Free MCAT-style practice, 2B
MCAT Virus Structure, Retrovirus and Prion Practice Questions
Eight free MCAT-style practice questions on viral envelopes, genome types, plaque assay titers, retroviruses, host range, prions and viroids, with full explanations.
Quick concept primer
A virus is a nucleic acid genome wrapped in a protein capsid, sometimes surrounded by a lipid envelope taken from a host membrane as the virus buds out. Viruses have no ribosomes and no metabolism of their own, so they replicate only inside a host cell. The genome type decides what the virus must bring: a positive-sense RNA genome can be read by host ribosomes at once, while a negative-sense RNA genome must first be copied, so the virion carries its own RNA-dependent RNA polymerase. Retroviruses carry reverse transcriptase, copy their RNA into DNA and insert that DNA into a host chromosome as a provirus. Host range depends largely on whether viral surface proteins can bind receptors on a given cell.
Prions are misfolded proteins with no nucleic acid; they template the misfolding of the normal host protein. Viroids are small circular RNAs with no capsid that infect plants and encode no protein.
Traps: envelopes come from the host, not the viral genome; reverse transcriptase is an RNA-dependent DNA polymerase, not an RNA polymerase; the prion protein gene belongs to the host. The 2B Prokaryotes, Viruses and Microbial Genetics pack adds flashcards and more practice across the whole category, and you can try a free sampler first.
8 practice questions
Question 1
A research team finds that a newly isolated virus loses nearly all infectivity after a brief treatment with a mild detergent that dissolves lipid bilayers but leaves proteins folded and nucleic acids intact. Which feature of the virus best explains this result?
- It has a double-stranded DNA genome.
- It lacks a protein capsid.
- It has a lipid envelope acquired from a host cell membrane.
- It replicates only inside the host nucleus.
Show answer and explanation
Answer: C. It has a lipid envelope acquired from a host cell membrane.
A detergent that dissolves lipid bilayers destroys an envelope, and enveloped viruses need their envelope proteins to attach and fuse with host cells. Genome type and replication site are unrelated to detergent sensitivity, and every virus has a capsid, so a virus lacking one would not be a virus at all.
Question 2
A drug is a nucleoside analog that lacks a 3' hydroxyl group. It is incorporated by a retrovirus's reverse transcriptase but is a poor substrate for host DNA polymerases. Which step of the retroviral life cycle does it most directly block?
- Synthesis of DNA using the viral RNA genome as a template
- Insertion of viral DNA into a host chromosome
- Transcription of the provirus into new viral RNA
- Budding of new virions from the plasma membrane
Show answer and explanation
Answer: A. Synthesis of DNA using the viral RNA genome as a template
Without a 3' hydroxyl, the analog terminates the growing DNA chain once reverse transcriptase adds it, so copying the RNA genome into DNA stops. Integration is carried out by integrase on DNA that already exists, transcription of the provirus uses host RNA polymerase II, and budding comes after assembly, so none of these is the direct target.
Question 3
A virus stock is diluted to 10-6, and 0.1 mL of that dilution is spread on a lawn of host bacteria. After incubation, 45 plaques are counted. What is the titer of the original stock in plaque-forming units (PFU) per mL?
- 4.5 × 106 PFU/mL
- 4.5 × 107 PFU/mL
- 4.5 × 109 PFU/mL
- 4.5 × 108 PFU/mL
Show answer and explanation
Answer: D. 4.5 × 108 PFU/mL
Titer equals plaques divided by (volume plated times dilution): 45 / (0.1 mL × 10-6) = 4.5 × 108 PFU/mL. Forgetting to divide by the 0.1 mL volume gives 4.5 × 107, and an error of one power of ten in either direction gives the other two choices.
Question 4
Each of the following statements about prions is correct EXCEPT:
- Prions consist of a misfolded form of a normal host protein.
- Prions carry a small RNA genome that encodes the prion protein.
- A prion can convert the normally folded host protein into the misfolded form.
- Prions resist some sterilization methods that inactivate viruses.
Show answer and explanation
Answer: B. Prions carry a small RNA genome that encodes the prion protein.
Prions contain no nucleic acid at all; the normal prion protein is encoded by a gene in the host's own genome, and disease spreads when misfolded protein templates the misfolding of normal copies. Choices A, C and D are correct, including the resistance of prions to standard procedures such as routine autoclaving cycles and ultraviolet light that destroy most viruses.
Question 5
A plant pathologist isolates the agent of a stunting disease in potato plants. It is a circular single-stranded RNA of about 350 nucleotides, has no protein coat and encodes no protein. The agent is best classified as a:
- viroid.
- prion.
- retrovirus.
- bacteriophage.
Show answer and explanation
Answer: A. viroid.
A viroid is a small, naked, circular RNA that encodes no protein and infects plants, which matches every detail. Prions are protein with no RNA, retroviruses have a capsid and encode reverse transcriptase, and bacteriophages infect bacteria and have protein capsids.
Question 6
A virus readily infects cultured human liver cells but not human kidney cells. When the kidney cells are engineered to express a single liver cell surface protein, they become infected at high rates. What does this result best support?
- Kidney cells lack the ribosomes needed to translate viral proteins.
- The viral genome is too large to enter kidney cells.
- Host range is set by binding of viral surface proteins to specific cell receptors.
- Kidney cells degrade all viral RNA before it can be translated.
Show answer and explanation
Answer: C. Host range is set by binding of viral surface proteins to specific cell receptors.
Adding one surface protein was enough to make kidney cells susceptible, so the missing step was attachment, which depends on a viral protein recognizing a specific receptor. Kidney cells have ribosomes, genome size does not change with a surface protein, and adding a receptor would not stop an RNA-degrading defense, so the other choices do not fit the result.
Question 7
A virology course compares four viruses by genome type.
| Virus | Genome |
|---|---|
| V1 | Double-stranded DNA |
| V2 | Positive-sense single-stranded RNA |
| V3 | Negative-sense single-stranded RNA |
| V4 | Single-stranded RNA retrovirus |
Which virus must package an RNA-dependent RNA polymerase inside its virion in order to begin infection?
- V1
- V3
- V2
- V4
Show answer and explanation
Answer: B. V3
A negative-sense RNA genome cannot be read by ribosomes and host cells have no enzyme to copy RNA from RNA, so the virion must bring its own RNA-dependent RNA polymerase. A positive-sense genome is translated directly to make that enzyme, a DNA virus can use host transcription machinery, and a retrovirus packages reverse transcriptase, which makes DNA rather than RNA.
Question 8
In a single-cycle growth experiment, 2 × 105 bacteria are each infected with one phage. When the cells lyse, a total of 3 × 107 new phage particles are released. What is the average burst size per infected cell?
- 15
- 0.0067
- 1500
- 150
Show answer and explanation
Answer: D. 150
Burst size is progeny released divided by infected cells: (3 × 107) / (2 × 105) = 150 phages per cell. A value of 15 or 1500 comes from a one-power-of-ten slip, and 0.0067 comes from inverting the ratio (infected cells divided by progeny).