Retrovirus
Also known as: retroviridae
A retrovirus is an RNA virus that uses the enzyme reverse transcriptase to copy its genome into DNA, which is then integrated into the host cell's chromosome. HIV and HTLV are the best-known human retroviruses.
Retroviruses carry two identical copies of a positive-sense single-stranded RNA genome inside an enveloped capsid — a diploid arrangement unique among viruses. Despite being positive-sense, that RNA is not translated on entry. Instead, the virion brings its own reverse transcriptase, which synthesizes double-stranded DNA from the RNA template. Viral integrase then splices that DNA into the host chromosome, where it is called a provirus and is copied along with the cell's own genes at every division.
Three genes define the family. gag encodes the structural matrix and capsid proteins, pol encodes the enzymes reverse transcriptase, integrase, and protease, and env encodes the surface glycoproteins that bind host receptors — in HIV, gp120 for attachment to CD4 and gp41 for membrane fusion. Once integrated, host RNA polymerase II transcribes the provirus, and viral protease cleaves the resulting polyproteins during maturation. Reverse transcriptase lacks proofreading, so retroviruses mutate rapidly, which drives both antigenic variation and drug resistance.
Retroviruses matter beyond infectious disease. Reverse transcription of RNA into DNA runs opposite to the usual flow of genetic information, and the same enzyme is the workhorse of laboratory cDNA synthesis. Integration also explains viral latency, since a provirus can sit transcriptionally quiet for years, and it underlies oncogenesis when a retrovirus inserts near a proto-oncogene or carries a transforming gene. Modified, replication-deficient retroviral and lentiviral vectors are standard tools for delivering genes in research and gene therapy.
The MCAT covers retroviruses within virus structure and life cycle, emphasizing reverse transcription as a genetic-information concept and its link to cDNA. USMLE Step 1 approaches the same virus clinically, testing HIV genome organization, the gp120 and CD4 interaction, integration and latency, and the enzyme targets of antiretroviral drug classes.
Key takeaways
- Retroviruses convert their RNA genome into DNA using reverse transcriptase.
- Integrase inserts the resulting DNA into the host chromosome as a provirus that persists through cell division.
- The gag, pol, and env genes encode structural proteins, viral enzymes, and envelope glycoproteins.
- Error-prone reverse transcription drives rapid mutation, antigenic variation, and antiretroviral drug resistance.