repressible enzymes are a type of enzyme that can be inhibited or repressed in their activity or synthesis in response to the presence of a specific molecule.

2. **Characteristics of Repressible Enzymes**:
- They are not always present; their synthesis is regulated based on the needs of the cell.
- When the concentration of the end product of a metabolic pathway is high, the synthesis of these enzymes is reduced or stopped. This prevents the overproduction of the end product.

3. **Corepressor Role**: The end product that leads to the repression of these enzymes is known as a corepressor. When the corepressor is present, it binds to a repressor protein, which then attaches to the operator region of the gene, blocking transcription and thus enzyme synthesis.

4. **Regulation by Repressor Molecules**: The regulation of repressible enzymes involves a repressor molecule that is produced by a regulatory gene. This repressor can inhibit the synthesis of mRNA for the enzymes, effectively controlling their production.

5. **Analyzing the Options**:
- Option 1: Present all the time - This is incorrect as repressible enzymes are not always present.
- Option 2: Functional almost all the time - This is also incorrect since their function is regulated.
- Option 3: Repressed in the presence of a specific chemical - This is correct as it describes the behavior of repressible enzymes.
- Option 4: All of the above - This is incorrect because not all previous statements are true.

6. **Conclusion**: The correct answer is that repressible enzymes are repressed in the presence of a specific chemical, which corresponds to option 3.

**Final Answer**: The correct answer is option 3: Repressed in presence of a specific chemical.

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