- A viral preparation is treated with an organic solvent, resulting in a 99.9% loss of infectivity. However, when the solvent-treated virions are incubated with a non-ionic detergent and purified viral glycoproteins, a significant portion of infectivity is restored. Which conclusion is most consistent with these findings?
Answer: The virus is enveloped, and its infectivity is critically dependent on its surface glycoproteins for host cell interaction.
The sensitivity to an organic solvent (like ether or chloroform) strongly indicates the presence of a lipid envelope, as these solvents dissolve lipids. The loss of infectivity implies the envelope is essential. The restoration of infectivity upon adding back purified viral glycoproteins demonstrates that these proteins, which are embedded in the envelope, are the key components required for initiating infection (e.g., attachment and entry). The detergent likely helps to properly orient the glycoproteins with the nucleocapsid. Distractor A is incorrect because organic solvents target lipids, not proteins directly, and the virus is enveloped. Distractor B is incorrect because while lipids are involved, the viral glycoproteins are the specific mediators of fusion, as shown by the restoration experiment. Distractor D is plausible but doesn't explain why adding back glycoproteins restores function; it focuses on nucleocapsid integrity, which wasn't addressed.
- Virus A is a non-enveloped virus with a dsDNA genome, while Virus B is an enveloped virus with a -ssRNA genome. Which prediction regarding their environmental stability and transmission is most accurate?
Answer: Virus A is more likely to be transmitted via contaminated inanimate objects (fomites) due to the resistance of its capsid to desiccation.
Non-enveloped viruses (like Virus A) have robust protein capsids that are resistant to drying (desiccation), detergents, and acidic conditions. This makes them highly stable in the environment and well-suited for transmission via fomites or the fecal-oral route. Enveloped viruses (like Virus B) have a fragile lipid envelope that is easily disrupted by detergents, solvents, acid, and drying, making them less stable environmentally and typically requiring transmission through more direct means (e.g., respiratory droplets, bodily fluids). Therefore, A and D are incorrect. C is incorrect because while ethanol is a good disinfectant, its primary mode of action against enveloped viruses is dissolving the lipid envelope, and some non-enveloped viruses are notoriously resistant to ethanol.
- A novel antiviral drug is a potent inhibitor of the viral protease found in a complex dsDNA virus. This virus assembles an immature procapsid containing scaffolding proteins, which are then cleaved by the protease to allow for DNA packaging and capsid maturation. In cells infected with this virus and treated with the drug, what would be the most likely observation?
Answer: Empty, immature procapsids accumulate, and no infectious progeny are produced.
The drug inhibits the protease required for maturation. Therefore, the viral life cycle will be arrested at the step just before protease activity. The structural proteins and scaffolding proteins would assemble into a procapsid, but the protease-dependent cleavage required for maturation and DNA packaging would be blocked. This leads to the accumulation of empty, non-infectious procapsids. Distractor A describes the effect of a protease inhibitor on a virus that uses a polyprotein strategy (like many RNA viruses), not the maturation of a complex dsDNA virus capsid. The genome would not be degraded (B); it simply wouldn't be packaged. Virions would not be infectious or fully assembled (C).
- If a host cell were co-infected with Influenza A virus (-ssRNA, enveloped, segmented) and Vesicular Stomatitis Virus (VSV) (-ssRNA, enveloped, non-segmented), which of the following describes a potential 'pseudotype' virion that could be produced through phenotypic mixing?
Answer: A virion containing the VSV genome enclosed within a capsid and envelope whose surface glycoproteins are influenza hemagglutinin (HA) and neuraminidase (NA).
Phenotypic mixing is a phenomenon where the genome of one virus is packaged within the structural proteins (capsid and/or envelope) of another virus coinfecting the same cell. A 'pseudotype' specifically refers to a virion core from one virus acquiring the surface proteins (which determine tropism) from another. Choice A correctly describes this: the genome and core are from VSV, but the envelope glycoproteins that mediate host cell entry are from Influenza virus. Choice B describes genetic reassortment, which cannot happen here as VSV is non-segmented. Choice C is incorrect because VSV is enveloped. Choice D describes complementation or polymerase sharing, not phenotypic mixing of structural components.
- Poxviruses are large dsDNA viruses that are unusual because they replicate entirely within the cytoplasm of host cells. To initiate their life cycle, what essential, virally-encoded enzyme must be packaged within the mature virion?
Answer: DNA-dependent RNA polymerase
Since poxviruses replicate in the cytoplasm, they cannot use the host cell's transcription and replication machinery, which is located in the nucleus. The first step in any viral life cycle is to produce mRNA to synthesize viral proteins. Because the poxvirus genome is DNA, it must be transcribed into mRNA. This requires a DNA-dependent RNA polymerase. As the host's enzyme is unavailable in the cytoplasm, the virus must bring its own copy in the virion to kick-start gene expression. The viral DNA-dependent DNA polymerase (A) is also required, but later, for replicating the genome; it is typically synthesized as an early gene product after initial transcription. The other enzymes (B, D) are for RNA viruses or retroviruses.
- A newly identified virus contains a single-stranded RNA genome and a virally-encoded RNA-dependent RNA polymerase (RdRp) within its mature virion. When the purified genomic RNA is introduced directly into a susceptible host cell's cytoplasm, no viral protein synthesis or replication occurs. Which of the following best describes the viral genome?
Answer: Negative-sense single-stranded RNA (-ssRNA)
The fact that purified genomic RNA is not infectious (cannot be translated) means it is not in the form of mRNA. This rules out +ssRNA (A), which would be immediately translated. The genome must be -ssRNA. For a -ssRNA virus, the packaged RdRp is essential to first transcribe the -ssRNA genome into +ssRNA (mRNA). Without the packaged enzyme, the bare -ssRNA genome is inert in the host cytoplasm. An ambisense genome (C) has both positive and negative sense regions; the positive-sense region could potentially be translated, which contradicts the observation of 'no viral protein synthesis'. A retroviral genome (D) is +ssRNA, but it requires a reverse transcriptase (an RNA-dependent DNA polymerase), not an RdRp, and it is not immediately translated.
- Which of the following statements correctly distinguishes a viroid from a satellite virus?
Answer: A viroid is an infectious agent composed solely of RNA, whereas a satellite virus possesses a genome encapsidated by a protein coat.
When distinguishing between different types of infectious agents, you need to focus on their structural composition and replication dependencies. Viroids and satellite viruses represent two fundamentally different categories of subviral particles.
A viroid is the simplest known infectious agent, consisting entirely of a short, circular, single-stranded RNA molecule with no protein coat whatsoever. These RNA molecules are highly structured with extensive secondary folding, but they contain no genes for proteins—not even capsid proteins. Satellite viruses, in contrast, do have their genetic material surrounded by a protein capsid, making them structurally more similar to conventional viruses.
Option A correctly captures this fundamental distinction: viroids are purely RNA-based infectious agents, while satellite viruses have encapsidated genomes within protein coats.
Option B is backwards—viroids cannot encode capsid proteins since they lack protein-coding genes entirely, while satellite viruses may encode their own structural proteins or rely on helper viruses depending on the specific type.
Option C reverses the replication dependencies. Viroids can replicate autonomously using the host cell's RNA polymerase II, while satellite viruses are definitionally dependent on helper viruses for replication.
Option D incorrectly describes viroid genetics—viroids contain ssRNA, not dsDNA, and satellite viruses can have various genome types (DNA or RNA, single or double-stranded).
Remember this key distinction: if you see "protein-free" or "naked RNA," think viroid. If you see "helper-dependent" with a capsid, think satellite virus.
- The principle of quasi-equivalence, proposed by Caspar and Klug, explains how large icosahedral viral capsids are built from numerous copies of an identical protein subunit. How is this achieved in a capsid with a T-number greater than 1?
Answer: The identical protein subunits occupy non-equivalent positions, forming slightly different non-covalent bonds with their neighbors.
When you encounter questions about viral capsid structure, focus on the elegant engineering problem viruses face: how to build large, stable protein shells using limited genetic information. The Caspar-Klug model of quasi-equivalence solves this puzzle brilliantly.
Option A is correct because quasi-equivalence allows identical protein subunits to adopt slightly different conformations based on their local environment within the capsid. In icosahedral capsids with T-numbers greater than 1, the same protein must function in both pentameric (5-fold) and hexameric (6-fold) arrangements. These identical subunits achieve this flexibility by forming different angles and bond strengths with their neighbors, creating "quasi-equivalent" (nearly equivalent but not identical) positions. This conformational adaptability is the key insight of the Caspar-Klug theory.
Option B is wrong because programmed frameshifting would create different proteins entirely, not the identical subunits that quasi-equivalence requires. Option C is incorrect because host chaperones don't post-translationally modify viral capsid proteins to create structural differences - the conformational flexibility is intrinsic to the protein sequence. Option D misses the point entirely - quasi-equivalence specifically explains how identical proteins build complex structures, not how different proteins work together.
Study tip: Remember that "quasi-equivalence" literally means "almost equivalent" - same protein, slightly different positions and interactions. This principle explains how viruses maximize structural complexity while minimizing genetic requirements, a hallmark of viral efficiency.
- A fundamental principle differentiating the self-assembly of helical versus icosahedral viral capsids is that the final dimensions of a helical capsid are directly determined by:
Answer: the length of the encapsidated viral nucleic acid molecule.
In helical capsid assembly, protein subunits (protomers) bind sequentially along the length of the viral genome (RNA or DNA). The assembly process terminates when the entire genome is coated. Therefore, the length of the helical nucleocapsid is directly proportional to the length of the nucleic acid. In contrast, icosahedral capsids assemble into a fixed-size, sphere-like structure whose size is determined by the number and arrangement of its capsomeres (B), not the genome length. Scaffolding proteins (A) are more common in icosahedral assembly, and while protein curvature (C) is important for both, it doesn't solely determine the final length of a helical structure.
- A host cell line is engineered with a mutation that inactivates a key component of the ESCRT (Endosomal Sorting Complexes Required for Transport) machinery. This cell line would most likely exhibit resistance to productive infection by which of the following viruses?
Answer: An enveloped virus that is released from the cell by budding from the plasma membrane.
The ESCRT machinery is a host cell system involved in membrane remodeling and scission, particularly in the formation of multivesicular bodies. Many enveloped viruses, including HIV and Ebola virus, hijack the ESCRT pathway to mediate the final 'pinching off' step of budding from the host cell membrane, which allows for the release of new virions. Therefore, inactivation of ESCRT would most directly inhibit the release of such viruses. ESCRT is not typically required for endocytic entry (A), nuclear replication (B), or bacteriophage assembly (D), which occurs in bacteria, not eukaryotic cells.
- A bacteriophage with a complex 'head-and-tail' structure experiences a mutation in the gene encoding its portal protein. This protein normally forms a dodecameric ring at one vertex of the icosahedral head. What is the most direct and immediate consequence of this mutation on the phage assembly pathway?
Answer: The viral dsDNA genome cannot be translocated into the pre-formed procapsid.
The portal protein is a critical component of the molecular motor that packages the viral DNA into the empty, pre-formed head (procapsid). It sits at a unique vertex and acts as the channel through which the DNA is actively pumped. A mutation in this protein would directly prevent DNA packaging. While the head might still form (ruling out D), it would remain empty. Failure to attach the tail (B) is a subsequent step, and failure to attach to the host (A) is a function of the tail fibers, not the portal. The most direct consequence is the failure of genome packaging.
- According to the taxonomic scheme of the International Committee on Taxonomy of Viruses (ICTV), which of the following characteristics is given the most weight when grouping viruses into a Family?
Answer: The type of nucleic acid genome and strategy for replication.
The ICTV uses a hierarchical system where the most fundamental and conserved properties define the higher taxonomic ranks like Orders and Families. The most fundamental properties of a virus are its genome composition (e.g., dsDNA, +ssRNA) and its basic replication strategy (how it produces mRNA and replicates its genome). These features are the basis of the Baltimore classification, which strongly aligns with ICTV Family-level classification. While receptor usage (A), disease pathology (B), and specific gene homology (C) are important criteria, they are typically used for classification at lower taxonomic levels (e.g., Genus, Species, or strain) as they tend to be less conserved than the overall replication strategy.
- An icosahedral viral capsid is described by a Triangulation number (T) of 4. According to the principles of quasi-equivalence, which of the following structural features is expected for this virus?
Answer: The capsid contains 12 pentameric capsomeres and 30 hexameric capsomeres.
The T-number describes the complexity of an icosahedral capsid. All icosahedral capsids have exactly 12 pentamers (pentons) at their vertices. The number of hexamers (hexons) is given by the formula 10(T-1). For T=4, the number of hexamers is 10(4-1) = 30. Therefore, the capsid is composed of 12 pentamers and 30 hexamers. Distractor A describes a T=1 capsid, the simplest icosahedron. Distractor C is incorrect because all icosahedral capsids must have 12 pentamers. Distractor D misinterprets the T-number; T=4 means each of the 20 primary triangular faces of the underlying icosahedron is subdivided into 4 smaller facets, not that it is made of 4 subunits.
- Many complex DNA viruses, such as adenoviruses and herpesviruses, utilize scaffolding proteins during capsid assembly. These proteins are essential for procapsid formation but are proteolytically removed during maturation and are absent from the final infectious virion. What is the primary role of these scaffolding proteins?
Answer: To provide a temporary internal framework that guides the correct geometric assembly of the procapsid.
Scaffolding proteins function as chaperones for capsid assembly. They co-assemble with the major capsid proteins to form a procapsid (or prohead), ensuring the correct size, shape, and curvature of the structure and preventing mis-assembly into incorrect forms. Once the procapsid is formed, the scaffolding protein is removed, typically by a viral protease, which often triggers a conformational change and maturation of the capsid. The scaffold is the substrate of the protease, not the protease itself (A). It does not link the capsid to the genome (C) or anchor it for budding (D).
- Many +ssRNA viruses, such as poliovirus, translate their genome into a single large polyprotein, which is subsequently cleaved by a viral protease into individual functional proteins. This strategy is a crucial adaptation primarily to overcome which host cell limitation?
Answer: The inability of eukaryotic ribosomes to efficiently initiate translation at internal sites on an mRNA molecule.
Eukaryotic translation machinery is generally monocistronic, meaning ribosomes bind at the 5' cap of an mRNA and scan for the first AUG start codon, terminating at the first stop codon. They typically do not re-initiate translation at internal start sites on the same mRNA. Since +ssRNA viral genomes must encode multiple proteins, they have evolved the polyprotein strategy. By having one start and one stop codon for a single large open reading frame, they ensure all their genetic information is translated. A virally-encoded protease then cleaves this polyprotein to release the individual mature proteins. This strategy directly circumvents the monocistronic nature of eukaryotic translation.
- Herpes Simplex Virus acquires its envelope by budding through the inner nuclear membrane of the host cell. In contrast, Influenza virus buds from the apical plasma membrane. This difference in origin implies that the lipid composition of the Herpesvirus envelope is...
Answer: relatively poor in cholesterol and more similar to that of the endoplasmic reticulum.
Cellular membranes have distinct lipid compositions. The plasma membrane is characterized by high concentrations of cholesterol and sphingolipids, which provide rigidity. In contrast, intracellular membranes like the nuclear envelope and the endoplasmic reticulum (which is contiguous with the outer nuclear membrane) are more fluid and have a lower cholesterol content. Since Herpesvirus buds from the inner nuclear membrane, its envelope will reflect this lipid composition, being relatively poor in cholesterol. Influenza virus, budding from the plasma membrane, will have a cholesterol-rich envelope. Therefore, A is incorrect. While viruses do modify membranes, they do not completely change their fundamental lipid composition (B). All viral envelopes are lipid bilayers (D).
- Both Influenza virus (an Orthomyxovirus) and Reovirus have segmented genomes. However, their replication strategies differ significantly based on their genome type. A key distinction is that the Reovirus dsRNA genome:
Answer: remains inside the protective viral core during transcription into mRNA.
When you encounter questions about viral replication strategies, focus on how different genome types (DNA vs RNA, single-strand vs double-strand) dictate where and how viruses can replicate.
Reoviruses have a unique double-stranded RNA (dsRNA) genome that creates a special challenge. Unlike single-stranded RNA viruses, dsRNA triggers the host cell's antiviral defenses if detected in the cytoplasm. To avoid this, reoviruses have evolved an elegant solution: they keep their dsRNA genome safely sequestered inside the protective viral core throughout the entire transcription process. The viral RNA-dependent RNA polymerase operates from within this core, transcribing only the positive-sense mRNA strands that exit through channels in the core to be translated by host ribosomes.
Answer A correctly describes this protective strategy. Answer B is wrong because dsRNA in the cytoplasm would activate host antiviral responses, and host RNA polymerase II cannot transcribe RNA from RNA templates anyway. Answer C incorrectly suggests nuclear involvement - reoviruses replicate entirely in the cytoplasm, and the dsRNA never needs nuclear transport. Answer D describes a retrovirus strategy, not a reovirus one - reoviruses don't use reverse transcriptase or create DNA intermediates.
This contrasts sharply with influenza viruses, which have single-stranded RNA genomes that must enter the nucleus for replication using the host's transcription machinery.
Study tip: Remember that dsRNA viruses face unique host detection challenges, so look for protective mechanisms like core retention when you see questions about reoviruses or other dsRNA viruses.
- The Baltimore classification system categorizes viruses based on their genome and pathway to synthesizing mRNA. A virus with a circular, partially double-stranded DNA genome that replicates using an RNA intermediate would be placed in which Baltimore class?
Answer: Class VII (dsDNA-RT viruses)
This description is the hallmark of Hepadnaviruses, such as Hepatitis B virus. These viruses have a 'gapped' or partially double-stranded DNA genome. Their replication strategy is unique: the DNA genome is repaired to form a complete circle, which is then transcribed into a pre-genomic RNA intermediate. This RNA is then reverse-transcribed back into the DNA genome of progeny virions. Because they are dsDNA viruses that use reverse transcriptase (RT), they are classified as Class VII. Class I viruses replicate their dsDNA genome without an RNA intermediate. Class VI viruses (retroviruses) have an RNA genome that is reverse-transcribed into DNA. This is a key distractor, as both VI and VII use RT, but their genome type and replication cycle differ.
- A virus belonging to Baltimore Class IV has a genome that can be used directly as mRNA by host ribosomes. However, for the virus to replicate, a full-length complementary copy of the genome must first be synthesized. This complementary copy then serves as a template. What is the nature of this template molecule?
Answer: A negative-sense ssRNA molecule
Baltimore Class IV viruses have positive-sense single-stranded RNA (+ssRNA) genomes. The input genome acts as mRNA for protein synthesis. To replicate this genome, the virus must make more +ssRNA. It does this by using the original +ssRNA genome as a template to synthesize a complementary negative-sense ssRNA (-ssRNA) strand via an RNA-dependent RNA polymerase. This newly synthesized -ssRNA strand then serves as the template for the synthesis of many new +ssRNA genomes. Therefore, the template for progeny genome synthesis is a negative-sense ssRNA molecule.
