cDNA is

Options

A. Formed by reverse transcriptase
B. Cloned DNA
C. Circular DNA
D. Recombinant DNA

Answer

To answer the question "cDNA is," we will analyze the options provided and determine the correct definition of cDNA.

### Step-by-Step Solution:

1. **Understanding cDNA**: cDNA stands for complementary DNA. It is synthesized from a messenger RNA (mRNA) template through a process called reverse transcription.

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

3. **Evaluating the Options**:
- **Option A**: "Formed by reverse transcriptase" - This is correct because cDNA is indeed synthesized by the action of reverse transcriptase on mRNA.
- **Option B**: "Clone DNA" - This is incorrect. While cDNA can be cloned, it is not defined as cloned DNA.
- **Option C**: "Circular DNA" - This is also incorrect. cDNA is typically linear and not circular.
- **Option D**: "Recombinant DNA" - This is incorrect as well. Recombinant DNA refers to DNA that has been artificially created by combining DNA from different sources, which is different from cDNA.

4. **Conclusion**: Based on the analysis of the options, the correct answer is **Option A: formed by reverse transcriptase**.

### Final Answer:
cDNA is formed by reverse transcriptase.