- A dsDNA virus replicates in the nucleus and produces mRNAs that are capped and polyadenylated. In cells treated with an inhibitor of host RNA polymerase II, viral mRNA production is strongly reduced, but viral DNA remains detectable shortly after infection. Which statement best reflects the viral replication strategy illustrated?
Answer: The virus likely relies on host transcription machinery for viral mRNA synthesis in the nucleus
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 dsDNA virus replicates in the nucleus and its mRNA production is blocked by RNA polymerase II inhibition. Choice D is correct because many nuclear DNA viruses use host transcription machinery for mRNA synthesis. Choice B is incorrect because positive-sense RNA viruses replicate in the cytosol, not relying on nuclear Pol II. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Determine replication site and host enzyme dependence from inhibitor effects.
- A virus enters cells via receptor-mediated endocytosis. When endosomal acidification is blocked, viral genomes are not released into the cytosol and infection fails, despite normal binding and uptake. Which conclusion is most consistent with the viral life cycle?
Answer: Low pH is likely required for a conformational change that enables uncoating or membrane fusion during entry
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, blocking endosomal acidification prevents genome release despite entry. Choice A is correct because low pH triggers uncoating or fusion in endocytic viruses. Choice B is incorrect because pH affects viral entry, not host DNA replication directly. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Examine entry pathways and environmental cues like pH for uncoating.
- 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?
Answer: Nuclear export is required to move viral genome segments or transcripts to the cytosol for translation and/or packaging
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.
- A DNA virus infecting dividing fibroblasts shows markedly reduced replication when cells are arrested in G0, but replication is restored when a viral early gene is overexpressed that increases host S-phase markers. Which outcome would be expected in the host cell?
Answer: The virus likely benefits from host DNA synthesis machinery and promotes entry into S phase to replicate its genome
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 DNA virus replicates poorly in G0-arrested fibroblasts but improves with an early gene promoting S-phase entry. Choice A is correct because many DNA viruses depend on host S-phase machinery for genome replication. Choice B is incorrect because translation occurs in the cytosol, not the nucleus, and S-phase does not increase nuclear ribosomes. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Link viral replication to host cell cycle phases, especially for DNA viruses exploiting synthesis machinery.
- 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?
Answer: The transducing particles are defective because they lack essential phage genes needed for productive lytic replication in recipients.
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).
- A lysogenic bacterial strain carries a prophage that encodes a toxin gene. The bacteria are grown without inducing conditions and show no detectable extracellular phage particles, yet toxin protein is measurable in the culture medium. A deletion is introduced that prevents prophage excision but leaves the toxin gene intact. Which outcome would be expected in the host cell?
Answer: Toxin expression can persist without phage particle production because some prophage genes may be expressed during lysogeny.
This question tests understanding of prophage gene expression during lysogeny (Foundational Concept 2). During lysogeny, most prophage genes are repressed, but some genes can be constitutively expressed from the integrated prophage, including virulence factors like toxins. This explains toxin production without detectable phage particles. Preventing prophage excision would maintain the lysogenic state indefinitely, allowing continued toxin expression without entering the lytic cycle. Choice A correctly describes this lysogenic gene expression, while choices B, C, and D incorrectly suggest toxin expression requires lytic replication, excision blockade induces lysis, or eukaryotic integration. To approach similar questions, remember that lysogenic conversion can confer new properties (like toxin production) to bacteria through prophage gene expression.
- A segmented negative-sense RNA virus is engineered to lack a functional polymerase gene. Purified virions can still bind and enter host cells, but no viral mRNA is detected after entry. Based on the information, which conclusion is most consistent with the viral life cycle?
Answer: The virion must carry or encode an RdRp activity to transcribe negative-sense RNA into mRNA after entry
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 negative-sense RNA virus lacking polymerase enters cells but produces no mRNA. Choice A is correct because virions must package RdRp to transcribe negative-sense RNA into mRNA. Choice B is incorrect because host polymerases cannot translate negative-sense RNA directly. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Identify requirements for packaged enzymes based on genome polarity.
- 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?
Answer: 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.
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.
- A positive-sense ssRNA virus is added to cultured hepatocytes. Within 30 minutes, viral proteins are detected even when the cells are treated with an inhibitor of nuclear import. However, treatment with a cytosolic translation inhibitor blocks both viral protein production and later increases in viral RNA. Which outcome would be expected in the host cell?
Answer: Viral genomic RNA can function as mRNA for immediate translation in the cytosol
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 ssRNA virus produces proteins rapidly in hepatocytes, unaffected by nuclear import inhibition but blocked by cytosolic translation inhibition. Choice B is correct because the genomic RNA acts directly as mRNA for translation in the cytosol, enabling immediate protein synthesis. Choice A is incorrect because it describes a retroviral process requiring reverse transcription, not applicable to positive-sense RNA viruses. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Consider genome polarity and location of initial translation to distinguish RNA virus classes.
- Cells are infected with a positive-sense single-stranded RNA virus. When translation initiation is blocked immediately after entry, no new RNA genomes are detected. When translation is allowed for 1 hour and then inhibited, RNA genome replication proceeds for several hours. Which outcome would be expected in the host cell given this replication strategy?
Answer: Early translation produces viral replicase proteins needed to synthesize a negative-sense template for genome amplification.
This question tests understanding of positive-sense RNA virus replication requirements (Foundational Concept 2). Positive-sense RNA viruses have genomes that can function directly as mRNA, but they still require viral proteins for genome replication. The inability to detect new RNA genomes when translation is blocked immediately indicates that viral replicase proteins must first be translated from the incoming genome. Once these proteins are produced (within 1 hour), they can continue RNA replication even after translation is inhibited, explaining continued genome synthesis. Choice B correctly identifies that early translation produces the viral replicase needed for genome amplification through a negative-sense intermediate. Choice D is incorrect because RNA viruses replicate using viral RNA-dependent RNA polymerases, not host DNA-dependent polymerases. When analyzing RNA virus replication, consider whether the virus must first produce its own replication machinery through translation.
- 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?
Answer: Immediate virion production followed by host lysis, consistent with a lytic replication program.
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.
- A negative-sense ssRNA virus infects airway epithelial cells. When cells are treated with a drug that specifically inhibits RNA-dependent RNA polymerase (RdRp), no viral proteins are detected, despite successful viral entry. Based on the information, which conclusion is most consistent with the viral life cycle?
Answer: The virus must generate a positive-sense RNA intermediate using RdRp before translation can occur
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 negative-sense ssRNA virus enters airway cells but fails to produce proteins when RdRp is inhibited. Choice C is correct because negative-sense RNA requires RdRp to synthesize positive-sense intermediates for translation. Choice A is incorrect because it describes positive-sense RNA viruses, which can be directly translated without prior transcription. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Evaluate the need for virion-associated enzymes based on genome sense to predict replication dependencies.
- A bacteriophage integrates into the host genome and remains quiescent. A mutation in the bacterial SOS response pathway prevents induction after UV exposure; prophage DNA remains integrated and no virions are released. Which statement best reflects the viral replication strategy illustrated?
Answer: Induction of the lytic cycle depends on host DNA damage signaling; blocking SOS prevents prophage activation
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, SOS mutation prevents prophage induction by UV, maintaining integration. Choice D is correct because SOS signaling triggers lytic induction in lysogenic phages. Choice B is incorrect because UV induces via host pathways, not direct capsid assembly. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Connect host stress responses like SOS to prophage induction mechanisms.
- A lytic bacteriophage infection is monitored in a synchronized E. coli culture. A translation inhibitor is added either 5 minutes or 25 minutes after adsorption. When added at 5 minutes, no infectious phage are produced. When added at 25 minutes, infectious phage still appear later, but at a reduced yield. Which conclusion is most consistent with the timing of viral replication events?
Answer: Early translation is required to produce proteins needed for genome replication and assembly, while later translation mainly increases burst size.
This question tests understanding of temporal gene expression during lytic bacteriophage replication (Foundational Concept 2). Lytic phages typically express early genes immediately after infection to produce proteins required for DNA replication and regulation. Late genes, encoding structural proteins, are expressed after genome replication begins. Translation inhibition at 5 minutes blocks early protein synthesis, preventing DNA replication and all subsequent steps. Inhibition at 25 minutes allows early events to proceed but reduces late protein synthesis, decreasing burst size without eliminating phage production. Choice A correctly describes this temporal program, while choices B, C, and D incorrectly suggest translation is only needed for entry, injection, or host defense. To solve similar problems, consider the sequential nature of phage gene expression: early genes enable replication, late genes produce virion components.
- 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?
Answer: Integration is required for long-term maintenance of viral gene expression across cell divisions.
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).
- A retrovirus infects activated T cells. Viral DNA is detected only after several hours, and integration into host chromosomal DNA is observed prior to robust viral mRNA production. An integrase inhibitor prevents sustained viral gene expression but does not prevent initial reverse transcription. Which statement best reflects the viral replication strategy illustrated?
Answer: Integration is required to produce stable viral transcripts from host transcription machinery
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 retrovirus integrates into T cell DNA before robust mRNA production, with integrase inhibition blocking expression but not reverse transcription. Choice D is correct because integration allows the use of host machinery for stable viral transcription. Choice B is incorrect because integration occurs early, before virion assembly, to enable gene expression. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Distinguish retroviral steps like reverse transcription and integration to understand inhibitor effects.
- A lytic phage infection is monitored in synchronized bacterial cells. A drug that inhibits bacterial DNA gyrase is added after phage adsorption but before phage DNA replication. Early phage proteins are still synthesized, but production of full-length phage genomes and mature virions is markedly reduced. Which outcome would be expected in the host cell?
Answer: Reduced phage genome replication due to impaired DNA topology management needed for efficient replication of double-stranded DNA.
This question tests understanding of DNA topology requirements in viral replication (Foundational Concept 2). DNA gyrase introduces negative supercoils and manages DNA topology during replication, which is crucial for both bacterial and phage DNA synthesis. Double-stranded DNA phages rely on proper DNA topology management for efficient replication fork progression and resolution of topological constraints. Inhibiting gyrase after adsorption but before DNA replication allows early protein synthesis (from incoming DNA) but impairs subsequent genome replication due to accumulation of positive supercoils and replication fork stalling. Choice B correctly identifies that impaired topology management reduces phage DNA replication efficiency. Choice C is incorrect because gyrase inhibition increases positive (not negative) supercoiling, which inhibits rather than promotes replication. To approach such questions, consider how DNA topology affects replication fork progression in double-stranded DNA genomes.
- A circular double-stranded DNA virus infects human epithelial cells. When cells are treated with a drug that blocks nuclear import, viral early proteins accumulate in the cytosol but viral DNA replication does not occur; however, when nuclear import is restored, viral DNA replication begins and late structural proteins are produced. Which outcome would be expected in the host cell under continued nuclear import blockade?
Answer: Viral genome replication is impaired because essential replication steps require access to nuclear machinery and/or nuclear factors.
This question tests understanding of DNA virus nuclear dependence for replication (Foundational Concept 2). Many DNA viruses, particularly those with small genomes, rely heavily on nuclear machinery for DNA replication, including access to host DNA polymerases, primases, and other replication factors. The observation that early proteins accumulate in the cytosol but DNA replication fails when nuclear import is blocked demonstrates this nuclear requirement. Once nuclear access is restored, the virus can proceed with DNA replication and late gene expression. Choice B correctly identifies that viral genome replication requires nuclear access for essential replication machinery. Choice D is incorrect because most DNA viruses replicate in the nucleus, not the cytosol, to access host replication machinery. To solve similar problems, consider where the host cell's DNA replication machinery is located and whether the virus encodes all its own replication proteins.
- A DNA virus is detected in a patient's epithelial cells as a circular episome maintained over many cell divisions. During immunosuppression, viral DNA copy number increases and virions are detected in secretions. Which outcome would be expected in the host cell?
Answer: The virus can persist in a latent-like state as an episome and later reactivate to productive replication when host control is reduced
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 DNA virus persists as an episome in epithelial cells, reactivating during immunosuppression. Choice D is correct because this mirrors latent episomal maintenance with reactivation, like in herpesviruses. Choice B is incorrect because episomal DNA indicates eukaryotic viral latency, not bacterial lysogeny. To approach similar questions, assess whether the viral strategy aligns with the host cell conditions and the viral replication phase described. Identify latency markers like episomes and triggers for reactivation.
- 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?
Answer: Early transcription requires a virion-packaged RNA-dependent RNA polymerase to generate positive-sense mRNA from the incoming genome.
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.
