Why are retroviruses placed in a separate class of RNA viruses?
- A. They always infect animal cells only
- B. Their RNA acts directly as mRNA
- C. They lack protein coats
- D. They use reverse transcriptase to make DNA from RNA
Explanation
A retrovirus reverses the usual flow of information by copying its RNA genome into DNA, which integrase then inserts into the host chromosome as a provirus, and that integration is why HIV infection is lifelong. In other RNA viruses the genome often does act directly as messenger RNA, which is the contrast the second option draws. Retroviruses have perfectly ordinary capsids and envelopes.
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