Repressor of lac-operon

Options

A. Is a tetrameric protein
B. Having a molecular weight of 16,000
C. Has only one side
D. Is made by operator gene

Answer

**Step-by-Step Solution:**

1. **Understanding the Lac Operon**: The lac operon is a set of genes in bacteria that are responsible for the metabolism of lactose. It is regulated by a repressor protein that inhibits gene expression when lactose is not present.

2. **Identify the Repressor**: The repressor of the lac operon is a protein that binds to the operator region of the operon, preventing transcription of the downstream genes that are necessary for lactose metabolism.

3. **Structure of the Repressor**: The lac repressor is a homotetrameric protein, meaning it is composed of four identical subunits (monomers). This structure allows it to effectively bind to the DNA.

4. **Molecular Weight**: The molecular weight of the lac repressor is approximately 15154-1520 grams per mole. This is important to note as it helps in identifying the protein correctly.

5. **Binding Mechanism**: When lactose is absent, the lac repressor binds to the operator region, blocking RNA polymerase from transcribing the genes needed for lactose metabolism. When lactose is present, it is converted to allolactose, which binds to the repressor, causing it to change shape and release from the operator, allowing transcription to occur.

6. **Conclusion**: The correct characterization of the lac repressor is that it is a tetrameric protein that inhibits gene expression in the absence of lactose.