Viral Life Cycles, Replication Strategies (2B)
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MCAT Biological and Biochemical Foundations of Living Systems › Viral Life Cycles, Replication Strategies (2B)
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
UV exposure prevents viral replication by blocking capsid assembly, explaining the reduced cell density
The viral genome remains episomal and is translated directly by host ribosomes without replication
The virus establishes a lysogenic state with a prophage that can be induced to enter the lytic cycle by DNA damage
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.
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?
The mutant phage is forced into latency because it cannot package its genome without the repressor
Loss of the repressor prevents adsorption to host receptors, reducing infection and causing culture clearing
The repressor is required for host DNA replication, so its loss triggers host cell cycle arrest and lysis
Loss of the repressor favors immediate entry into lytic replication rather than maintenance of lysogeny
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.
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?
dsRNA intermediates indicate lysogeny; reducing them should increase proviral integration
Reduced dsRNA sensing would be expected to decrease antiviral signaling and allow more efficient viral replication
Preventing dsRNA accumulation would block host DNA replication, indirectly increasing viral yield
Higher titers require more dsRNA sensing because innate signaling directly catalyzes virion assembly
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.
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?
This is consistent with bacterial transformation, where cells take up naked DNA released by phage budding
This is consistent with lysogeny, where host DNA is packaged only after prophage integration into the recipient genome
This is consistent with transduction, where phage can mediate horizontal gene transfer between bacteria
This is consistent with conjugation, where bacteria transfer DNA via a pilus encoded by the phage
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.
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?
Loss of the integration site biases infection toward the lytic pathway because stable lysogeny cannot be established
Deletion of att prevents genome replication, so lysis must be caused by host apoptosis unrelated to the phage
Loss of the integration site forces the phage into lysogeny because excision is prevented
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.
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?
Polyprotein processing is required to generate functional viral proteins necessary for assembly and productive infection
Protease loss prevents viral genome integration into host DNA, blocking transcription of viral mRNAs
Polyproteins are translated only in the nucleus; protease mutation would be rescued by nuclear import inhibitors
Protease loss increases capsid stability, preventing uncoating and thereby increasing viral RNA abundance
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.
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?
Suppression of phage replication because high multiplicity favors bacterial sporulation over viral assembly.
Immediate virion production followed by host lysis, consistent with a lytic replication program.
Conversion of viral RNA to DNA by reverse transcriptase prior to genome replication.
Long-term survival with prophage maintenance due to integration at a specific attachment site.
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.
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?
Positive-sense RNA viruses require integration into the host genome before any viral proteins can be produced.
Positive-sense RNA can function as mRNA upon entry, whereas negative-sense RNA requires a viral polymerase to generate translatable mRNA.
Negative-sense RNA is directly translated by host ribosomes, while positive-sense RNA must first be copied into DNA in the nucleus.
Both positive- and negative-sense RNA require host DNA-dependent RNA polymerase II to transcribe viral mRNA from RNA templates.
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).
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?
Stable lysogen formation with prophage maintenance, with lytic replication suppressed until an inducing stress occurs.
Integration of viral RNA into the bacterial chromosome via integrase after reverse transcription in the cytosol.
Immediate lysis because nutrient limitation increases ATP availability for phage assembly.
Productive infection only if host RNA polymerase II is inhibited, shifting transcription to viral genes.
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.
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?
UV primarily increases host ribosome synthesis, which directly causes capsid assembly without viral genome replication.
The phage genome is maintained as a prophage during initial growth and UV triggers induction into a lytic program.
The phage immediately lyses host cells, and UV exposure is required for adsorption to the bacterial surface.
The phage replicates only by binary fission with the host chromosome, so extracellular phage should not increase after UV.
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.