DNA Viruses: Usually replicate in the host nucleus using host DNA-dependent DNA polymerase. They often follow the standard central dogma: .
RNA Viruses: Most replicate in the cytoplasm. Since host cells lack enzymes to copy RNA from RNA, these viruses must provide their own RNA-dependent RNA polymerase (RdRp).
Retroviruses: These unique RNA viruses use the enzyme Reverse Transcriptase to convert their RNA genome into DNA (), which then integrates into the host's genome as a provirus.
| Feature | Lytic Cycle | Lysogenic Cycle |
|---|---|---|
| Immediate Outcome | Rapid production of virions and host cell death. | Viral genome integrates into host DNA and remains dormant. |
| Host Survival | Host cell is lysed (destroyed). | Host cell survives and reproduces normally. |
| Viral State | Active replication. | Prophage or Provirus state. |
| Trigger | Occurs immediately upon infection. | Can be induced to enter the lytic cycle by environmental stress. |
Lytic Cycle: This is the "standard" replication pathway where the virus takes over the cell immediately to produce as many offspring as possible, ending in cell rupture.
Lysogenic Cycle: This is a strategy of persistence. The virus "hides" within the host's own genetic code, allowing it to be copied every time the cell divides without killing the host.
Identify the Enzyme: Always check if the virus is DNA or RNA based. If it is an RNA virus (except retroviruses), it MUST carry or encode for RNA-dependent RNA polymerase because the host cell cannot replicate RNA.
Source of the Envelope: Remember that the viral envelope is derived from the host cell's membrane (plasma membrane, ER, or Golgi) during the budding process, but the proteins in that envelope are viral-encoded.
The 'Central Dogma' Exception: Be prepared to explain how Retroviruses reverse the flow of genetic information using Reverse Transcriptase.
Verification: If a question asks about the "first step" of replication, ensure you distinguish between attachment (binding) and penetration (entering).
Ribosome Myth: A common mistake is thinking viruses have their own ribosomes. They do NOT; they are entirely dependent on host ribosomes for protein synthesis.
Lysis vs. Budding: Students often think all viruses kill the host cell immediately. While lysis does, budding allows the host cell to remain alive for a period while it continuously "sheds" new viruses.
Transcription vs. Replication: Do not confuse the two. Transcription is the creation of mRNA for protein production; replication is the creation of new full-length genomes for the next generation of viruses.
