cDNA is

Options

A. Circular DNA
B. Coiled DNA
C. Cytoplasmic DNA
D. Complementary DNA

Answer

### Step-by-Step Solution:

1. **Understanding cDNA**:
- cDNA stands for complementary DNA. It is a form of DNA that is synthesized from a messenger RNA (mRNA) template.

2. **Synthesis of cDNA**:
- cDNA is synthesized through a process called reverse transcription. This process is catalyzed by the enzyme reverse transcriptase, which converts single-stranded RNA into double-stranded DNA.

3. **Source of cDNA**:
- The RNA template used for synthesizing cDNA is typically mRNA, which carries the genetic information from DNA to the ribosome for protein synthesis.

4. **Applications of cDNA**:
- cDNA is often used in molecular biology for cloning eukaryotic genes in prokaryotic organisms. This is important for studying gene function and expression.
- Additionally, cDNA is produced naturally by retroviruses, which use their own reverse transcriptase enzyme to convert their RNA genome into DNA, which can then integrate into the host genome.

5. **Elimination of Incorrect Options**:
- The other options provided (circular DNA, coil DNA, and cytoplasmic DNA) do not accurately describe cDNA.
- Circular DNA refers to DNA that forms a closed loop, commonly found in plasmids.
- Coil DNA is not a standard term in molecular biology.
- Cytoplasmic DNA refers to DNA located outside the nucleus, which is not the definition of cDNA.

6. **Conclusion**:
- Therefore, the correct answer to the question "cDNA is" is **complementary DNA**.