retroviral information flow
Summary
Retroviruses (such as HIV) move genetic information RNA → DNA → RNA → protein: reverse transcriptase copies the RNA genome into DNA, which integrates into the host genome and is expressed by host machinery. The testable step is RNA → DNA.
Detail
Meaning: A retrovirus carries a single-stranded RNA genome and the enzyme reverse transcriptase, an RNA-dependent DNA polymerase. After the virus enters a cell, this enzyme copies the RNA into double-stranded DNA. The enzyme integrase then inserts that DNA into a host chromosome, where it is called a provirus. The host's RNA polymerase transcribes the provirus into new genomic RNA and viral mRNA, and host ribosomes translate the mRNA into viral proteins. Reverse transcriptase lacks proofreading, so retroviruses mutate quickly.
On the MCAT: A passage describes HIV or a similar virus and asks which step a compound blocks, which enzyme a step needs, or where viral nucleic acid is found. If reverse transcriptase is inhibited, no viral DNA forms and no provirus is made. If integrase is inhibited, viral DNA forms but never joins the host genome. Telomerase is also a reverse transcriptase, since it extends telomeres by copying an RNA template into DNA.
Common confusion: Students say retroviruses break the central dogma. They do not, because the dogma forbids transfer from protein back to nucleic acid, not from RNA to DNA. Also, not every RNA virus is a retrovirus. Influenza copies RNA to RNA with an RNA-dependent RNA polymerase and does not make a DNA intermediate.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.