Compare replication strategies of DNA viruses versus RNA viruses.

• A. DNA viruses replicate in the cytoplasm; RNA viruses replicate in the nucleus.
• B. DNA viruses replicate in the nucleus using host or viral DNA polymerases; RNA viruses replicate in the cytoplasm using RNA-dependent RNA polymerases (and some reverse-transcribing viruses).
• C. DNA viruses replicate in mitochondria; RNA viruses replicate outside the cell.
• D. DNA viruses require no polymerases; RNA viruses do not need polymerases.

Answer: (B)

Replication location and enzymes distinguish DNA and RNA viruses. DNA viruses generally move into the nucleus to copy their genomes, using host DNA polymerases—and sometimes their own viral polymerases—to replicate. RNA viruses, on the other hand, replicate in the cytoplasm and rely on RNA-dependent RNA polymerases encoded by the virus to synthesize new RNA genomes. Some RNA viruses are retroviruses, which use reverse transcriptase to convert their RNA into DNA and integrate it into the host genome, introducing a DNA phase. This combination—nuclear replication with DNA polymerases for DNA viruses and cytoplasmic replication with RNA-dependent RNA polymerases (plus reverse transcription in retroviruses) for RNA viruses—best matches how these two classes reproduce.