A provirus is

Options

A. Free virus
B. Free DNA
C. Primitive virus
D. Integrated virus genome

Answer

**Step-by-Step Solution:**

1. **Understanding Provirus**: A provirus is a viral genome that has integrated itself into the DNA of a host cell. This means that the viral DNA is not free-floating but is incorporated into the host's genetic material.

2. **Identifying the Correct Option**: Among the options provided:
- Free virus: Incorrect, as a provirus is integrated into the host DNA.
- Free DNA: Incorrect, as it does not represent the state of a provirus.
- Primitive virus: Incorrect, as this term does not accurately describe a provirus.
- Integrated virus: Correct, as it describes the state of a provirus.

3. **Role of Retroviruses**: One type of virus that can become a provirus is retroviruses. These viruses can reverse transcribe their RNA into DNA, which can then integrate into the host genome.

4. **Replication of Provirus**: Once integrated, the provirus is replicated passively along with the host's DNA during cell division. This means that every time the host cell divides, the provirus is also copied and passed on to the daughter cells.

5. **Types of Infections**: The presence of a provirus can lead to two types of infections:
- **Latent Infection**: The provirus remains transcriptionally silent, meaning it does not produce new viral particles.
- **Productive Infection**: The provirus is transcribed into messenger RNA, leading to the production of new viruses.

6. **Activation of Provirus**: Changes in the host environment can trigger the activation of a provirus, leading to the transcription of the viral genome and potentially resulting in the production of new viral particles.

7. **Conclusion**: Therefore, the correct answer to the question "A provirus is" is **integrated virus**.

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