Retroviruses are implicated as a cause of cancer in man because they

Options

A. carry genes for reverse transcriptase
B. carry ssRNA in their genomes
C. may carry protooncogenes in their genomes
D. may carry viral oncogenes in their genomes

Answer

### Step-by-Step Solution:

1. **Understanding Retroviruses**: Retroviruses are a type of virus that have RNA as their genetic material. They are known for their ability to reverse transcribe their RNA into DNA once they infect a host cell.

2. **Role of Proto-oncogenes**: Proto-oncogenes are normal genes that can become oncogenes (cancer-causing genes) when they undergo mutations or are expressed at high levels. These mutations can lead to uncontrolled cell growth, which is a hallmark of cancer.

3. **Mechanism of Cancer Induction**: Retroviruses can integrate their genetic material into the host genome. If they carry proto-oncogenes, these genes can be altered or overexpressed, leading to the transformation of normal cells into cancerous cells.

4. **Identifying the Correct Option**: The question asks why retroviruses are implicated as a cause of cancer in humans. The options given include:
- A) Carry genes of reverse transcriptase
- B) Carry ssRNA in their genomes
- C) May carry proto-oncogenes in their genomes
- D) May carry viral oncogenes in their genomes

Among these, option C is the most accurate because it directly relates to the mechanism by which retroviruses can induce cancer through the presence of proto-oncogenes.

5. **Conclusion**: Therefore, the correct answer to the question is option C: "may carry proto-oncogenes in their genomes."