Genes that are involved in turning on or off the transcription of set of structural genes are called Or Functioning of structural genes is controlled by

Options

A. polymorphic genes
B. operator genes
C. reductant genes
D. regulatory genes

Explanation

To answer the question "Genes that are involved in turning on or off the transcription of a set of structural genes are called" or "Functioning of structural genes is controlled by," we need to understand the concept of operons and their components.

### Step-by-Step Solution:

1. **Understanding Operons**:
- An operon is a cluster of genes that are regulated together. It typically includes structural genes, a promoter, an operator, and regulatory genes.

2. **Components of an Operon**:
- **Structural Genes**: These are the genes that code for proteins. They are responsible for the metabolic functions of the cell.
- **Promoter**: This is the region where RNA polymerase binds to initiate transcription of the structural genes.
- **Operator**: This is a segment of DNA that acts as a regulatory switch. It can either allow or block the transcription of the structural genes based on the presence of certain proteins.

3. **Regulatory Genes**:
- These genes produce regulatory proteins (activators or repressors) that interact with the operator. The regulatory protein can either promote (activate) or inhibit (repress) the transcription of the structural genes.

4. **Example: Lac Operon**:
- In the lac operon, the presence of lactose leads to the inactivation of a repressor protein, allowing RNA polymerase to transcribe the structural genes (Z, Y, A) necessary for lactose metabolism.
- If lactose is absent, the repressor binds to the operator, preventing transcription of the structural genes.

5. **Conclusion**:
- Therefore, the genes that are involved in turning on or off the transcription of a set of structural genes are known as **regulatory genes**. The functioning of structural genes is controlled by the **operator**.

### Final Answer:
- The genes that are involved in turning on or off the transcription of a set of structural genes are called **regulatory genes**. The functioning of structural genes is controlled by the **operator**.