The RNA polymerase holoenzyme transcribes

Options

A. the promoter, structural gene and the terminator region
B. the structural gene and the terminator regions
C. the structural gene and the terminator regions
D. the structural gene only

Explanation

### Step-by-Step Solution:

1. **Understanding Transcription**:
- Transcription is the process of copying genetic information from DNA to RNA. The enzyme responsible for this process is RNA polymerase.

2. **Identifying the Components of a Transcription Unit**:
- A transcription unit consists of three main parts: the promoter, the structural gene, and the terminator region.
- The promoter is the region where RNA polymerase binds to initiate transcription.
- The structural gene is the segment of DNA that is transcribed into RNA.
- The terminator region signals the end of transcription.

3. **Role of RNA Polymerase**:
- RNA polymerase binds to the promoter region to start transcription.
- It synthesizes RNA by transcribing only the structural gene, which lies between the promoter and the terminator.

4. **Analyzing the Options**:
- Option A: Promoter, structural gene, and terminator region - Incorrect, as the promoter and terminator are not transcribed.
- Option B: Structural gene and terminator regions - Incorrect, as the terminator is not transcribed.
- Option C: Structural gene and terminator region - Incorrect, as only the structural gene is transcribed.
- Option D: Structural gene only - Correct, as RNA polymerase transcribes only the structural gene.

5. **Conclusion**:
- The correct answer is that RNA polymerase transcribes the structural gene only.

### Final Answer:
- The RNA polymerase holoenzyme transcribes the **structural gene only** (Option D).

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