A retrovirus is engineered with a mutated integrase that can bind viral DNA but cannot catalyze strand transfer into host DNA. In infected cells, reverse-transcribed viral DNA is detected, and viral proteins are produced for a short period, but the signal rapidly declines over subsequent cell divisions. No integrated proviral DNA is detected.
Based on the vignette, which conclusion is most consistent with retroviral integration?
MCAT Biological and Biochemical Foundations of Living Systems Quiz
Practice 2b Viral Genetics Transduction Retroviruses 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.
This quiz focuses on 2b Viral Genetics Transduction Retroviruses, 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.
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.
A retrovirus is engineered with a mutated integrase that can bind viral DNA but cannot catalyze strand transfer into host DNA. In infected cells, reverse-transcribed viral DNA is detected, and viral proteins are produced for a short period, but the signal rapidly declines over subsequent cell divisions. No integrated proviral DNA is detected.
Based on the vignette, which conclusion is most consistent with retroviral integration?
Explanation: This question tests understanding of integrase function in retroviral persistence. The mutant integrase can bind DNA but cannot catalyze the strand transfer reaction needed for chromosomal insertion. Without integration, the reverse-transcribed viral DNA remains episomal (unintegrated) in the nucleus, where it can be transiently transcribed but is not replicated during cell division. The correct answer A accurately explains that this unintegrated DNA is diluted through cell divisions and eventually lost or degraded. Answer C incorrectly claims integration is required for reverse transcription, when the vignette clearly shows DNA is made without functional integrase. Answer B incorrectly invokes bacterial transduction in a mammalian cell context. A critical principle is that retroviral DNA must integrate to establish a stable provirus that replicates with host chromosomes.
A research team tracks the replication of an enveloped retrovirus in permissive mammalian cells. They observe: (1) early synthesis of viral DNA from an RNA template, (2) later detection of viral DNA in the nucleus, (3) persistent viral DNA integrated into host chromosomes, and (4) production of viral proteins followed by release of new enveloped virions from the plasma membrane. A drug blocks the viral protease, and virions are still released but are poorly infectious.
Which outcome would be expected during the viral replication cycle described when the protease is inhibited?
Explanation: This question tests understanding of retroviral protease function in the viral life cycle. Retroviruses initially produce polyproteins that must be cleaved by viral protease to generate mature, functional viral proteins necessary for infectivity. The vignette shows that protease inhibition still allows virion budding but reduces infectivity, indicating that improperly processed polyproteins are packaged into particles. The correct answer B accurately describes this outcome where virions still form and bud but contain uncleaved polyproteins that compromise their ability to infect new cells. Answer A incorrectly claims protease is needed for reverse transcription, while C incorrectly invokes bacterial transduction which is unrelated to retroviruses. A key principle is that retroviral protease acts late in the replication cycle to process structural proteins, not early steps like reverse transcription or integration.
Researchers study generalized transduction using a lytic bacteriophage that infects two E. coli strains: Strain 1 is Lac+ (functional lactose operon) and Strain 2 is Lac− due to a deletion in lacZ. The phage is first grown on Strain 1, then the resulting phage lysate is used to infect Strain 2. After infection, bacteria are plated on minimal medium with lactose as the only carbon source; a small number of colonies grow. The investigators confirm these colonies are still Strain 2 by an unrelated chromosomal marker but now express β-galactosidase.
Which statement best describes the process of viral transduction in the scenario?
Explanation: This question tests understanding of generalized transduction, a mechanism where bacteriophages accidentally package random fragments of host bacterial DNA instead of viral DNA. In generalized transduction, during lytic infection of Strain 1, some phage particles mistakenly encapsidate pieces of the Strain 1 chromosome (including the functional lacZ gene) into phage heads. When these transducing particles infect Strain 2, they inject bacterial DNA rather than viral DNA, and this DNA can recombine with the recipient chromosome through homologous recombination, restoring lacZ function. The correct answer B accurately describes this random packaging and subsequent recombination. Answer A incorrectly describes specialized transduction involving prophage excision, while D incorrectly invokes reverse transcription which is a retroviral mechanism not used by bacteriophages. A key check for generalized transduction is that any bacterial gene can potentially be transferred, not just genes adjacent to a prophage insertion site.
A retroviral vector is engineered to deliver a therapeutic gene into dividing human cells. The vector retains long terminal repeats (LTRs) and the packaging signal but lacks functional genes for viral structural proteins. Producer cells supply the missing structural proteins in trans, allowing formation of viral particles that can infect target cells once. After infection, the therapeutic gene is detected in host genomic DNA for months.
Based on the vignette, which conclusion is most consistent with retroviral integration?
Explanation: This question tests understanding of retroviral integration for gene therapy applications. Retroviral vectors exploit the natural retroviral mechanism where reverse transcriptase converts the RNA genome to DNA, which then integrates into host chromosomes via integrase (using the LTRs as integration signals). The vignette describes a replication-defective vector that can undergo one round of infection, reverse transcription, and integration, but cannot produce new virions without helper proteins. The correct answer A accurately explains that long-term persistence results from chromosomal integration of the reverse-transcribed DNA. Answer B incorrectly invokes repeated lytic cycles which cannot occur with this defective vector, while D incorrectly mentions bacterial transduction which is irrelevant to mammalian cells. A key principle for retroviral vectors is that stable integration enables persistent gene expression even through many cell divisions.
A temperate phage integrates into the bacterial chromosome at a specific attachment site. Upon induction, a small fraction of phage particles carry a bacterial gene located immediately adjacent to the integration site, but not genes located far away. Which statement best describes the process of viral transduction in this scenario?
Explanation: This question tests understanding of viral transduction mechanisms in bacteria, distinguishing specialized from generalized transduction. Specialized transduction occurs in temperate phages when imprecise excision captures bacterial genes adjacent to the integration site, limiting transfer to nearby loci. In this vignette, a temperate phage integrates at a specific site, and upon induction, transfers only adjacent genes, not distant ones. Choice B is consistent because it describes imprecise excision enabling specialized transduction of nearby genes. A common distractor, choice A, fails by misapplying generalized transduction's random transfer, a misconception as proximity to the integration site biases gene capture. In evaluating transduction types, assess gene location relative to phage site: adjacency suggests specialized. Additionally, confirm the phage is temperate, as lytic phages typically perform generalized transduction.
A lytic phage infects bacteria at high multiplicity of infection. Some phage particles produced are noninfectious but can still deliver bacterial DNA to new cells. When these particles infect recipients, the introduced DNA is detected briefly but is lost unless it recombines into the chromosome. Which statement best describes the process of viral transduction in this scenario?
Explanation: This question tests understanding of viral transduction mechanisms in bacteria, particularly the fate of transduced DNA in recipients. In generalized transduction, delivered bacterial DNA is transient unless it recombines homologously into the recipient's chromosome for stability. In this vignette, noninfectious phage particles deliver DNA that is detected briefly but lost without recombination. Choice A is consistent because it emphasizes the need for recombination for stable genetic alteration, aligning with transduction. A common distractor, choice C, fails by assuming all transduced DNA acts as plasmids, a misconception as bacterial fragments lack replication origins. To reason through similar cases, check if DNA stability requires recombination, indicating transduction over plasmid transfer. Also, note if particles are noninfectious, suggesting they carry host DNA instead of full phage genomes.
A retrovirus enters a cell and must access the host genome to establish long-term infection. In nondividing cells, long-term infection is markedly reduced compared to dividing cells in this experimental system. Which outcome would be expected during the viral replication cycle described?
Explanation: This question tests understanding of retroviral replication and nuclear access for integration. Integration requires nuclear entry, often facilitated in dividing cells by membrane breakdown, reducing efficiency in nondividing cells. In this vignette, long-term infection decreases in nondividing cells. Choice D is consistent because it links reduced access to limited provirus formation. A common distractor, choice C, fails due to the misconception that RT depends on replication, as RT is cytoplasmic. Assess cell cycle effects on long-term outcomes for integration dependency. Differentiate from viruses entering nuclei independently of division.
Researchers compare two conditions for a retrovirus: Condition 1 uses a drug that blocks viral entry; Condition 2 uses a drug that blocks reverse transcriptase after entry. In Condition 2, viral proteins are not detected at late time points. Which outcome would be expected during the viral replication cycle described?
Explanation: This question tests understanding of retroviral replication and inhibitor effects on steps. Blocking reverse transcriptase prevents DNA synthesis, halting integration and late gene expression from provirus. In this vignette, post-entry RT inhibition eliminates late proteins. Choice D is consistent because it links RT block to prevented integration and output. A common distractor, choice B, fails due to the misconception of direct RNA translation, ignoring DNA requirement. Map inhibitors to cycle points for outcomes. Compare with entry blocks affecting all steps.
A temperate phage integrates into a bacterial chromosome and remains latent. Later, UV exposure induces the prophage, leading to excision and production of new phage particles. A subset of these particles carry a bacterial toxin gene that was adjacent to the integration site. Which statement best describes the process of viral transduction in this scenario?
Explanation: This question tests understanding of viral transduction mechanisms in bacteria, focusing on specialized transduction triggered by prophage induction. Specialized transduction arises from imprecise prophage excision, incorporating adjacent bacterial genes into phage particles for transfer. In this vignette, UV induction of a temperate phage leads to particles carrying an adjacent toxin gene. Choice B is consistent because it describes imprecise excision capturing the nearby gene, hallmark of specialized transduction. A common distractor, choice A, fails by attributing it to random packaging in generalized transduction, a misconception as induction from lysogeny favors site-specific errors. When analyzing induction-related transduction, check for gene adjacency to integration sites. Also, differentiate from lytic cycles where random errors predominate.
A retrovirus enters a cell, uncoats, and begins synthesizing DNA from its RNA genome. Investigators then detect a DNA intermediate that is double-stranded and later find viral sequences embedded within host chromosomal DNA. Which outcome would be expected during the viral replication cycle described?
Explanation: This question tests understanding of retroviral replication, including reverse transcription and integration. Retroviruses synthesize dsDNA from RNA via reverse transcriptase, followed by integrase-mediated insertion into host chromosomes for stable maintenance. In this vignette, post-entry DNA synthesis leads to a dsDNA intermediate and subsequent chromosomal viral sequences. Choice B is consistent because it outlines RT followed by integration, explaining the observed DNA forms. A common distractor, choice A, fails due to the misconception that RNA integrates directly, ignoring the obligatory DNA intermediate. For similar replication timelines, confirm DNA detection precedes integration evidence. Also, contrast with DNA viruses that lack RT steps.
A temperate bacteriophage normally integrates into a bacterial chromosome and can later excise to enter a lytic cycle. A mutant phage is engineered to lack the gene required for integration. When this mutant infects bacteria, it can still replicate lytically and lyse cells, but it does not form stable lysogens. Researchers ask whether this mutant can still mediate transfer of bacterial genes to new bacteria.
Which statement is most consistent with the transduction outcome expected for this mutant phage?
Explanation: This question tests understanding of how prophage integration enables specialized transduction. Specialized transduction occurs when integrated prophages excise imprecisely, carrying adjacent bacterial genes. A phage that cannot integrate cannot form prophages and thus cannot perform specialized transduction. However, generalized transduction (random DNA packaging during lytic infection) remains possible. The correct answer A recognizes that specialized transduction requires integration and would be reduced. Answer B incorrectly states generalized transduction requires integration (it does not), while C incorrectly suggests lack of integration increases specialized transduction. The key distinction is that specialized transduction is prophage-dependent while generalized transduction occurs during any lytic infection through packaging errors.
A retroviral infection is initiated in cultured cells, and viral cDNA is detected inside cells within hours. However, long-term viral gene expression is observed only in cells that pass through mitosis during the first day after infection. Investigators conclude that nuclear envelope breakdown during mitosis facilitates a key step. Based on the vignette, which conclusion is most consistent with retroviral integration?
Explanation: This question tests understanding of retroviral integration and cell cycle requirements. Some retroviruses (like MLV) require nuclear envelope breakdown during mitosis for their pre-integration complex to access chromosomes, while others (like HIV) can traverse intact nuclear pores. The observation that only mitotic cells show long-term expression indicates this virus requires mitosis for nuclear entry and subsequent integration. The correct answer A explains this cell cycle-dependent integration, while option B incorrectly claims reverse transcription requires mitosis. Option C confuses retroviral and bacteriophage mechanisms, and option D incorrectly suggests direct RNA integration. The key principle is that integration accessibility can be a rate-limiting step for some retroviruses, explaining why they preferentially infect dividing cells.
A lytic bacteriophage is propagated on a donor bacterial strain that carries a plasmid encoding ampicillin resistance (AmpR). The phage lysate is used to infect a recipient strain lacking the plasmid. After infection, some recipient colonies become AmpR. However, when researchers isolate DNA from AmpR recipients, the ampicillin-resistance gene is found integrated into the bacterial chromosome rather than as a plasmid. Which statement best describes the process of viral transduction in this scenario?
Explanation: This question tests understanding of generalized transduction and chromosomal integration of transduced DNA. During generalized transduction, phages can package random fragments of donor DNA, including portions of plasmids. When this DNA is injected into recipients, it must recombine with the chromosome to be stably maintained (since it lacks a plasmid origin of replication). The correct answer B explains that the Amp^R gene was delivered via phage and integrated by homologous recombination, while option A incorrectly suggests intact plasmid transfer. Option C incorrectly attributes resistance to phage genes, and option D incorrectly invokes reverse transcription of plasmid RNA. The key insight is that transduced DNA fragments lacking replication origins must integrate into the chromosome for stable inheritance, distinguishing this from plasmid transformation.
A retrovirus with an RNA genome infects a population of dividing human cells. In one condition, a small-molecule inhibitor of integrase is added immediately after viral entry; in another condition, the inhibitor is added 24 hours later. Only the early-addition condition markedly reduces the number of cells with long-term viral gene expression. Which outcome would be expected during the viral replication cycle described?
Explanation: This question tests understanding of the temporal sequence of retroviral replication steps and the role of integrase. After viral entry, reverse transcription converts viral RNA to DNA, followed by integration into the host chromosome via integrase. Early integrase inhibition prevents the viral DNA from integrating, resulting in unstable episomal DNA that is diluted or degraded over time, reducing long-term expression. The correct answer A explains this mechanism, while option B incorrectly suggests integrase affects capsid uncoating. Option C incorrectly claims integrase affects translation of the RNA genome, and option D misunderstands integrase function as excision rather than integration. The key principle is that integration must occur soon after reverse transcription for stable provirus formation, making early inhibition more effective than late inhibition.
A lab compares two outcomes after exposing bacteria to a phage lysate prepared from a donor strain carrying a functional toxin gene (tox+). In Condition 1, the phage is known to be strictly lytic. In Condition 2, the phage is temperate and can lysogenize recipients. After infection, tox+ recipients are recovered in both conditions, but only Condition 2 recipients show stable maintenance of tox+ even without selection.
Which statement is most consistent with the viral mechanism or outcome discussed?
Explanation: This question tests understanding of lysogenic conversion versus transient gene transfer. In Condition 2, the temperate phage can establish lysogeny by integrating as a prophage, and if this prophage carries the tox gene, it confers a stable new trait that is replicated with the bacterial chromosome. This explains why tox+ is stably maintained without selection. The correct answer A accurately describes this lysogenic conversion mechanism. In contrast, Condition 1 with a strictly lytic phage can only achieve transient gene transfer through generalized transduction, requiring recombination for stability. Answer B incorrectly assigns retroviral mechanisms to bacteria, while D incorrectly suggests plasmid maintenance for what is clearly chromosomal integration. A key principle is that lysogenic conversion provides stable inheritance of new traits through prophage integration, distinguishing it from transient transduction.
A bacteriophage infecting a bacterial population is used to move an antibiotic-resistance gene (ampR) between strains. After infection, some recipient bacteria become AmpR, but only when the recipients are recombination-proficient. When recipients lack homologous recombination functions, AmpR colonies are not recovered, even though phage adsorption and DNA injection still occur.
Which statement best describes the process of viral transduction in the scenario?
Explanation: This question tests understanding of generalized transduction and the requirement for homologous recombination. In generalized transduction, phage particles accidentally package random fragments of donor bacterial DNA (including ampR) instead of phage DNA. When these transducing particles inject this DNA into recipients, it exists as linear DNA that must recombine with the recipient chromosome to be stably maintained and replicated. The correct answer B accurately describes this process and explains why recombination-deficient recipients cannot establish stable ampR. Answer A incorrectly suggests prophage integration which would not require host recombination, while D incorrectly invokes retroviral mechanisms in bacteria. A critical check for generalized transduction is that the transferred DNA is bacterial in origin and requires the recipient's recombination machinery for chromosomal integration.
A retrovirus infects epithelial cells. Investigators measure viral products over time and find: viral RNA enters the cell first; shortly after, a DNA form of the viral genome appears; later, integrated viral DNA is detected; finally, viral mRNA and proteins increase. A student proposes that integration happens before the DNA form appears.
Which outcome would be expected during the viral replication cycle described if the student's proposed order were correct?
Explanation: This question tests understanding of the temporal sequence in retroviral replication, specifically that reverse transcription must precede integration. The correct retroviral sequence is: RNA entry → reverse transcription to DNA → integration → transcription of viral mRNA. The student's proposal that integration happens before DNA synthesis is impossible because integrase requires a DNA substrate to insert into chromosomes. If the student were correct, we would observe integrated viral sequences before any DNA intermediate exists, which is biochemically impossible. Answer A correctly identifies this logical impossibility. Answer C incorrectly invokes bacterial transduction in a retroviral context, while D misunderstands reverse transcriptase function. A fundamental check for retroviral biology is that you cannot integrate RNA directly into DNA chromosomes - the RNA must first be converted to DNA.
A retrovirus with an RNA genome infects cultured human T cells. Within hours, investigators detect a DNA copy of the viral genome in the cytoplasm. Two days later, they detect viral DNA integrated at multiple chromosomal sites, and viral mRNA is transcribed using host RNA polymerase II. When the viral integrase gene is inactivated, viral DNA is still synthesized but stable chromosomal integration is not detected.
Based on the vignette, which conclusion is most consistent with retroviral integration?
Explanation: This question tests understanding of retroviral integration mechanisms and the role of integrase enzyme. Retroviruses use reverse transcriptase to synthesize a DNA copy of their RNA genome in the cytoplasm, then this DNA must integrate into host chromosomes for stable, long-term expression. The vignette shows that when integrase is inactivated, viral DNA is still made (confirming reverse transcription occurs) but integration fails, demonstrating that integrase is specifically required for chromosomal insertion. The correct answer A accurately describes this integrase-dependent integration process that enables persistent viral gene expression. Answer C incorrectly suggests host polymerase can replicate RNA directly, while D reverses the correct order by claiming integration precedes reverse transcription. A critical check for retroviral biology is that reverse transcription (RNA→DNA) must occur before integration, and integrase is essential for stable chromosomal insertion.
To compare specialized versus generalized transduction, investigators use a temperate phage that integrates at a specific site near the gal operon in E. coli. Rarely, induction of the lysogen produces phage particles that carry gal DNA and can transfer it to a gal− recipient, generating gal+ transductants. Sequencing of the transferred bacterial DNA shows it is consistently derived from the chromosomal region adjacent to the original prophage insertion site.
Which statement best describes the process of viral transduction in the scenario?
Explanation: This question tests understanding of specialized transduction, where temperate phages transfer specific bacterial genes adjacent to their integration site. In specialized transduction, when a lysogenic phage is induced to enter the lytic cycle, imprecise excision can occur where the prophage takes adjacent bacterial DNA (like gal genes) while leaving behind some viral DNA. The resulting defective phage particles carry this specific bacterial DNA to new hosts where it can recombine and confer the gal+ phenotype. The correct answer B accurately describes this imprecise excision mechanism that consistently transfers genes from the prophage insertion site region. Answer A incorrectly describes generalized transduction with random gene transfer, while D wrongly suggests the phage genome itself encodes bacterial metabolic genes. A diagnostic feature of specialized transduction is that the same chromosomal region is consistently transferred, reflecting the specific prophage integration site.
A retrovirus is engineered to carry a GFP reporter but lacks genes required for producing new viral particles. Human cells are exposed to the virus along with a helper system that supplies missing viral proteins in trans. After infection, some cells show stable GFP expression for many weeks, even after the helper system is removed. Based on the vignette, which conclusion is most consistent with retroviral integration?
Explanation: This question tests understanding of retroviral replication and integration into host genomes. Retroviruses reverse transcribe their RNA genome into DNA, which integrates into the host chromosome as a provirus, allowing stable inheritance and expression during cell division. In this vignette, a replication-defective retrovirus carrying GFP infects human cells with a helper system, resulting in stable GFP expression persisting for weeks even after helper removal. Choice B is consistent because it explains the conversion to DNA and integration, enabling heritable GFP expression in dividing cells without new virion production. A common distractor, choice A, fails due to the misconception that retroviral RNA is directly translated long-term without integration, ignoring that stable expression requires chromosomal insertion. For similar retroviral scenarios, confirm if stable expression aligns with integration versus transient episomal DNA. Also, assess if the outcome depends on cell division, as unintegrated DNA dilutes over time.