Overview of Viral Reproduction
Viruses are not alive in the traditional sense. They cannot reproduce on their own. Instead, they invade a host cell and hijack its machinery to make copies of themselves. The method of reproduction depends on the type of viral genome (DNA or RNA), and whether it uses reverse transcription.
Virus Classification Based on Genome
Viruses are classified into seven groups (Baltimore Classification) based on the nature of their genome and replication method:
1. Group I: Double-Stranded DNA (dsDNA) Viruses
- Examples: Adenoviruses, Herpesviruses, Poxviruses
- Reproduction: Use host DNA polymerase in the nucleus for replication and transcription.
2. Group II: Single-Stranded DNA (ssDNA) Viruses
- Examples: Parvoviruses
- Reproduction: Must first synthesize a complementary strand to become dsDNA before replicating.
3. Group III: Double-Stranded RNA (dsRNA) Viruses
- Examples: Reoviruses (e.g., Rotavirus)
- Reproduction: Replicate in the cytoplasm; avoid exposing dsRNA to prevent immune activation.
4. Group IV: Positive-Sense Single-Stranded RNA ((+)ssRNA) Viruses
- Examples: Coronaviruses, Picornaviruses
- Reproduction: Genome acts directly as mRNA and is translated into viral proteins.
5. Group V: Negative-Sense Single-Stranded RNA ((−)ssRNA) Viruses
- Examples: Orthomyxoviruses (Influenza), Rhabdoviruses (Rabies)
- Reproduction: Must be transcribed into (+)RNA using a viral RNA polymerase.
6. Group VI: Single-Stranded RNA Viruses with Reverse Transcription (ssRNA-RT)
- Example: HIV
- Reproduction: Reverse transcribes RNA into DNA, which integrates into the host genome.
7. Group VII: Double-Stranded DNA Viruses with Reverse Transcription (dsDNA-RT)
- Example: Hepatitis B virus
- Reproduction: DNA is transcribed into RNA, which is reverse transcribed back into DNA for packaging.