The enzyme that is used to make the first strand of cDNA from mRNA is

Options

A. Restriction endonuclease
B. Reverse transcriptase
C. DNA polymerase
D. `T_(4)` DNA ligase

Answer

To answer the question about the enzyme used to make the first strand of cDNA from mRNA, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding cDNA**: cDNA stands for complementary DNA. It is synthesized from a messenger RNA (mRNA) template. This process is called reverse transcription, which is the opposite of transcription where DNA is used to synthesize RNA.

2. **Identifying the Enzyme**: The key enzyme responsible for synthesizing cDNA from mRNA is called reverse transcriptase. This enzyme catalyzes the conversion of RNA into DNA.

3. **Source of Reverse Transcriptase**: Reverse transcriptase is typically found in retroviruses. These viruses can convert their RNA genome into DNA upon infecting a host cell, which is a crucial step in their replication cycle.

4. **Other Enzymes**: It is important to differentiate reverse transcriptase from other enzymes:
- **Restriction Endonuclease**: These enzymes cut DNA at specific sequences but do not synthesize cDNA.
- **DNA Polymerase**: This enzyme synthesizes DNA from a DNA template but is not used to convert RNA into DNA.
- **T4 DNA Ligase**: This enzyme joins DNA fragments together but does not play a role in the synthesis of cDNA.

5. **Conclusion**: Therefore, the enzyme that is used to make the first strand of cDNA from mRNA is **reverse transcriptase**.

### Final Answer:
The enzyme that is used to make the first strand of cDNA from mRNA is **reverse transcriptase**.

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