Initiation of transcription is by

Options

A. Only `sigma` (sigma) factor
B. Only RNA polymerase
C. Only `rho` (rho) factor
D. RNA polymerase when associated with `sigma` factor

Answer

To solve the question "Initiation of transcription is by," we can break down the process of transcription and identify the key components involved in its initiation.

### Step-by-Step Solution:

1. **Understanding Transcription**:
- Transcription is the process by which the genetic information in DNA is copied into RNA. This process is crucial for gene expression.

2. **Key Enzyme**:
- The main enzyme involved in transcription is **RNA polymerase**. It synthesizes RNA from the DNA template.

3. **Role of Sigma Factor**:
- During the initiation phase of transcription, RNA polymerase requires a protein called the **sigma factor**. The sigma factor helps RNA polymerase to recognize the promoter region of the gene, which is the starting point for transcription.

4. **Formation of the RNA Polymerase-Sigma Complex**:
- The sigma factor binds to RNA polymerase, forming a complex that can then attach to the promoter region of the DNA. This complex is essential for the initiation of transcription.

5. **Initiation Process**:
- Once the RNA polymerase-sigma factor complex binds to the promoter, RNA polymerase unwinds the DNA and begins synthesizing RNA. This marks the beginning of the transcription process.

6. **Conclusion**:
- Therefore, the initiation of transcription is primarily facilitated by the **RNA polymerase when associated with the sigma factor**.

### Final Answer:
The initiation of transcription is by **RNA polymerase when associated with sigma factor**.

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