Define transcription unit

Explanation

**Step-by-Step Solution:**

1. **Definition of Transcription Unit**: A transcription unit is a specific segment of DNA that is transcribed into RNA during the process of transcription.

2. **Template Strand**: In the context of transcription, only one of the two strands of the DNA double helix is used as a template for RNA synthesis. This strand is referred to as the template strand.

3. **Components of the Transcription Unit**: The transcription unit consists of three main parts:
- **Promoter**: This is a region of DNA that initiates transcription. It is located upstream of the structural gene and is the binding site for RNA polymerase.
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