MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2b Viral Life Cycles Replication
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2b Viral Life Cycles ReplicationQuestion 1 of 20

In a bacteriophage study, E. coli cultures infected at low multiplicity of infection showed no immediate drop in optical density, but PCR of host DNA detected a stable viral genome segment at a specific chromosomal locus for >20 generations. When the culture was exposed to UV light, extracellular phage particles increased sharply and cell density declined. Which conclusion is most consistent with the viral life cycle?

The virus replicates only by immediate host lysis and cannot persist without killing the cell
The virus establishes a lysogenic state with a prophage that can be induced to enter the lytic cycle by DNA damage
The viral genome remains episomal and is translated directly by host ribosomes without replication
UV exposure prevents viral replication by blocking capsid assembly, explaining the reduced cell density
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2b Viral Life Cycles Replication

Practice 2b Viral Life Cycles Replication in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on 2b Viral Life Cycles Replication, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

In a bacteriophage study, E. coli cultures infected at low multiplicity of infection showed no immediate drop in optical density, but PCR of host DNA detected a stable viral genome segment at a specific chromosomal locus for >20 generations. When the culture was exposed to UV light, extracellular phage particles increased sharply and cell density declined. Which conclusion is most consistent with the viral life cycle?

  1. The virus replicates only by immediate host lysis and cannot persist without killing the cell
  2. The virus establishes a lysogenic state with a prophage that can be induced to enter the lytic cycle by DNA damage (correct answer)
  3. The viral genome remains episomal and is translated directly by host ribosomes without replication
  4. UV exposure prevents viral replication by blocking capsid assembly, explaining the reduced cell density

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, the bacteriophage infects E. coli without immediate lysis, but the viral genome is stably integrated into the host chromosome over multiple generations. Choice B is correct because it describes the establishment of lysogeny with a prophage that can be induced into the lytic cycle by UV-induced DNA damage, leading to phage production and cell density decline. Choice A is incorrect because it implies an obligate lytic cycle, which would cause immediate host lysis not observed here. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. For instance, stable genome detection without lysis points to lysogeny, while stress-induced virion release indicates a switch to lytic replication.

Question 2

A dsDNA bacteriophage encodes a repressor protein. Infected bacterial colonies appear normal in growth, and the viral genome is found integrated into the host chromosome. A mutant phage lacking the repressor causes rapid culture clearing within hours of infection. Which statement best reflects the viral replication strategy illustrated?

  1. Loss of the repressor favors immediate entry into lytic replication rather than maintenance of lysogeny (correct answer)
  2. Loss of the repressor prevents adsorption to host receptors, reducing infection and causing culture clearing
  3. The repressor is required for host DNA replication, so its loss triggers host cell cycle arrest and lysis
  4. The mutant phage is forced into latency because it cannot package its genome without the repressor

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, the wild-type bacteriophage integrates into the host chromosome, allowing normal bacterial growth, indicative of lysogeny maintained by a repressor. Choice A is correct because loss of the repressor prevents lysogeny, favoring immediate lytic replication and rapid culture clearing. Choice B is incorrect because it suggests the repressor is needed for adsorption, but the mutant still infects and causes lysis. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. For example, mutations affecting regulatory proteins like repressors can shift the balance between lytic and lysogenic paths.

Question 3

Cells infected with an RNA virus produce double-stranded RNA (dsRNA) intermediates detectable in the cytosol. Activation of a dsRNA-sensing pathway correlates with reduced viral yield. A viral mutant that prevents dsRNA accumulation produces higher viral titers. Which outcome would be expected in the host cell?

  1. Higher titers require more dsRNA sensing because innate signaling directly catalyzes virion assembly
  2. Preventing dsRNA accumulation would block host DNA replication, indirectly increasing viral yield
  3. dsRNA intermediates indicate lysogeny; reducing them should increase proviral integration
  4. Reduced dsRNA sensing would be expected to decrease antiviral signaling and allow more efficient viral replication (correct answer)

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, the RNA virus produces dsRNA intermediates that activate antiviral pathways, reducing yield, but a mutant avoiding dsRNA increases titers. Choice D is correct because minimizing dsRNA reduces innate sensing, enhancing replication. Choice B is incorrect because preventing dsRNA would not block host DNA replication but rather evade immune responses. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Consider how viral intermediates trigger host defenses and how mutations evade them.

Question 4

A dsDNA bacteriophage is found to package a short piece of host bacterial DNA in some virions. When these virions infect new bacteria, the host DNA fragment can recombine into the recipient genome without producing phage progeny. Which outcome would be expected in the host cell?

  1. This is consistent with lysogeny, where host DNA is packaged only after prophage integration into the recipient genome
  2. This is consistent with conjugation, where bacteria transfer DNA via a pilus encoded by the phage
  3. This is consistent with bacterial transformation, where cells take up naked DNA released by phage budding
  4. This is consistent with transduction, where phage can mediate horizontal gene transfer between bacteria (correct answer)

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, the bacteriophage packages host DNA, transferring it to new bacteria for recombination without phage production. Choice D is correct because this describes specialized transduction. Choice B is incorrect because conjugation involves direct cell contact via pili, not phage-mediated. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Differentiate gene transfer mechanisms like transduction from conjugation or transformation.

Question 5

In a clinical isolate of a temperate phage, sequencing shows deletion of the attachment (att) site required for integration. Infected bacteria show rapid virion production and culture lysis, and no stable prophage is detected. Which statement best reflects the viral replication strategy illustrated?

  1. Loss of the integration site biases infection toward the lytic pathway because stable lysogeny cannot be established (correct answer)
  2. Loss of the integration site forces the phage into lysogeny because excision is prevented
  3. Deletion of att prevents genome replication, so lysis must be caused by host apoptosis unrelated to the phage
  4. Integration occurs after virion release; deleting att should only affect transmission, not intracellular replication

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, deletion of the att site in the temperate phage leads to lytic infection without stable prophage. Choice A is correct because lacking integration forces a lytic pathway. Choice B is incorrect because without att, lysogeny is prevented, not forced. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Evaluate how integration site mutations bias temperate phage cycles.

Question 6

A positive-sense RNA virus is engineered with a mutation that prevents synthesis of a viral protease required to cleave a polyprotein. Infected cells show abundant viral RNA but minimal accumulation of functional viral structural proteins and low infectious particle production. Which conclusion is most consistent with the viral life cycle?

  1. Polyprotein processing is required to generate functional viral proteins necessary for assembly and productive infection (correct answer)
  2. Protease loss prevents viral genome integration into host DNA, blocking transcription of viral mRNAs
  3. Protease loss increases capsid stability, preventing uncoating and thereby increasing viral RNA abundance
  4. Polyproteins are translated only in the nucleus; protease mutation would be rescued by nuclear import inhibitors

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, the positive-sense RNA virus with protease mutation accumulates RNA but lacks functional proteins and particles. Choice A is correct because protease cleaves polyproteins into functional units for assembly. Choice B is incorrect because these viruses do not integrate; protease is for post-translational processing. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Focus on polyprotein processing in RNA viruses for protein maturation.

Question 7

A double-stranded DNA bacteriophage is added to a log-phase bacterial culture at high multiplicity of infection. Within 30 minutes, host chromosomal DNA becomes fragmented, and incorporation of radiolabeled nucleotides is detected predominantly in phage DNA. No stable lysogens are recovered when infected cells are plated. Which outcome would be expected in the host cell?

  1. Long-term survival with prophage maintenance due to integration at a specific attachment site.
  2. Immediate virion production followed by host lysis, consistent with a lytic replication program. (correct answer)
  3. Conversion of viral RNA to DNA by reverse transcriptase prior to genome replication.
  4. Suppression of phage replication because high multiplicity favors bacterial sporulation over viral assembly.

Explanation: This question tests understanding of lytic bacteriophage replication (Foundational Concept 2). The experimental observations - host DNA fragmentation, phage DNA synthesis, and absence of stable lysogens - are hallmarks of the lytic cycle. During lytic replication, phages hijack host machinery, often degrading host DNA to provide nucleotides for viral genome synthesis. The high multiplicity of infection ensures most bacteria are infected, leading to synchronized lysis. Choice B is correct because all evidence points to immediate virion production followed by host lysis, characteristic of obligate lytic phages or lytic decisions by temperate phages. Choice A is incorrect because no stable lysogens form, ruling out prophage maintenance. To approach similar questions, look for key indicators: DNA fragmentation suggests lytic cycle, while stable colony formation would indicate lysogeny.

Question 8

A positive-sense single-stranded RNA virus is used to infect hepatocyte-like cells. When purified viral RNA (without capsid proteins) is directly transfected into cells, infectious virions are produced. However, transfection of purified negative-sense RNA from a related virus does not yield infectious particles unless viral polymerase proteins are co-transfected. Which statement best reflects the viral replication strategy illustrated?

  1. Positive-sense RNA can function as mRNA upon entry, whereas negative-sense RNA requires a viral polymerase to generate translatable mRNA. (correct answer)
  2. Negative-sense RNA is directly translated by host ribosomes, while positive-sense RNA must first be copied into DNA in the nucleus.
  3. Both positive- and negative-sense RNA require host DNA-dependent RNA polymerase II to transcribe viral mRNA from RNA templates.
  4. Positive-sense RNA viruses require integration into the host genome before any viral proteins can be produced.

Explanation: This question tests understanding of positive-sense versus negative-sense RNA virus replication strategies (Foundational Concept 2). Positive-sense RNA viruses have genomes that can function directly as mRNA upon entry, allowing immediate translation by host ribosomes. The experiment demonstrates this by showing infectious virion production from naked positive-sense RNA alone. In contrast, negative-sense RNA cannot be translated and requires viral polymerase to synthesize positive-sense mRNA first. Choice A is correct because it accurately distinguishes these fundamental differences in replication strategy. Choice B is incorrect because it reverses the properties - negative-sense RNA cannot be directly translated, and positive-sense RNA viruses don't require DNA intermediates. To approach similar questions, remember that positive-sense RNA = mRNA-like (can be translated), while negative-sense RNA = anti-mRNA (requires transcription first).

Question 9

A lab compares infection outcomes of a temperate phage in nutrient-rich versus nutrient-poor bacterial cultures. In rich medium, most infected cells lyse within one hour. In poor medium, infected cells remain viable, and phage DNA is detected in the host chromosome without extracellular virions. No other changes are introduced. Which outcome would be expected in the host cell in poor medium?

  1. Immediate lysis because nutrient limitation increases ATP availability for phage assembly.
  2. Stable lysogen formation with prophage maintenance, with lytic replication suppressed until an inducing stress occurs. (correct answer)
  3. Productive infection only if host RNA polymerase II is inhibited, shifting transcription to viral genes.
  4. Integration of viral RNA into the bacterial chromosome via integrase after reverse transcription in the cytosol.

Explanation: This question tests understanding of environmental influences on the lysogenic-lytic decision in temperate phages (Foundational Concept 2). Temperate phages can sense host physiological state to optimize their replication strategy. In nutrient-rich conditions, hosts support robust lytic replication, while nutrient-poor conditions favor lysogeny to ensure phage survival when host resources are limited. The detection of phage DNA in the chromosome without extracellular virions confirms lysogenic integration. Choice B is correct because it describes stable lysogen formation under poor conditions, with lytic genes suppressed until inducing stress occurs. Choice D is incorrect because it describes retroviral replication (RNA genome, reverse transcription, integrase) rather than temperate phage biology. To solve similar problems, remember that favorable host conditions promote lytic cycles, while stress or poor conditions favor lysogenic maintenance.

Question 10

A temperate bacteriophage infects an E. coli strain carrying a reporter that fluoresces only when the bacterial SOS response is activated. Infected cultures show low fluorescence and stable bacterial growth for ~8 generations, then after brief UV exposure the culture rapidly clears and extracellular phage particles increase. Which conclusion is most consistent with the viral life cycle described?

  1. The phage genome is maintained as a prophage during initial growth and UV triggers induction into a lytic program. (correct answer)
  2. The phage immediately lyses host cells, and UV exposure is required for adsorption to the bacterial surface.
  3. The phage replicates only by binary fission with the host chromosome, so extracellular phage should not increase after UV.
  4. UV primarily increases host ribosome synthesis, which directly causes capsid assembly without viral genome replication.

Explanation: This question tests understanding of temperate bacteriophage life cycles and the lysogenic-to-lytic switch (Foundational Concept 2). Temperate phages can establish lysogeny, where the viral genome integrates as a prophage and replicates with the host chromosome without causing immediate lysis. The observation of stable bacterial growth for ~8 generations with low SOS response indicates lysogenic maintenance. UV exposure is a classic inducer of the SOS response, which triggers prophage excision and entry into the lytic cycle, explaining the rapid culture clearing and phage particle release. Choice A correctly describes this lysogenic-to-lytic transition triggered by UV-induced SOS response. Choice B is incorrect because it suggests immediate lysis, contradicting the observed stable growth period. To approach similar questions, identify whether the phage shows delayed lysis and responds to DNA damage signals, which are hallmarks of temperate phage behavior.

Question 11

A retrovirus infects a dividing T-cell line. Infections are performed in the presence or absence of an integrase inhibitor. Viral proteins are detectable in both conditions at 12 hours, but only untreated cells show stable viral gene expression after 10 days of passaging. Based on the information, which conclusion is most consistent with the viral life cycle?

  1. Integrase activity is required for initial entry and uncoating, explaining the loss of early viral protein with inhibitor.
  2. Integration is required for long-term maintenance of viral gene expression across cell divisions. (correct answer)
  3. Integrase inhibition prevents reverse transcription, so no viral proteins should be detectable at 12 hours.
  4. Stable viral expression after passaging reflects host plasmid replication of the viral RNA genome in the cytosol.

Explanation: This question tests understanding of retroviral integration and its role in persistent infection (Foundational Concept 2). Retroviruses reverse transcribe their RNA genome into DNA, which then integrates into the host chromosome via integrase enzyme. The detection of viral proteins at 12 hours in both conditions indicates that reverse transcription and early gene expression can occur without integration. However, the loss of viral gene expression after 10 days of passaging in integrase-inhibited cells demonstrates that integration is essential for long-term maintenance, as unintegrated viral DNA is diluted or degraded during cell division. Choice B correctly identifies that integration ensures viral genome persistence across cell divisions. Choice C is incorrect because integrase acts after reverse transcription, not before it. When analyzing retroviral questions, distinguish between early events (reverse transcription, early expression) and requirements for persistent infection (integration).

Question 12

Mammalian cells are infected with an enveloped, negative-sense single-stranded RNA virus. In a pulse-labeling experiment, viral mRNAs are detected in the cytosol within 30 minutes even when host nuclear transcription is inhibited, but no viral mRNA is detected when virions are treated to inactivate a virion-associated enzyme before infection. Which statement best reflects the viral replication strategy illustrated?

  1. Viral mRNA is produced only after reverse transcription of the RNA genome into DNA followed by integration.
  2. Early transcription requires host DNA-dependent RNA polymerase II to transcribe the viral RNA genome in the nucleus.
  3. The viral genome functions directly as mRNA, so translation should proceed without any viral enzyme activity.
  4. Early transcription requires a virion-packaged RNA-dependent RNA polymerase to generate positive-sense mRNA from the incoming genome. (correct answer)

Explanation: This question tests understanding of negative-sense RNA virus replication strategies (Foundational Concept 2). Negative-sense RNA viruses cannot directly serve as mRNA templates; they require conversion to positive-sense RNA for translation. The rapid detection of viral mRNA in the cytosol despite nuclear transcription inhibition indicates the virus carries its own transcription machinery. The requirement for a virion-associated enzyme confirms that the virus packages an RNA-dependent RNA polymerase (RdRp) to transcribe its negative-sense genome into positive-sense mRNA immediately upon entry. Choice D correctly identifies this virion-packaged RdRp requirement for early transcription. Choice C is incorrect because negative-sense RNA cannot function directly as mRNA due to its complementary orientation. When analyzing RNA virus questions, consider the genome polarity and whether the virus must provide its own polymerase for initial transcription.

Question 13

A DNA virus that replicates in the nucleus is studied in mammalian cells. When cells are treated with a drug that blocks nuclear export of mRNA, viral DNA replication still occurs, but production of late structural proteins and infectious particles is markedly reduced. Based on the information, which conclusion is most consistent with the viral life cycle?

  1. Viral genome replication can proceed in the nucleus, but late gene expression depends on export of viral mRNAs for cytosolic translation. (correct answer)
  2. Nuclear export is required for viral DNA polymerase to enter the nucleus, so blocking export should prevent viral DNA replication.
  3. Blocking nuclear export prevents uncoating at the plasma membrane, so neither viral DNA replication nor protein synthesis should occur.
  4. Late structural proteins are produced in the nucleus by host ribosomes, so blocking nuclear export should increase particle formation.

Explanation: This question tests understanding of nuclear-replicating DNA viruses and the compartmentalization of viral processes (Foundational Concept 2). DNA viruses that replicate in the nucleus can utilize host nuclear machinery for genome replication and early transcription. However, viral mRNAs must be exported to the cytoplasm for translation by ribosomes, as eukaryotic translation occurs exclusively in the cytoplasm. Blocking nuclear export prevents viral mRNA from reaching ribosomes, inhibiting late protein synthesis and particle assembly while allowing nuclear DNA replication to continue. Choice A correctly identifies this dependence on mRNA export for translation, while choices B, C, and D incorrectly describe polymerase import, membrane uncoating, or nuclear translation. To approach similar questions, remember that eukaryotic cells separate transcription (nuclear) from translation (cytoplasmic), requiring mRNA export.

Question 14

A researcher infects mammalian cells with an enveloped RNA virus. Treating cells with a drug that prevents acidification of endosomes reduces infection only when the drug is added during the first 30 minutes after virus exposure; adding it 2 hours after exposure has no effect on viral protein production. Which statement best reflects the viral replication strategy illustrated?

  1. The virus requires low pH in endosomes for an early uncoating/fusion step, so blocking acidification prevents productive entry. (correct answer)
  2. The virus requires low pH to activate host RNA polymerase II in the nucleus, so blocking acidification prevents late viral transcription.
  3. The drug blocks assembly of the viral capsid in the cytosol, but only if capsid proteins have not yet accumulated.
  4. The drug prevents budding by disrupting host membrane cholesterol, which only matters before viral proteins are translated.

Explanation: This question tests understanding of enveloped virus entry mechanisms and the role of endosomal acidification in viral replication (Foundational Concept 2). Many enveloped viruses enter cells through receptor-mediated endocytosis and require the acidic pH of endosomes to trigger conformational changes in viral fusion proteins. The observation that the drug only works when added within 30 minutes indicates it targets an early entry step, not late transcription or assembly. After 2 hours, the virus has already completed pH-dependent fusion and released its genome into the cytoplasm, making pH blockade ineffective. Choice A correctly identifies this pH-dependent uncoating/fusion mechanism, while choices B, C, and D incorrectly suggest effects on late transcription, capsid assembly, or budding. To solve similar problems, consider the timing of drug effects: early-acting drugs typically target entry/uncoating, while late-acting drugs affect assembly/release.

Question 15

A double-stranded DNA bacteriophage is used for generalized transduction. After infection of donor bacteria, some released phage particles package fragments of host bacterial DNA instead of phage DNA. These particles can deliver donor genes to recipient bacteria but do not produce plaques unless co-infected with wild-type phage. Which statement best reflects the replication strategy illustrated?

  1. Plaque formation fails because recipient bacteria cannot translate bacterial DNA delivered by phage capsids into phage proteins.
  2. The transducing particles establish lysogeny in recipients by integrating donor bacterial DNA at the phage attachment site.
  3. The transducing particles replicate by binary fission with recipients, so plaque formation should occur without wild-type phage.
  4. The transducing particles are defective because they lack essential phage genes needed for productive lytic replication in recipients. (correct answer)

Explanation: This question tests understanding of generalized transduction and defective phage particles (Foundational Concept 2). During generalized transduction, phage packaging machinery occasionally packages random fragments of host DNA instead of phage DNA into virions. These transducing particles can inject their DNA cargo into recipient cells but cannot establish productive infection because they lack essential phage genes required for replication, transcription of late genes, and virion assembly. Co-infection with wild-type phage provides these missing functions in trans, allowing plaque formation. Choice D correctly identifies that transducing particles are defective due to lacking essential phage genes. Choice B is incorrect because generalized transduction involves random DNA packaging, not site-specific integration. When analyzing transduction questions, distinguish between particles carrying phage DNA (infectious) versus host DNA (defective for lytic growth).

Question 16

A double-stranded DNA bacteriophage is engineered to lack a functional integrase. The mutant phage adsorbs to E. coli and injects its genome normally. Compared with wild-type phage, infected cultures show earlier appearance of extracellular virions and a more rapid decrease in optical density (cell lysis). Based on the information, which conclusion is most consistent with the viral life cycle?

  1. Loss of integrase biases the phage away from lysogeny, increasing the proportion of infections that proceed directly to lytic replication. (correct answer)
  2. Loss of integrase prevents viral genome replication, so the phage compensates by budding from the host membrane without lysis.
  3. Loss of integrase increases host chromosomal replication speed, which accelerates phage capsid assembly and release.
  4. Loss of integrase causes immediate uncoating defects, so only preformed virions are detected earlier in the supernatant.

Explanation: This question tests understanding of bacteriophage decision-making between lysogenic and lytic pathways (Foundational Concept 2). Integrase is essential for establishing lysogeny by catalyzing the integration of phage DNA into the host chromosome. Without functional integrase, the phage cannot establish lysogeny and is forced to proceed directly to lytic replication. This explains both the earlier appearance of virions and more rapid cell lysis, as all infections immediately enter the lytic cycle rather than some establishing lysogeny. Choice A correctly identifies this shift from lysogeny to obligate lytic replication, while choices B, C, and D incorrectly suggest effects on budding, host replication, or uncoating. To approach similar questions, consider how loss of lysogeny-specific functions (like integrase) would force phages into the alternative lytic pathway.

Question 17

In a study of influenza-like viruses, researchers observe that viral RNA synthesis decreases when nuclear export is inhibited, and viral ribonucleoprotein complexes accumulate in the nucleus. Protein translation machinery remains intact in the cytosol. Based on the information, which conclusion is most consistent with the viral life cycle?

  1. Influenza-like viruses replicate exclusively in the cytosol, so nuclear export should have no effect on RNA synthesis
  2. Nuclear export inhibition increases viral replication by concentrating ribosomes in the nucleus
  3. Accumulation in the nucleus indicates the virus is lysogenic and should integrate into host DNA
  4. Nuclear export is required to move viral genome segments or transcripts to the cytosol for translation and/or packaging (correct answer)

Explanation: This question tests understanding of viral life cycles and replication strategies (Foundational Concept 2). Viruses can replicate through lytic or lysogenic cycles, each with distinct phases. In the vignette, nuclear export inhibition causes RNP accumulation and reduced RNA synthesis in influenza-like viruses. Choice D is correct because export is needed for cytosolic translation and packaging, indirectly affecting nuclear replication. Choice B is incorrect because ribosomes are cytosolic; inhibition does not concentrate them in the nucleus. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Trace compartmental requirements for segmented RNA viruses like influenza.

Question 18

A temperate bacteriophage infects an E. coli strain carrying a reporter downstream of the phage attachment site (attB). After infection at low multiplicity, most colonies remain viable and PCR across the attB locus yields a larger amplicon consistent with phage DNA insertion. When the same lysogenic colonies are exposed to UV light, culture turbidity drops within 90 minutes and plaque-forming units increase in the supernatant. Which conclusion is most consistent with the viral life cycle?

  1. The phage establishes a prophage by integrating into the host genome and UV exposure induces entry into the lytic cycle with virion production and host lysis. (correct answer)
  2. The phage immediately enters the lytic cycle after infection, and UV light increases plaque counts by enhancing bacterial binary fission.
  3. The phage remains episomal and UV light triggers reverse transcription to generate DNA for integration.
  4. The phage integrates only after UV exposure, and the drop in turbidity reflects slowed host metabolism rather than lysis.

Explanation: This question tests understanding of temperate bacteriophage life cycles and the lysogenic-to-lytic switch (Foundational Concept 2). Temperate phages can establish lysogeny by integrating their DNA into the host chromosome at specific attachment sites, forming a prophage that replicates with the host. The PCR data showing a larger amplicon confirms phage DNA insertion at the attB site, indicating lysogenic integration. UV light is a classic inducer that damages DNA and triggers the SOS response, causing prophage excision and entry into the lytic cycle. Choice A is correct because it accurately describes both the initial lysogenic state (prophage integration) and UV-induced switch to lytic replication with virion production and host lysis. Choice B is incorrect because it suggests immediate lytic infection, contradicting the PCR evidence of integration. To approach similar questions, identify key experimental evidence (PCR showing integration, UV-induced lysis) and match it to the appropriate viral life cycle phase.

Question 19

A retrovirus infects activated CD4+ T cells. Early after infection, viral cDNA is detected in the cytoplasm, and later a host genomic locus contains an inserted proviral sequence. Treatment with an integrase inhibitor prevents stable insertion but does not prevent initial cDNA formation. Based on the information, which conclusion is most consistent with the viral life cycle?

  1. Integrase is required for reverse transcription, so blocking integrase prevents cDNA synthesis in the cytoplasm.
  2. Integration is required for entry, so integrase inhibition prevents the virus from binding to CD4 and fusing with the membrane.
  3. Reverse transcription can occur before integration, and integrase is specifically required to insert viral DNA into the host genome. (correct answer)
  4. Proviral insertion occurs through host ribosomes, so integrase inhibition blocks viral translation rather than genome insertion.

Explanation: This question tests understanding of retroviral replication, specifically the roles of reverse transcriptase and integrase (Foundational Concept 2). Retroviruses use reverse transcriptase to synthesize DNA from their RNA genome in the cytoplasm, then integrase to insert this DNA into the host chromosome. The experiment shows cDNA forms even with integrase inhibition, but stable insertion is blocked. This demonstrates that reverse transcription and integration are separate steps requiring different enzymes. Choice C is correct because it accurately describes this two-step process: reverse transcription occurs first in the cytoplasm, then integrase specifically catalyzes proviral insertion. Choice A is incorrect because it wrongly claims integrase is needed for reverse transcription, contradicting the experimental evidence of cDNA formation with integrase inhibition. To solve similar problems, remember the retroviral replication sequence: entry → reverse transcription → integration → transcription.

Question 20

A bacteriophage encodes a repressor protein that binds operator sequences in the phage genome. In bacteria expressing a nonfunctional repressor (loss-of-function mutation), infection results in rapid cell lysis and high phage yield. In bacteria expressing a hyperstable repressor, infected cells remain viable and phage genomes are maintained without detectable virion release. Which statement best reflects the viral replication strategy illustrated?

  1. Repressor activity determines bacterial transcription globally, so changes in lysis reflect altered host operon regulation rather than phage decisions.
  2. A stable repressor promotes lytic replication by enhancing capsid assembly, whereas loss of repressor function causes integration into the host genome.
  3. A stable repressor favors lysogeny by suppressing lytic gene expression, whereas loss of repressor function permits lytic replication and host lysis. (correct answer)
  4. Repressor proteins are required only for viral entry, so repressor mutations affect adsorption but not replication strategy.

Explanation: This question tests understanding of lysogeny maintenance and the lytic-lysogenic decision in temperate phages (Foundational Concept 2). The repressor protein is central to maintaining lysogeny by binding operators and suppressing lytic gene expression. Loss-of-function repressor mutations prevent lysogeny establishment, defaulting to lytic replication with rapid lysis. Conversely, hyperstable repressors maintain lysogeny even under conditions that might normally induce the lytic cycle. Choice C is correct because it accurately describes how repressor stability controls the lysogenic-lytic switch. Choice B is incorrect because it reverses the repressor's role - repressors suppress lytic genes, not promote them. To approach similar questions, remember that in lysogeny, repressor = lysogenic maintenance, while repressor loss/inactivation = lytic induction.