MCAT content library / Biology & biochemistry / Viral structure and replication
Locate a viral replication block by following genome, nucleic-acid intermediate, protein, and particle readouts in the order they must appear.
About 17 minutes, including the retroviral pathway experiment and checks.
Useful foundations
On this page
- Separate viral parts from host machinery
- Concept: genome strategy determines the required information flow
- Worked application: localize three blocks in a retroviral cycle
- Pattern: a later product requires the earlier steps
- Rule: not all RNA viruses use reverse transcription
- Check your reasoning
- Continue learning
- Sources
Separate viral parts from host machinery
A virus contains a nucleic-acid genome packaged in a protein capsid; some viruses also have a lipid envelope acquired from host membranes. Viruses depend on host cells for translation and many metabolic needs, but they can encode specialized enzymes that the host does not provide for the required replication strategy.
Attachment, entry, uncoating, genome replication, gene expression, assembly, and release are separable stages. A defect at an early stage removes downstream products; a defect late in the cycle can leave upstream nucleic acids and proteins intact while reducing new infectious particles.
Concept: genome strategy determines the required information flow
Viral genomes can be DNA or RNA, single- or double-stranded, and organized in different ways. The key question is how the virus obtains a translatable mRNA and how it copies its genome. Host ribosomes translate viral mRNA, but human cells do not normally provide every viral polymerase activity. A virus may therefore need to encode or package an RNA-dependent RNA polymerase, reverse transcriptase, or other specialized enzyme.
Retroviruses are a specific case: their virions contain RNA, but the replication cycle includes reverse transcription into DNA and integration of that DNA into the host genome as a provirus. Viral gene expression then proceeds from the DNA intermediate.2
Worked application: localize three blocks in a retroviral cycle
An original retrovirus-like model enters cells normally in all conditions. Investigators measure incoming viral RNA at 1 hour, reverse-transcribed viral DNA at 4 hours, integrated proviral DNA at 8 hours, viral protein at 12 hours, and released infectious particles at 18 hours. Each intervention selectively blocks one stage.
| Condition | Incoming RNA | Viral DNA | Integrated DNA | Viral protein | Infectious particles |
|---|---|---|---|---|---|
| Control | + | + | + | + | + |
| Intervention A | + | − | − | − | − |
| Intervention B | + | + | − | − | − |
| Intervention C | + | + | + | + | − |
Intervention A acts after entry but before viral DNA appears, so it is consistent with a block in reverse transcription. Intervention B allows viral DNA to form but prevents integration and every later readout, so it is consistent with an integration block. Intervention C allows genome conversion, integration, and protein production but prevents infectious-particle output, so it acts at a late assembly, maturation, or release step.
The data localize each intervention to a stage. They do not identify a precise protein target unless the experiment supplies additional molecular evidence.
Pattern: a later product requires the earlier steps
Pathway order creates logical constraints. If integrated proviral DNA is detected, reverse transcription must already have occurred in this model. If viral proteins are abundant but infectious particles are absent, the block cannot be explained solely by failure to translate the viral mRNA.
Time-of-addition experiments use the same logic. A drug that loses effectiveness when added after the viral-DNA intermediate has already formed is more consistent with an early replication target than with a late assembly target.
Rule: not all RNA viruses use reverse transcription
RNA genome does not mean retrovirus. Many RNA viruses copy RNA using RNA-dependent RNA polymerases and never make a DNA intermediate. Retroviruses are distinguished by their RNA-to-DNA step and integrated provirus. Some other virus families can also use reverse transcription in different genome strategies, so the passage’s supplied pathway matters more than a simple RNA-versus-DNA label.3
Also keep subviral agents distinct. A prion is an infectious protein conformer without a nucleic-acid genome. A viroid is a small infectious RNA agent of plants and does not represent a conventional capsid-containing animal virus. These cases reinforce the rule that “infectious” does not always mean “virus.”
ACTIVE PRACTICE · EVIDENCE AND MECHANISM
Localize a Retroviral Block From Ordered Readouts
In the original retrovirus-like model, entry occurs normally. The measured sequence is incoming RNA at 1 h → reverse-transcribed viral DNA at 4 h → integrated DNA at 8 h → viral protein at 12 h → infectious particles at 18 h. A plus sign means detectable product, not a quantitative viral load.
The experimental result supports a bounded mechanism, not proof of every possible molecular cause. Teaching practice is not a calibrated MCAT score.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
A retroviral intervention leaves incoming RNA intact but eliminates viral DNA. Which step is the best match?
A. Reverse transcription
Correct. RNA enters, but the DNA intermediate does not form.
B. Translation only
Review this choice. The missing product is DNA, which lies upstream of late protein production in the supplied cycle.
C. Virion release only
Review this choice. A release defect would not prevent viral DNA formation.
D. Host-cell mitosis only
Review this choice. The readout specifically localizes the failure to viral RNA-to-DNA conversion.
Check 2
A virus has a negative-sense RNA genome, and the host cell lacks an enzyme that can copy RNA from RNA. The passage states that viral mRNA appears immediately after entry. What must the virion or viral genome provide?
A. An RNA-dependent RNA polymerase activity
Correct. The host lacks the required activity, so the virus must supply a way to generate translatable RNA.
B. A host ribosome encoded inside the viral genome
Review this choice. Viruses use host ribosomes for translation; the missing activity is RNA-directed RNA synthesis.
C. A mitochondrial electron-transport chain
Review this choice. That does not solve the RNA-replication problem.
D. A bacterial conjugation pilus
Review this choice. Conjugation is a bacterial DNA-transfer mechanism.
Check 3
An infectious agent contains no detectable nucleic acid and propagates by inducing abnormal folding of a host protein. What is it most consistent with?
A. Prion
Correct. Prions are infectious protein conformers without nucleic-acid genomes.
B. Retrovirus
Review this choice. Retroviruses contain RNA genomes and use reverse transcription.
C. Plasmid
Review this choice. A plasmid is DNA.
D. Viroid
Review this choice. Viroids contain RNA.
Continue learning
- Translation and the Genetic Code
- Prokaryotic cells and growth
- DNA replication and repair
- Browse Biology & Biochemistry MCAT lessons