MCAT Biological and Biochemical Foundations of Living Systems Quiz: 2b Virus Structure Classification

What this quiz covers

This quiz focuses on 2b Virus Structure Classification, 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. Non-enveloped, double-stranded DNA virus that replicates in the nucleus using host DNA polymerase
2. Enveloped, positive-sense single-stranded RNA virus that requires an RdRP for genome replication (correct answer)
3. Non-enveloped, positive-sense single-stranded RNA virus whose capsid protects it from detergent disruption
4. Enveloped, double-stranded RNA virus that is resistant to detergents but sensitive to extracellular RNase

Explanation: This question tests the ability to classify viruses based on structural features and replication strategy. Virus classification relies on key characteristics: presence/absence of envelope (lipid bilayer), genome type (DNA vs RNA, single vs double-stranded), and replication mechanism. The passage data shows Virus X loses infectivity with detergent (Triton X-100), indicating an enveloped virus whose lipid bilayer is disrupted. RNase only works after detergent treatment, confirming the genome is protected by the envelope and is RNA-based. The RdRP inhibitor blocking replication indicates the virus must synthesize RNA from an RNA template, characteristic of positive-sense ssRNA viruses. Choice B correctly identifies all these features, while choice A incorrectly suggests DNA genome, choice C incorrectly states non-enveloped (contradicting detergent sensitivity), and choice D incorrectly combines envelope presence with RNase sensitivity of intact virions. When classifying viruses experimentally, always check: detergent sensitivity (envelope), nuclease sensitivity patterns (genome type), and polymerase requirements (replication strategy).

Question 2

1. Non-enveloped double-stranded DNA virus that replicates in the nucleus using host DNA polymerase
2. Enveloped RNA virus whose genome is protected within a lipid envelope and capsid prior to entry (correct answer)
3. Non-enveloped RNA virus that is ethanol-resistant due to a stable protein capsid
4. Enveloped double-stranded DNA virus that requires host nuclear DNA synthesis to replicate

Explanation: This question tests the ability to classify viruses based on structural features and genome type using experimental data. Virus classification relies on key characteristics: presence/absence of an envelope (lipid bilayer), genome type (DNA vs RNA), and replication strategy. The passage data shows that Virus X loses infectivity with ethanol (indicating an envelope), contains RNA (becomes RNase-sensitive after detergent disrupts the envelope), and doesn't require host DNA synthesis (ruling out DNA viruses dependent on S-phase). These findings point to an enveloped RNA virus whose genome is protected by both the envelope and capsid until detergent treatment exposes it. Choice A incorrectly describes a non-enveloped DNA virus, contradicting both the ethanol sensitivity and RNase data. A key strategy is to systematically evaluate each experimental result against the structural features claimed in each answer choice.

Question 3

1. Bacterium with an outer membrane and plasmid DNA
2. Non-enveloped virus with genome exposed on the surface
3. Prion lacking nucleic acid
4. Enveloped virus with a protein capsid protecting its genome (correct answer)

Explanation: The skill being tested is using treatment assays to classify virus structure for MCAT. Enveloped viruses protect genomes via lipid and capsid, detergent exposes to enzymes, protease targets surface proteins. Detergent greatly reduces infectivity, protease moderately, RNase only post-detergent, indicating envelope and internal capsid. Answer D is correct as this pattern shows envelope-mediated protection with protein involvement. Distractor B fails by suggesting exposed genome, ignoring RNase resistance intact, a protection misconception. Verify by sequential treatments. Strategize by assessing enzyme access pre/post-disruption.

Question 4

1. Virus A requires endosomal acidification for membrane fusion; Virus B does not rely on a lipid envelope (correct answer)
2. Virus A is acid-stable due to its envelope; Virus B is acid-labile due to its capsid
3. Virus B must use reverse transcriptase because it lacks an envelope
4. Virus B enters only by membrane fusion at the plasma membrane because it lacks spikes

Explanation: The skill being tested is understanding how virus structure influences entry mechanisms and pathogenicity in MCAT biological contexts. Virus classification involves envelope status (affecting solvent sensitivity and entry pathways) and capsid features (like spikes for fusion). The passage details Virus A as ether-sensitive with a membrane and spikes, requiring normal endosomal pH for plaques, while Virus B is ether-resistant with only a protein shell and unaffected by pH changes. The correct answer A is supported because Virus A's envelope necessitates low-pH endosomal fusion, while Virus B's non-enveloped nature allows pH-independent entry, directly linking structure to pathogenicity. A distractor like B fails by reversing acid stability, misconstruing envelopes as protective against acid when they often confer lability. To verify, compare solvent sensitivity with entry inhibitors. Reason by correlating structural assays (EM, solvents) with functional assays (plaque formation) for entry mechanisms.

Question 5

1. Classify as a positive-sense RNA virus because polymerase activity is only produced after translation in host cells
2. Classify as a negative-sense RNA virus because the virion packages an RdRP detectable without new protein synthesis (correct answer)
3. Classify as a DNA virus because cycloheximide blocks host ribosomes needed for viral DNA replication
4. Classify by capsid symmetry because polymerase detection uniquely identifies helical nucleocapsids

Explanation: This question tests classification based on polymerase packaging patterns. Virus classification distinguishes positive-sense from negative-sense RNA viruses by whether they package polymerase in virions. The passage shows polymerase activity in purified virions without new synthesis (pre-packaged enzyme) but no polymerase accumulation when translation is blocked immediately after infection. This pattern is diagnostic of negative-sense RNA viruses, which must package RdRP because their genome cannot be translated directly. Choice B correctly identifies this classification method. Choice A incorrectly suggests positive-sense (which don't package polymerase), choice C incorrectly identifies DNA virus, and choice D incorrectly links polymerase detection to capsid symmetry rather than genome polarity. The key principle: negative-sense RNA viruses must package functional polymerase for initial transcription, while positive-sense RNA viruses synthesize polymerase after translation of their genome.

Question 6

1. A lipid envelope promotes survival in acidic environments, increasing fecal–oral transmission
2. A stable protein capsid supports environmental and GI stability, facilitating fecal–oral transmission (correct answer)
3. A segmented genome enables survival in bile salts, facilitating fecal–oral transmission
4. Surface peptidoglycan protects against chloroform, facilitating fecal–oral transmission

Explanation: This question tests understanding of how viral structure influences transmission routes and environmental stability. Virus classification reveals that non-enveloped viruses with stable protein capsids are particularly suited for fecal-oral transmission due to their resistance to harsh environmental conditions. The data shows Virus Z survives low pH (stomach acid), bile salts (intestinal detergents), and chloroform (lipid solvent), all indicating a non-enveloped structure confirmed by electron microscopy showing only a protein shell. This robust capsid enables survival through the gastrointestinal tract and in the environment between hosts. Choice A incorrectly claims lipid envelopes promote acid stability when they actually make viruses more fragile. Choice D mentions peptidoglycan, a bacterial cell wall component not found in viruses. The key insight is that fecal-oral transmission selects for structurally stable, non-enveloped viruses.

Question 7

1. A rigid protein capsid alone mediates receptor binding, so glycosidase increases infectivity by exposing binding sites
2. Envelope glycoproteins (and their glycans) mediate attachment/entry, so disrupting the envelope or glycosylation reduces infectivity (correct answer)
3. Viral peptidoglycan is the major antigenic determinant, so chloroform reduces infectivity by dissolving the cell wall
4. Host antibodies primarily neutralize by degrading viral genomes extracellularly, so glycosidase has no effect on infectivity

Explanation: This question tests understanding of how viral envelope glycoproteins mediate pathogenicity through attachment and entry. Virus classification recognizes that enveloped viruses use glycoproteins embedded in their lipid bilayer for host cell recognition and entry. The data shows antibody blocks a surface glycoprotein reducing binding, glycosidase treatment (removing sugar modifications) reduces infectivity, and chloroform (disrupting lipids) destroys infectivity - all pointing to an enveloped virus whose glycoproteins are critical for infection. The glycan modifications on these proteins often determine tissue tropism and immune evasion. Choice A incorrectly describes a non-enveloped virus and misinterprets glycosidase effects. Choice C mentions peptidoglycan, a bacterial component not found in viruses. The key insight is that envelope glycoproteins and their post-translational modifications are primary determinants of viral pathogenicity.

Question 8

1. Positive-sense single-stranded RNA virus (correct answer)
2. Negative-sense single-stranded RNA virus
3. Double-stranded DNA virus
4. Satellite virus requiring a helper virus for replication

Explanation: The skill being tested is classifying RNA viruses by genome sense and replication autonomy in MCAT biochemistry. Positive-sense ssRNA viruses have genomes acting as mRNA, directly producing proteins and progeny without added enzymes, unlike others needing polymerases. The passage shows purified RNA alone yields proteins and particles, with host pol II inhibition not affecting early synthesis, indicating direct translation. Answer A fits as +ssRNA viruses like poliovirus replicate cytoplasmically without host transcription. Distractor B fails for -ssRNA, requiring polymerase, misconstruing autonomy as negative-sense trait. Verify by transfecting naked genome and monitoring protein production. Reason by assessing if genome functions immediately as mRNA.

Question 9

1. Enveloped RNA virus requiring an RNA-dependent RNA polymerase (correct answer)
2. Non-enveloped DNA virus requiring host RNA polymerase II
3. Prion composed solely of protein
4. Enveloped DNA virus requiring reverse transcriptase

Explanation: The skill being tested is virus classification via inhibitor responses and structural features for MCAT biological foundations. Viruses are classified by genome (requiring specific polymerases) and envelope (conferring chloroform sensitivity and helical nucleocapsids). The virus is chloroform-sensitive with helical nucleocapsid, unaffected by actinomycin D but blocked by RdRp inhibitor, indicating RNA genome independent of host transcription. Answer A fits as enveloped RNA viruses like paramyxoviruses use viral RdRp for mRNA, matching inhibitor data. Distractor B errs by suggesting host RNA pol II dependence, ignoring actinomycin D insensitivity, a misconception for DNA viruses. Confirm by assaying polymerase inhibitors' effects on replication. Reason by excluding classifications mismatched with structural (EM) and functional (inhibitor) evidence.

Question 10

1. Non-enveloped double-stranded DNA virus
2. Enveloped positive-sense single-stranded RNA virus (correct answer)
3. Enveloped double-stranded RNA virus
4. Non-enveloped negative-sense single-stranded RNA virus

Explanation: The skill being tested is the classification of viruses based on their structural features and genomic properties as per MCAT biological foundations. Viruses are classified by envelope presence (affecting detergent sensitivity), genome type (DNA vs RNA, strandedness), and RNA sense (positive vs negative, determining direct translatability). In this passage, Virus X shows detergent sensitivity indicating an envelope, acid stability consistent with some enveloped viruses, and direct translation of its nucleic acid into a polyprotein using only ribosomes, pointing to RNA that functions as mRNA. The correct answer B follows from the data because enveloped viruses are detergent-sensitive, and positive-sense ssRNA genomes can be directly translated without additional polymerases, matching the cell-free system's requirements. A distractor like A fails by assuming a dsDNA genome, misconstruing the lack of need for DNA-dependent RNA polymerase and direct polyprotein production as DNA-based replication. A transferable check is to assess detergent effects for envelopes and translation assays for RNA sense. Additionally, reason by eliminating options that mismatch genome functionality with experimental outcomes.

Question 11

1. Antibodies block attachment by degrading the lipid bilayer through enzymatic activity
2. Internal capsid proteins mediate attachment because they are closest to the genome
3. Detergent sensitivity indicates attachment is mediated by exposed RNA interacting with receptors
4. Envelope-associated surface proteins mediate attachment; loss of these proteins reduces binding and infection (correct answer)

Explanation: This question tests understanding of virus structure and classification, specifically how enveloped viruses utilize surface proteins for host cell attachment. Viruses are classified based on genome type, presence of an envelope, and structural components, with enveloped viruses featuring a lipid bilayer derived from the host cell that is sensitive to detergents and often studded with viral glycoproteins essential for infection. The passage describes Virus JJ as a detergent-sensitive RNA virus, indicating it is enveloped, and provides data on antibody blocking and protease treatment affecting attachment. The correct answer, A, follows from the data because antibodies against surface proteins block attachment and protease removal of envelope proteins reduces binding, demonstrating these proteins mediate host cell interaction and thus pathogenicity through infection efficiency. A common distractor like B fails due to the misconception that internal capsid proteins, rather than exposed envelope glycoproteins, are primarily responsible for attachment, ignoring that capsid proteins are shielded in enveloped viruses. To verify similar questions, always cross-reference experimental outcomes, such as sensitivity to treatments, with structural features like envelope presence. A useful strategy is to eliminate choices contradicting data, ensuring the explanation aligns with observed effects on viral entry.

Question 12

1. Non-enveloped DNA virus with an exposed genome
2. Enveloped RNA virus with genome protected inside a capsid (correct answer)
3. Enveloped DNA virus with reverse transcription
4. Non-enveloped RNA virus whose genome is exposed on the surface

Explanation: The skill being tested is determining virus envelope and genome via protection assays for MCAT classification. Enveloped viruses have lipid bilayers protecting internal capsids and genomes from external enzymes until disrupted. Virus Q's nucleic acid is nuclease-resistant intact but RNase-sensitive post-detergent, with infectivity detergent-labile but protease-resistant intact, suggesting enveloped RNA. Answer B is correct as envelopes shield RNA genomes inside capsids from nucleases, matching degradation patterns. Distractor D errs by implying exposed RNA on non-enveloped surface, ignoring protection until disruption, a capsid misconception. Check nuclease effects pre/post-disruption for genome type/protection. Strategize by using detergent to distinguish enveloped from non-enveloped.

Question 13

1. Negative-sense single-stranded RNA virus (correct answer)
2. Positive-sense single-stranded RNA virus
3. Single-stranded DNA virus
4. Double-stranded DNA virus

Explanation: The skill being tested is classifying RNA viruses by replication timing and enzymes for MCAT. Negative-sense ssRNA viruses delay mRNA via RdRp-dependent transcription, unlike immediate positive-sense. Virus CC has delayed mRNA, virion RdRp, transfection failure, indicating -ssRNA. Answer A fits as RdRp synthesizes mRNA from template, explaining delay and transfection. Distractor B errs for +ssRNA, directly translatable, misconstruing delay. Check mRNA kinetics and transfection. Reason by evaluating enzyme packaging and genome functionality.

Question 14

1. Enveloped virus whose lipid membrane contributes to buoyant density and infectivity (correct answer)
2. Non-enveloped virus with two different capsid symmetries
3. Prion that separates by protein density only
4. Bacteriophage classified by tail length rather than lipid content

Explanation: The skill being tested is using biophysical methods to classify enveloped viruses for MCAT. Enveloped viruses have lower density due to lipids, essential for infectivity. Virus II separates into lipid-rich low-density infectious and high-density non-infectious, detergent eliminates low-density/infectivity. Answer A is correct as lipid envelope contributes density and function. Distractor B fails by suggesting two symmetries, misconstruing density peaks. Verify with density and treatment. Strategize by correlating fractions with components/infectivity.

Question 15

1. Detergent sensitivity indicates the virus is non-enveloped and relies on capsid pores for entry
2. Helical capsids prevent membrane fusion, so UV treatment should increase infectivity
3. Envelope glycoproteins can drive fusion independent of genome integrity, enabling entry steps even when replication is blocked (correct answer)
4. Bullet shape implies the genome is double-stranded DNA and must enter the nucleus for transcription

Explanation: The skill being tested is relating virus structure to entry and replication in pathogenicity for MCAT. Enveloped viruses use glycoproteins for fusion, separable from genome integrity, unlike non-enveloped relying on capsids. Virus R is bullet-shaped, helical, detergent-sensitive, and UV-inactivated particles still fuse membranes, indicating envelope-mediated entry. Answer C is correct as envelopes enable fusion via proteins, independent of nucleic acid, influencing pathogenicity. Distractor B fails by claiming helical capsids prevent fusion, misconstruing shape as inhibitory when it describes nucleocapsid. Verify by testing inactivated virions in fusion assays. Reason by separating structural functions from replicative ones.

Question 16

1. Viral surface proteins embedded in a lipid envelope mediate receptor binding; disrupting proteins reduces attachment (correct answer)
2. Host phospholipids directly bind sialic acid; protease increases binding by exposing lipids
3. Budding indicates a non-enveloped virus exiting by cell lysis
4. Neuraminidase treatment reduces attachment by degrading viral RNA

Explanation: The skill being tested is understanding enveloped virus attachment mechanisms in pathogenicity for MCAT. Enveloped viruses embed glycoproteins in lipid bilayers for receptor binding, with budding incorporating host lipids. Virus W buds with host phospholipids, neuraminidase reduces attachment, protease disrupts without genome harm. Answer A is correct as envelope proteins bind sialic acid, protease sensitivity confirms protein-mediated attachment. Distractor D errs by claiming neuraminidase degrades RNA, misconstruing enzyme specificity. Verify by treating virions/cells separately. Strategize by using enzyme treatments to identify binding components.

Question 17

1. Virus V is likely enveloped, making it detergent- and desiccation-sensitive; Virus U is likely non-enveloped and more stable (correct answer)
2. Virus U is likely enveloped because envelopes protect against drying; Virus V is non-enveloped because detergents target capsids
3. Both viruses are necessarily DNA viruses because they persist on surfaces
4. Virus V must be a prion because detergents reduce infectivity

Explanation: The skill being tested is how structure affects environmental stability and transmission in pathogenicity for MCAT. Enveloped viruses are sensitive to drying and detergents due to lipid fragility, unlike stable non-enveloped. Virus V loses infectivity upon drying/detergent, while Virus U retains it, indicating structural differences. Answer A is correct as envelopes confer sensitivity, reducing persistence and altering pathogenicity. Distractor B reverses this, misconstruing envelopes as protective against drying. Check persistence assays for stability. Reason by linking physical treatments to structural vulnerabilities.

Question 18

1. RNA virus that depends on an RdRp during replication (correct answer)
2. DNA virus that uses host RNA polymerase II to replicate its genome
3. Double-stranded DNA virus that packages reverse transcriptase in the virion
4. Prion-like infectious protein lacking a nucleic acid genome

Explanation: This question tests the ability to determine viral genome type through nuclease protection assays and replication inhibitor studies. Virus classification fundamentally distinguishes RNA viruses from DNA viruses based on genome composition and replication strategy. The intact virion's resistance to both nucleases indicates the genome is protected within the capsid, while post-disruption RNase sensitivity (but not DNase sensitivity) definitively identifies an RNA genome. The 95% reduction with RdRp inhibitor confirms this RNA virus uses RNA-dependent RNA polymerase for replication, a hallmark of RNA viruses. Choice B incorrectly suggests a DNA virus, contradicting the RNase sensitivity data. Choice C describes a retrovirus scenario not supported by the data. A key principle is that RNA viruses must encode their own RNA polymerase since host cells lack the machinery to replicate RNA genomes.

Question 19

1. Enveloped virus with detergent-sensitive lipid membrane that shields protein components from protease until disrupted (correct answer)
2. Non-enveloped virus whose capsid is dissolved by detergent but protected from protease by a polysaccharide capsule
3. Enveloped virus whose genome is directly degraded by trypsin unless detergent is present
4. Non-enveloped virus that is detergent-resistant; the data indicate Triton X-100 inactivates cells rather than virions

Explanation: This question tests the ability to interpret protease protection assays in virus classification. Virus classification uses the principle that enveloped viruses have lipid membranes shielding internal proteins from proteases until the membrane is disrupted. The data shows Triton X-100 (a detergent) reduces infectivity alone, indicating membrane dependence, while trypsin only works after detergent treatment, confirming proteins are normally protected by the envelope. This pattern is diagnostic for enveloped viruses. Choice B incorrectly describes non-enveloped viruses as having polysaccharide capsules, a bacterial feature. Choice D misinterprets the data by claiming cellular rather than viral effects. The key strategy is recognizing that sequential treatment (detergent then protease) specifically tests for envelope-mediated protection of viral proteins.

Question 20

1. Classify as enveloped vs. non-enveloped using solvent sensitivity and fusion dependence (correct answer)
2. Classify using Gram staining to determine the composition of the outer layer
3. Classify by presence of a 70S ribosome and sensitivity to translation inhibitors
4. Classify by whether the virus forms endospores under nutrient limitation

Explanation: This question tests the ability to use experimental approaches to classify viruses based on structural features. Virus classification fundamentally distinguishes enveloped from non-enveloped viruses using their differential sensitivity to lipid solvents and entry mechanisms. The data shows ether (a lipid solvent) destroys infectivity, indicating an envelope, while the membrane fusion requirement and independence from clathrin-mediated endocytosis suggest direct fusion at the plasma membrane. These are classic markers for enveloped virus classification. Choice B incorrectly suggests Gram staining, a bacterial technique irrelevant to viruses. Choices C and D describe bacterial features (ribosomes, endospores) absent in viruses. The key principle is that enveloped viruses are uniquely sensitive to lipid solvents and often use membrane fusion for entry.