Subviral Particles: Prions and Viroids (2B)
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MCAT Biological and Biochemical Foundations of Living Systems › Subviral Particles: Prions and Viroids (2B)
A comparative microbiology course lab examines two subviral particles. Sample X retains infectivity after RNase and DNase treatment but loses infectivity after strong denaturation that disrupts protein conformation. Sample Y loses infectivity after RNase but is unaffected by protease treatment and shows no detectable protein by mass spectrometry. In host tissue, X is associated with progressive neurodegeneration, while Y is associated with reduced plant vigor and altered gene expression.
Which statement best distinguishes replication/propagation between these subviral particles within cells?
X requires ribosomes to translate a capsid protein; Y requires a host chaperone to refold its protein genome into an infectious state
X replicates by reverse transcription into DNA; Y replicates by budding from the plasma membrane using a host-derived envelope
X forms only after neuronal death releases proteases; Y forms only after plant wilting releases nucleases that circularize RNA
X propagates by templating conformational change of a host protein; Y accumulates via host-driven RNA copying and can trigger RNA silencing
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins causing diseases by inducing misfolding in normal proteins, while viroids are small RNA particles affecting plants. In the vignette, sample X represents prions with protein-based infectivity, and Y represents viroids with RNA-based propagation. Choice C is correct because it distinguishes prion templating from viroid RNA copying and silencing. Choice B is incorrect because it misattributes replication mechanisms to the wrong particles. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as treatment sensitivities.
A neurology clinic evaluates subviral particles in patients with progressive ataxia and cognitive decline. Brain biopsy homogenate shows seeding activity in a real-time conversion assay that increases with serial dilution, consistent with self-propagating conformational templating. Treatment with UV light (to damage nucleic acids) does not reduce seeding, but treatment with a chaotropic agent that unfolds proteins abolishes activity. Early pathology shows synaptic loss and gliosis without prominent inflammatory infiltrates.
Which outcome would be expected from prion accumulation in affected brain regions?
Production of double-stranded DNA intermediates that integrate into neuronal chromosomes and activate interferon signaling
Nuclear replication of a circular RNA genome that is processed into siRNAs targeting neuronal mRNAs
Neuronal symptoms that directly cause misfolding of the host protein, making prion formation a consequence rather than a driver
Progressive accumulation of protease-resistant aggregates that impair synaptic function and cellular organization before cell death
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins causing diseases by inducing misfolding in normal proteins, while viroids are small RNA particles affecting plants. In the vignette, prions cause synaptic loss via protein templating in brain tissue. Choice A is correct because it captures prion aggregate impairment of neuronal function. Choice D is incorrect because it describes viroid RNA replication, not prions. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as seeding assays.
A horticulture team detects a subviral particle in cucumber plants showing mosaic-like leaf patterns and reduced fruit set. The agent is a small, circular RNA with extensive intramolecular base pairing and no open reading frames. In infected plants, Dicer-like enzyme activity is elevated and a subset of host transcripts involved in cell wall expansion are decreased, while total rRNA levels are unchanged. Based on the vignette, which cellular consequence is most consistent with viroid infection?
Insertion of the circular RNA into host chromosomes, causing stable loss of rRNA transcription
Accumulation of protease-resistant host proteins that seed further misfolding in neighboring cells
Production of small RNAs derived from the circular RNA that guide sequence-specific suppression of host mRNAs, altering growth-related cellular organization
Assembly of new capsid proteins in the nucleus that package the circular RNA for export
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Viroids are circular RNAs that replicate in plants and can trigger RNA silencing pathways affecting host gene expression. The vignette describes a circular RNA with no coding capacity, elevated Dicer activity, and decreased cell wall expansion transcripts. Choice B is correct because it accurately describes how viroids produce small RNAs that guide sequence-specific suppression of host mRNAs, altering cellular organization. Choice A describes prion mechanisms inappropriate for RNA agents. Choice C incorrectly suggests chromosomal integration. When analyzing viroid infections, look for RNA silencing mechanisms and altered host gene expression patterns without direct protein coding.
In a mouse neurodegeneration study, investigators inoculate two groups with brain homogenate from affected animals. Group 1 receives untreated homogenate; Group 2 receives the same homogenate after nuclease treatment that degrades DNA and RNA. Both groups develop progressive ataxia and neuronal vacuolization over weeks. In cultured neurons exposed to the homogenate, immunostaining shows intracellular aggregates of a host-encoded protein that becomes increasingly protease-resistant over time. Which outcome would be expected from prion accumulation in these neurons, based on the findings?
(Assume the infectious agent is a subviral particle.)
Increased expression of viral capsid proteins that assemble into new infectious particles in the cytosol
Neurodegeneration primarily driven by small circular RNA genomes that are processed into siRNAs to silence neuronal mRNAs
Formation of misfolded protein aggregates as a downstream consequence of neuronal death, rather than a contributing cause
Reduced synaptic function due to progressive sequestration and misfolding of host proteins, without a requirement for nucleic acid replication
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins that cause disease by inducing normal proteins to misfold, while viroids are small RNA particles affecting plants. In the vignette, the infectious agent survives nuclease treatment (ruling out nucleic acid-based agents) and causes protease-resistant protein aggregates, characteristic of prions. Choice A is correct because it accurately describes prion pathology: progressive protein misfolding and sequestration disrupts synaptic function without requiring nucleic acid replication. Choice B is incorrect because prions don't produce viral capsid proteins. Choice C incorrectly describes viroids (RNA-based plant pathogens) rather than prions. Choice D reverses causality - prion aggregates cause neurodegeneration, not vice versa.
A greenhouse outbreak causes stunting and leaf curling in tomato plants. Extracts from symptomatic plants remain infectious after protease treatment but lose infectivity after RNase treatment. No virion-like particles are observed, and sequencing reveals a small, circular, noncoding RNA. Infected leaf tissue shows increased accumulation of 21–24 nt small RNAs that map to the circular RNA sequence. Based on the vignette, what is most consistent with viroid replication in the infected plant cells?
(Assume the infectious agent is a subviral particle.)
Template-directed RNA synthesis by host RNA polymerase using the circular RNA as a template, with processing into small RNAs that can guide RNA silencing
Assembly of a protein capsid around the circular RNA to enable receptor-mediated entry into adjacent cells
Propagation by inducing conformational conversion of a host protein into a protease-resistant isoform
Reverse transcription of the circular RNA into DNA followed by integration into the plant genome as a provirus
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Viroids are small circular RNA molecules that infect plants and replicate using host machinery, while prions are misfolded proteins affecting animals. In the vignette, the agent is sensitive to RNase but not protease, contains circular noncoding RNA, and generates small RNAs - all characteristic of viroids. Choice A is correct because viroids replicate through rolling-circle replication using host RNA polymerase, producing RNA intermediates that are processed into small interfering RNAs. Choice B is incorrect as viroids don't undergo reverse transcription. Choice C describes prion replication, not viroid replication. Choice D is wrong because viroids lack protein-coding capacity and don't form capsids.
Researchers compare two subviral agents in cell-free and cellular systems. Agent X retains infectivity after nuclease treatment but is reduced by harsh protein-denaturing conditions; in neurons it correlates with protease-resistant host-protein aggregates. Agent Y loses infectivity after RNase treatment; in plants it correlates with abundant 21–24 nt small RNAs complementary to the agent’s sequence and no detectable protein-coding capacity. Which statement most accurately describes the impact of prions on cellular function relative to viroids in these systems?
Prions replicate by host RNA polymerase in the nucleus, while viroids replicate by converting host proteins into protease-resistant conformations
Prion aggregates form only after widespread cell death, so they are markers rather than drivers of cellular dysfunction, unlike viroids
Prions and viroids both require translation of their genomes into structural proteins to spread between cells
Prions primarily disrupt cellular function by templating misfolding and aggregation of a host protein, whereas viroids primarily perturb gene expression via RNA-based silencing pathways
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins causing disease through protein aggregation, while viroids are RNA molecules affecting plants through RNA interference. The vignette describes Agent X (nuclease-resistant, protein-sensitive, causes protein aggregates) as a prion and Agent Y (RNase-sensitive, generates small RNAs) as a viroid. Choice A is correct because it accurately distinguishes prion pathology (protein misfolding/aggregation) from viroid pathology (RNA-based gene silencing). Choice B is incorrect as neither agent requires genome translation. Choice C reverses the mechanisms. Choice D misrepresents prion aggregates as consequences rather than causes of dysfunction.
A comparative experiment evaluates two subviral particles. In preparation A, infectivity is retained after RNase treatment but reduced after protease treatment; infected neurons accumulate protease-resistant aggregates of a host protein. In preparation B, infectivity is lost after RNase treatment but retained after protease treatment; infected plant cells show abundant 21–24 nt small RNAs mapping to the agent sequence. Which statement most accurately describes the impact of prions on cellular function as supported by preparation A?
Prions primarily impair cells by encoding a polymerase that replicates their RNA genome and triggers RNA interference
Prions act by templating a conformational change in a host protein, promoting aggregation that disrupts cellular organization and function
Prion aggregates appear only after neurons degenerate, indicating aggregation is an effect of pathology rather than a contributor
Prion infectivity depends on a small circular RNA genome that is processed into siRNAs in neurons
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins that template conformational changes in host proteins, while viroids are RNA molecules affecting plants. Preparation A shows RNase resistance, protease sensitivity, and protein aggregates - characteristic of prions. Choice A is correct because it accurately describes prion mechanism: templating conformational changes in host proteins, leading to aggregation that disrupts cellular function. Choice B incorrectly attributes RNA polymerase activity to prions. Choice C wrongly suggests prions have RNA genomes. Choice D misrepresents aggregation as an effect rather than cause of pathology.
In a prion exposure model, two neuronal lines are compared. Line 1 expresses normal levels of the host prion protein; Line 2 has markedly reduced expression of the host prion protein. After identical exposure to infectious material, Line 1 develops protease-resistant aggregates and synaptic loss, while Line 2 shows minimal aggregation and preserved synaptic markers. Which outcome would be expected from prion accumulation and best explains the difference between the lines?
Prion propagation depends on the presence of the host prion protein substrate, so lowering its abundance reduces aggregate formation and downstream cellular dysfunction
Prion infectivity requires assembly of capsid proteins; reduced host prion protein expression prevents capsid formation and blocks spread
Prion replication depends on copying a circular RNA genome, so lowering host prion protein expression should not change aggregation
Synaptic loss triggers formation of infectious prions de novo, so preserved synapses in Line 2 should increase prion accumulation
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins that require normal host prion protein as substrate for propagation. The vignette shows that reducing host prion protein expression (Line 2) protects against aggregate formation and synaptic loss compared to normal expression (Line 1). Choice A is correct because prion propagation absolutely requires host prion protein as substrate - reducing its abundance limits the material available for conversion to the misfolded form, thereby reducing aggregation and cellular dysfunction. Choice B incorrectly suggests prions have RNA genomes. Choice C reverses causality. Choice D wrongly attributes capsid formation to prions.
A neuroscience group examines how a subviral particle affects cellular organization. Neurons exposed to a prion-containing preparation show increased detergent-resistant membrane fractions and mislocalization of a glycosylphosphatidylinositol (GPI)-anchored host protein from the cell surface to intracellular aggregates. These changes occur without detectable viral nucleic acids and persist despite nucleases, but are prevented by compounds that stabilize the host protein’s native conformation. Which outcome would be expected from prion accumulation in this context?
Membrane microdomain disruption that precedes and causes prion formation, making protein stabilization ineffective.
Enhanced production of viral capsid proteins that bud from lipid rafts, increasing detergent-resistant membrane fractions.
RNA-guided cleavage of host mRNAs by a circular RNA genome, directly generating detergent-resistant membranes as a defense response.
Templated misfolding and aggregation of a host membrane-associated protein, leading to disrupted membrane microdomain organization and trafficking.
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions are misfolded proteins that template the conversion of normal proteins into pathogenic forms, often affecting membrane-associated proteins. The vignette describes prion effects on neurons, including detergent-resistant membrane fractions and mislocalization of a GPI-anchored protein into aggregates. Choice B is correct because it accurately describes templated misfolding and aggregation of host membrane proteins leading to disrupted membrane organization and trafficking. Choice A is incorrect because prions do not produce viral capsid proteins. The prevention of these changes by protein-stabilizing compounds confirms the protein misfolding mechanism, and the absence of viral nucleic acids rules out conventional viral mechanisms.
In a prion-focused cell study, two neuronal lines are compared: Line 1 expresses a normal level of a GPI-anchored host protein implicated in prion propagation; Line 2 has markedly reduced expression of the same host protein. After exposure to identical prion-containing inocula, Line 1 develops abundant insoluble aggregates and progressive loss of synaptic marker staining, while Line 2 shows minimal aggregation and preserved synaptic markers. Which conclusion is most consistent with these results regarding the subviral particle’s dependence on cellular components?
Synaptic marker loss causes prion formation; Line 2 is protected because it maintains synapses longer after exposure.
Prion propagation depends primarily on antibody neutralization of viral capsids; Line 2 is protected due to altered antigen presentation.
Prion infectivity requires host expression of a convertible protein substrate, consistent with templated misfolding rather than nucleic acid replication.
Prion infectivity requires host RNA polymerase to replicate its circular RNA genome, explaining reduced aggregates in Line 2.
Explanation
This question tests understanding of subviral particles: prions and viroids, and their role in cellular organization. Prions require a host protein substrate (often $PrP^C$) that can be converted to the misfolded form; without this substrate, prions cannot propagate. The vignette shows that Line 2, with reduced expression of the GPI-anchored host protein, resists prion infection. Choice A is correct because it accurately describes prion dependence on host expression of a convertible protein substrate for templated misfolding. Choice B is incorrect because prions do not have RNA genomes or require RNA polymerase. The differential susceptibility based on host protein expression levels is a hallmark of prion biology, confirming that the infectious agent requires the host protein as a substrate for propagation.