The promoter site and the terminator site for transcription are located at

Options

A. 3' (downstream) end and 5' (upstream) end, respectively of the transcription unit
B. 5' (upstream) end and 3' (downstream ) end, respectively of the transcription unit
C. the 5' (upstream) end
D. the 3' (downstream) end

Answer

To solve the question about the locations of the promoter site and the terminator site for transcription, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the transcription unit**: The transcription unit consists of the DNA segment that is transcribed into RNA. It includes the promoter, the coding region, and the terminator.

2. **Identify the promoter site**: The promoter is a specific region of DNA that initiates transcription. It is located upstream of the transcription start site. This means it is found before (or to the left of) the start of the coding sequence when looking at the DNA strand in the 5' to 3' direction.

3. **Identify the terminator site**: The terminator is the sequence that signals the end of transcription. It is located downstream of the transcription unit, meaning it is found after (or to the right of) the coding sequence.

4. **Determine the positions**:
- The promoter is located at the 5' end (upstream) of the transcription unit.
- The terminator is located at the 3' end (downstream) of the transcription unit.

5. **Select the correct option**: Based on the above understanding, the correct answer is that the promoter site is at the 5' upstream and the terminator site is at the 3' downstream of the transcription unit. Therefore, the correct option is **B**.

### Final Answer:
The promoter site and the terminator site for transcription are located at **5' upstream and 3' downstream respectively of the transcription unit**.

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