retroviruses
Summary
Retroviruses (such as HIV) are enveloped RNA viruses that carry reverse transcriptase, which copies their RNA genome into DNA. That DNA integrates into the host chromosome as a provirus. The testable point is the reversed flow of information, RNA → DNA.
Detail
Meaning: A retrovirus has a single-stranded RNA genome (two copies per virion) inside a capsid and a lipid envelope taken from the host membrane. The virion carries reverse transcriptase, an RNA-dependent DNA polymerase. After entry, this enzyme makes double-stranded DNA from the viral RNA. Integrase then inserts the DNA into a host chromosome, where it is called a provirus. Host RNA polymerase transcribes the provirus into viral mRNA and new genomic RNA, and the host ribosomes translate that mRNA. Reverse transcriptase has no proofreading, so retroviruses mutate quickly. This helps explain variation and drug resistance.
On the MCAT: Passages usually use HIV. Questions ask you to place steps in order (binding, entry, reverse transcription, integration, transcription, translation, budding). They ask which step an inhibitor of reverse transcriptase or integrase would block. They also ask why the virus can stay latent: the provirus can sit silently in the host genome and is copied along with host DNA. Reverse transcriptase is also used in the lab to make cDNA from mRNA, as in RT-PCR.
Common confusion: Not every RNA virus is a retrovirus. Influenza and poliovirus copy their RNA with an RNA-dependent RNA polymerase and never make DNA. Retroviruses are defined by the RNA → DNA step. Reverse transcription also does not break the central dogma. It adds a reverse path.
Sources
- Campbell Biology
- Molecular Biology of the Cell
- Microbiology: An Introduction
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.