MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2b Prions Viroids

What this quiz covers

This quiz focuses on 2b Prions Viroids, 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.

How to use this quiz

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.

Question 1

1. X requires ribosomes to translate a capsid protein; Y requires a host chaperone to refold its protein genome into an infectious state
2. X replicates by reverse transcription into DNA; Y replicates by budding from the plasma membrane using a host-derived envelope
3. X propagates by templating conformational change of a host protein; Y accumulates via host-driven RNA copying and can trigger RNA silencing (correct answer)
4. X forms only after neuronal death releases proteases; Y forms only after plant wilting releases nucleases that circularize RNA

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.

Question 2

1. Progressive accumulation of protease-resistant aggregates that impair synaptic function and cellular organization before cell death (correct answer)
2. Production of double-stranded DNA intermediates that integrate into neuronal chromosomes and activate interferon signaling
3. Neuronal symptoms that directly cause misfolding of the host protein, making prion formation a consequence rather than a driver
4. Nuclear replication of a circular RNA genome that is processed into siRNAs targeting neuronal mRNAs

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.

Question 3

1. Accumulation of protease-resistant host proteins that seed further misfolding in neighboring cells
2. Production of small RNAs derived from the circular RNA that guide sequence-specific suppression of host mRNAs, altering growth-related cellular organization (correct answer)
3. Insertion of the circular RNA into host chromosomes, causing stable loss of rRNA transcription
4. 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.

Question 4

1. Reduced synaptic function due to progressive sequestration and misfolding of host proteins, without a requirement for nucleic acid replication (correct answer)
2. Increased expression of viral capsid proteins that assemble into new infectious particles in the cytosol
3. Neurodegeneration primarily driven by small circular RNA genomes that are processed into siRNAs to silence neuronal mRNAs
4. Formation of misfolded protein aggregates as a downstream consequence of neuronal death, rather than a contributing cause

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.

Question 5

1. 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 (correct answer)
2. Reverse transcription of the circular RNA into DNA followed by integration into the plant genome as a provirus
3. Propagation by inducing conformational conversion of a host protein into a protease-resistant isoform
4. Assembly of a protein capsid around the circular RNA to enable receptor-mediated entry into adjacent cells

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.

Question 6

1. 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 (correct answer)
2. Prions and viroids both require translation of their genomes into structural proteins to spread between cells
3. Prions replicate by host RNA polymerase in the nucleus, while viroids replicate by converting host proteins into protease-resistant conformations
4. Prion aggregates form only after widespread cell death, so they are markers rather than drivers of cellular dysfunction, unlike viroids

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.

Question 7

1. Prions act by templating a conformational change in a host protein, promoting aggregation that disrupts cellular organization and function (correct answer)
2. Prions primarily impair cells by encoding a polymerase that replicates their RNA genome and triggers RNA interference
3. Prion infectivity depends on a small circular RNA genome that is processed into siRNAs in neurons
4. Prion aggregates appear only after neurons degenerate, indicating aggregation is an effect of pathology rather than a contributor

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.

Question 8

1. Prion propagation depends on the presence of the host prion protein substrate, so lowering its abundance reduces aggregate formation and downstream cellular dysfunction (correct answer)
2. Prion replication depends on copying a circular RNA genome, so lowering host prion protein expression should not change aggregation
3. Synaptic loss triggers formation of infectious prions de novo, so preserved synapses in Line 2 should increase prion accumulation
4. Prion infectivity requires assembly of capsid proteins; reduced host prion protein expression prevents capsid formation and blocks spread

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.

Question 9

1. Enhanced production of viral capsid proteins that bud from lipid rafts, increasing detergent-resistant membrane fractions.
2. Templated misfolding and aggregation of a host membrane-associated protein, leading to disrupted membrane microdomain organization and trafficking. (correct answer)
3. RNA-guided cleavage of host mRNAs by a circular RNA genome, directly generating detergent-resistant membranes as a defense response.
4. Membrane microdomain disruption that precedes and causes prion formation, making protein stabilization ineffective.

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.

Question 10

1. Prion infectivity requires host expression of a convertible protein substrate, consistent with templated misfolding rather than nucleic acid replication. (correct answer)
2. Prion infectivity requires host RNA polymerase to replicate its circular RNA genome, explaining reduced aggregates in Line 2.
3. Synaptic marker loss causes prion formation; Line 2 is protected because it maintains synapses longer after exposure.
4. Prion propagation depends primarily on antibody neutralization of viral capsids; Line 2 is protected due to altered antigen presentation.

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.

Question 11

1. The protein-based agent exploits host protein quality control and membrane trafficking for spread; the RNA-based agent exploits nuclear RNA processing and RNA silencing pathways (correct answer)
2. The protein-based agent requires host ribosomes to translate a polymerase; the RNA-based agent requires host chaperones to refold its capsid
3. The protein-based agent replicates by copying its RNA genome in the nucleus; the RNA-based agent replicates by converting host proteins in the cytosol
4. Both agents propagate primarily by budding from the plasma membrane, differing only in host range

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, agents differ in exploiting cellular pathways for propagation. Choice A is correct because it distinguishes protein vs. RNA dependencies. Choice C is incorrect because it swaps the mechanisms. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as β-sheet shifts.

Question 12

1. They require a viral envelope to fuse with the plasma membrane and deliver a polymerase for replication
2. They replicate by synthesizing complementary DNA that inserts into the genome, altering ion channel transcription
3. They are circular RNAs that are diced into siRNAs to directly change membrane potential
4. They drive a self-propagating protein misfolding process that perturbs membrane trafficking and ion homeostasis (correct answer)

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 affect neuronal membranes and potentials. Choice D is correct because it describes self-propagating misfolding disrupting homeostasis. Choice C is incorrect because prions are not circular RNAs. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette like nuclease persistence.

Question 13

1. The brain agent propagates via protein conformation templating; the plant agent propagates as a noncoding circular RNA amplified by host enzymes (correct answer)
2. Both agents require host ribosomes to translate their genomes, but only the plant agent is sensitive to RNase because it is single-stranded
3. The brain agent is an RNA virus without a capsid; the plant agent is a prion that is resistant to proteases
4. The brain agent forms only after neurodegeneration begins; the plant agent forms only after leaf chlorosis damages RNA

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, the brain agent is a prion and plant agent a viroid based on stability. Choice A is correct because it links propagation to protein templating vs. RNA copying. Choice C is incorrect because it swaps the identities. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as denaturant effects.

Question 14

1. Delayed onset neurodegeneration due to progressive accumulation of misfolded host protein aggregates that disrupt cellular organization (correct answer)
2. Acute febrile illness driven by rapid replication of an RNA genome and strong interferon response
3. Immediate neurologic symptoms that trigger formation of the misfolded protein as a secondary effect
4. Plant-like gene silencing in neurons due to a circular RNA genome processed into siRNAs

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 from contaminated instruments cause delayed neurodegeneration. Choice A is correct because it describes progressive aggregate accumulation. Choice B is incorrect because prions lack RNA genomes. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as decontamination failures.

Question 15

1. A forms only after neurons die and release lysosomal enzymes; B forms only after leaves senesce and fragment rRNA
2. A increases by translating a polyprotein that self-cleaves; B increases by translating a polymerase from its circular RNA
3. A increases by integration into neuronal DNA; B increases by budding from the nuclear membrane as enveloped particles
4. A increases by templated conversion of host protein conformation; B increases by host-mediated RNA copying that can generate RNA silencing triggers (correct answer)

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, particle A is prion-like and B viroid-like in propagation. Choice D is correct because it compares templated conversion to RNA copying. Choice B is incorrect because neither translates polymerases. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as lack of polymerases.

Question 16

1. Insertion of viral glycoproteins into the plasma membrane followed by budding of enveloped virions
2. Translation of viroid-encoded proteins that inhibit chloroplast ribosomes
3. Templated misfolding of a host membrane protein leading to protease-resistant aggregates
4. RNA interference pathways processing viroid-derived RNA into small RNAs that guide suppression of host mRNAs (correct answer)

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, viroids affect potato development through RNA interference. Choice D is correct because it links viroid-derived small RNAs to host mRNA suppression. Choice C is incorrect because it confuses viroids with prion misfolding. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette like RNA transmission.

Question 17

1. Host RNA polymerase can amplify the circular RNA, and RNA silencing normally limits accumulation by processing replicative RNA into small RNAs (correct answer)
2. The agent's capsid is required for nuclear import, and reduced Dicer activity prevents capsid disassembly, increasing replication
3. Symptoms worsen because reduced Dicer activity causes host proteins to misfold into prion conformers
4. The circular RNA is translated into a polymerase; reduced Dicer increases translation efficiency and thereby increases replication

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, viroids accumulate more with reduced Dicer, worsening symptoms. Choice A is correct because it explains host amplification and silencing limitation. Choice B is incorrect because viroids lack capsids. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as small RNA changes.

Question 18

1. Increased host protein conversion to a β-sheet–rich isoform that templates further misfolding and disrupts membrane-associated trafficking (correct answer)
2. Insertion of a viral envelope into neuronal membranes, leading to budding and release of infectious particles
3. Primary neuronal injury that induces de novo synthesis of the misfolded protein as a downstream stress response
4. Replication of a small circular RNA genome in the nucleus followed by RNA interference–mediated silencing of synaptic genes

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 were shown to disrupt neuronal cell function through protein aggregation, as evidenced by β-sheet–rich isoforms and nuclease-resistant infectivity. Choice A is correct because it accurately describes prion-induced neurodegeneration as supported by the vignette. Choice B is incorrect because it confuses prion mechanisms with viral budding processes. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as resistance to nucleases indicating prions.

Question 19

1. They assemble into capsids that perforate membranes during viral egress
2. They integrate into the host genome and alter transcription of ion channel genes
3. They are circular RNAs that recruit Dicer to silence genes controlling membrane permeability
4. They template misfolding of a host protein, promoting aggregation and perturbing membrane trafficking and homeostasis (correct answer)

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 alter neuronal membrane dynamics through misfolded protein accumulation. Choice D is correct because it describes prion templating and aggregation disrupting membranes. Choice B is incorrect because prions do not integrate into genomes. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette like nuclease resistance.

Question 20

1. Host neurodegeneration that produces the protease-resistant fragment as a consequence, making the agent non-causal
2. Replication of a circular RNA genome in neuronal nuclei followed by siRNA-mediated silencing of motor genes
3. Integration of a DNA intermediate that activates oncogenes in neurons
4. Self-propagating conformational conversion of a host protein leading to aggregate accumulation and neurodegeneration (correct answer)

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 drive pathology in mice via protein conversion. Choice D is correct because it describes templated misfolding and aggregation. Choice B is incorrect because prions lack RNA genomes. Ensure understanding of prion vs. viroid mechanisms; look for context clues in vignette such as vacuolation.