The precursor of mRNA is transcribed by

Options

A. RNA polymerase I
B. RNA polymerase II
C. RNA polymerase III
D. Topoisomerase

Answer

To solve the question about which enzyme transcribes the precursor of mRNA, we can follow these steps:

### Step 1: Understand the Types of RNA Polymerases
In eukaryotic cells, there are three main types of RNA polymerases that are responsible for synthesizing different types of RNA:
- **RNA Polymerase I**: This enzyme is primarily responsible for synthesizing ribosomal RNA (rRNA).
- **RNA Polymerase II**: This enzyme is responsible for synthesizing precursor mRNA (also known as heterogeneous nuclear RNA or hnRNA).
- **RNA Polymerase III**: This enzyme synthesizes transfer RNA (tRNA), 5S rRNA, and small nuclear RNAs (snRNAs).

### Step 2: Identify the Precursor of mRNA
The precursor of mRNA is known as heterogeneous nuclear RNA (hnRNA). This is the initial transcript that undergoes processing to become mature mRNA.

### Step 3: Determine Which RNA Polymerase Synthesizes hnRNA
From our understanding of the roles of the different RNA polymerases:
- **RNA Polymerase II** is the enzyme that transcribes the precursor of mRNA (hnRNA).

### Conclusion
Therefore, the correct answer to the question is **RNA Polymerase II**.