Viral Genetics, Transduction, and Retroviruses (2B)
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MCAT Biological and Biochemical Foundations of Living Systems › Viral Genetics, Transduction, and Retroviruses (2B)
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?
Without integrase-catalyzed insertion, viral DNA fails to become a stable provirus and is diluted or degraded as cells divide
Without integrase, the virus will compensate by packaging random host DNA and transferring it to other bacteria via transduction
Without integrase, the virus will still integrate because host DNA ligase can directly splice viral RNA into chromosomal DNA
Without integrase, reverse transcription cannot occur because integration is required to generate the DNA template
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?
No viral DNA will be produced because protease is required for reverse transcription of the RNA genome
Virions will bud from the membrane but will contain improperly processed polyproteins, reducing infectivity of progeny particles
Viral integration will increase because protease inhibition enhances integrase activity and promotes chromosomal insertion
The virus will switch to generalized transduction, packaging random host DNA fragments instead of viral genomes
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?
The phage reverse-transcribes its RNA genome into DNA that integrates into Strain 2, restoring lacZ expression from the provirus
Some phage particles mistakenly package random fragments of Strain 1 chromosomal DNA, which recombine into the Strain 2 chromosome
A prophage excises imprecisely from Strain 1 and carries adjacent lac genes into Strain 2 after lysogeny is established
Strain 2 acquires a plasmid encoding lacZ that replicates independently after phage infection and is maintained without recombination
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?
Persistence is most consistent with generalized transduction, in which bacterial host DNA is packaged and transferred between bacteria
Long-term persistence of the therapeutic gene is most consistent with integration of a DNA copy into host chromosomes following reverse transcription
Persistence occurs because the incoming RNA genome is replicated directly by host DNA polymerase without a DNA intermediate
Long-term persistence is best explained by repeated lytic cycles that continually reinfect the same target cells to maintain gene expression
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?
The phage is performing retroviral integration, converting bacterial mRNA into DNA and inserting it near the attachment site.
The phage is performing conjugation, transferring adjacent DNA through a pilus formed during lysogeny.
The phage is performing specialized transduction due to imprecise excision, which can capture adjacent bacterial genes.
The phage is performing generalized transduction, which preferentially transfers genes near the attachment site.
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?
The delivered DNA is maintained as a plasmid because all phage-delivered DNA contains a bacterial origin of replication.
The delivered DNA must integrate via viral integrase, which is packaged in the noninfectious particles.
The delivered DNA is translated immediately into proteins, and these proteins permanently change the recipient genotype.
The recipients become genetically altered only if the delivered bacterial DNA undergoes homologous recombination, consistent with generalized transduction.
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?
Reduced long-term infection indicates the virus is performing generalized transduction, which requires bacterial cell division.
Reduced long-term infection indicates reverse transcription occurs only after host DNA replication, so DNA cannot form in nondividing cells.
Reduced long-term infection indicates the virus cannot bind receptors on nondividing cells, because receptors are expressed only during S phase.
Reduced long-term infection is consistent with decreased access to host chromosomal DNA for integration in this system, limiting stable provirus formation.
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?
Blocking reverse transcriptase after entry prevents viral binding to host receptors, so no capsid proteins should be detectable early either.
Blocking reverse transcriptase after entry has no effect because retroviruses translate their RNA genome immediately into all viral proteins.
Blocking reverse transcriptase after entry prevents formation of viral DNA, thereby preventing integration and subsequent production of viral mRNA and proteins.
Blocking reverse transcriptase after entry increases generalized transduction by forcing the virus to package host DNA instead of viral DNA.
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?
The toxin gene is transferred because imprecise prophage excision can capture neighboring bacterial DNA, consistent with specialized transduction.
The toxin gene is transferred because random bacterial DNA fragments are packaged during lytic growth, a hallmark of generalized transduction.
The toxin gene is transferred because the phage reverse transcribes bacterial mRNA into DNA and inserts it into recipients.
The toxin gene is transferred because UV causes targeted mutations that convert recipients into toxin producers without DNA transfer.
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?
Integration occurs first, followed by reverse transcription of the integrated RNA into DNA as the final step of replication.
After reverse transcription, integrase-mediated insertion of viral DNA into host chromosomes enables stable maintenance of viral sequences.
Host DNA is packaged into viral capsids and transferred to other cells, explaining chromosomal viral sequences as transduction products.
Viral RNA is directly integrated into host DNA without conversion to DNA, so a DNA intermediate would not be expected.
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.