Question 1

Viral families and hepatitis: Which hepatitis virus belongs to the family Hepadnaviridae?

Accepted Answer

Introduction / Context:Knowing viral taxonomy links genome type to replication strategy and clinical behavior. Hepadnaviridae are partially double-stranded DNA viruses with reverse transcription in their life cycle.Given Data / Assumptions:Hepatitis viruses A–E are unrelated families with different genomes.Hepadnaviridae hallmark: relaxed circular, partially double-stranded DNA and reverse transcriptase.Hepatitis D is a defective RNA agent requiring coinfection with HBV.Concept / Approach:HBV (Hepatitis B virus) is the prototypical Hepadnavirus. HAV is a picornavirus (nonenveloped, positive-sense RNA). HDV is a small circular RNA satellite (deltavirus) that depends on HBV surface antigen to assemble; it is not a member of Hepadnaviridae.Step-by-Step Solution:Identify which hepatitis virus has a DNA genome with reverse transcription.Select Hepatitis B virus as the sole correct member of Hepadnaviridae.Exclude HAV and HDV based on family and genome.Verification / Alternative check:HBsAg/HBcAg serology and antiviral targets (reverse transcriptase inhibitors) corroborate HBV’s hepadnaviral nature.Why Other Options Are Wrong:HAV and HDV are different families/agents; “All of these” is therefore incorrect.Common Pitfalls:Assuming all hepatitis viruses are related because of disease site; in reality, their taxonomy diverges widely.Final Answer:Hepatitis B virus

Question 2

Pock formation on chorioallantoic membrane (CAM): Which virus classically produces visible lesions (pocks) on the CAM of embryonated eggs used in diagnostic virology?

Accepted Answer

Introduction / Context:The chorioallantoic membrane (CAM) of embryonated eggs has long been used to culture viruses and visualize pock lesions. Recognizing which viruses yield characteristic CAM pocks is a staple of classical diagnostic virology.Given Data / Assumptions:CAM inoculation can reveal discrete, opaque “pocks.”Poxviruses (e.g., vaccinia, variola) produce prominent, countable pocks.Some herpesviruses can produce focal lesions, but the classic teaching example for CAM pocks is vaccinia.Concept / Approach:Poxviruses replicate in the cytoplasm and form large virions; upon CAM inoculation, vaccinia yields conspicuous pocks used for titration. While herpes simplex can produce lesions in certain systems, “pocks on CAM” traditionally points to vaccinia/variola in basic exam questions. Herpes B is a zoonotic primate virus and is not the standard CAM reference.Step-by-Step Solution:Recall hallmark: CAM pock assay is a classic for poxviruses.Identify vaccinia virus as the standard laboratory poxvirus used for pock formation and titration.Select Vaccinia virus as the best single answer.Verification / Alternative check:Historic smallpox vaccine production and lab titrations used vaccinia on CAM to count pocks.Why Other Options Are Wrong:Herpes simplex may cause lesions, but the archetypal CAM pock reference is vaccinia; Herpes B is not used classically for CAM pock titration in basic coursework; “all of these” overgeneralizes.Common Pitfalls:Assuming any virus that forms lesions in tissues will generate classic CAM pocks; conflating “lesion” with “pock” in exam wording.Final Answer:Vaccinia virus

Question 3

Enveloped versus nonenveloped viruses: Which one of the following viruses is enveloped?

Accepted Answer

Introduction / Context:Envelope status influences environmental stability, disinfection, and entry mechanisms. Enveloped viruses bud through host membranes and carry lipid bilayers; nonenveloped viruses rely on robust capsids.Given Data / Assumptions:Adenoviruses: nonenveloped, icosahedral DNA viruses.Herpesviruses: enveloped, large DNA viruses with tegument.Polioviruses: nonenveloped, positive-sense RNA (Picornaviridae).Concept / Approach:Among the listed choices, only herpesviruses possess a lipid envelope surrounding their capsid. This envelope bears glycoproteins essential for attachment and fusion but also makes the virion more susceptible to detergents and desiccation.Step-by-Step Solution:Match each virus family to envelope status.Identify herpesviruses as enveloped; others listed are nonenveloped.Choose “Herpes.”Verification / Alternative check:Electron micrographs reveal a membrane bilayer and spikes surrounding herpesvirus nucleocapsids, unlike adeno or polio.Why Other Options Are Wrong:Adeno and polio lack envelopes; “None of these” is incorrect because herpes is enveloped.Common Pitfalls:Equating “has spikes” with “is enveloped”—nonenveloped viruses can have protruding capsid proteins but no lipid bilayer.Final Answer:Herpes

Question 4

Modes of virus release and persistence: How is the process called in which infected cells continue to divide and simultaneously release mature virions from their surface for an extended period (for example, by budding)?

Accepted Answer

Introduction / Context:Viruses can exit cells by lysis, budding, or exocytosis. Some infections are acute and cytolytic; others are persistent, with cells remaining viable while releasing virions. Naming these patterns correctly is key to understanding pathogenesis and antiviral targets.Given Data / Assumptions:Cells continue reproducing (non-lytic infection).Virions are continuously extruded (e.g., budding through the plasma membrane).The term sought should reflect ongoing virion production.Concept / Approach:A productive infection is one in which new virions are produced. When this production is sustained without immediate cell death, the scenario is often called a persistent productive infection. Among the given options, “productive infection” is the correct, standard term; the other phrases are not established virology terms.Step-by-Step Solution:Recognize that virion output occurs continuously and the cell survives.Map to the established terminology: productive (and persistent) infection.Choose “productive infection” as the best fitting option.Verification / Alternative check:Many enveloped viruses (e.g., hepatitis B, some retroviruses) bud from cells without immediate lysis, exemplifying productive persistence.Why Other Options Are Wrong:“Inductive infection” and “resultant infection” are not standard terms; “all of these” therefore cannot be correct.Common Pitfalls:Confusing productive, persistent, latent, and chronic terminology; assuming all productive infections are lytic.Final Answer:productive infection

Question 5

Why bacteriophages are model systems: Bacteriophages are widely used in genetic research because they are the smallest and simplest biological entities capable of what property?

Accepted Answer

Introduction / Context:Bacteriophages (viruses that infect bacteria) have powered milestones in molecular biology—from the genetic code to DNA replication mechanisms. Their utility comes from genetic simplicity paired with the ability to replicate using host machinery.Given Data / Assumptions:Phages carry nucleic acid genomes encased in protein coats.They cannot metabolize independently but direct host systems to produce progeny virions.The question asks for the key capability that makes them powerful genetic tools.Concept / Approach:Despite metabolic dependence on hosts, phages are capable of self-replication in the biological sense: their genomes encode the information required to direct the production of more of themselves within suitable hosts. Terms like “duplication” or “multiplication” are colloquial and imprecise; “self-replication” is the standard biological term emphasizing template-directed propagation.Step-by-Step Solution:Focus on biological property: information-directed replication of the entity.Select the precise term “self-replication.”Reject synonyms that lack precise biological usage in this context.Verification / Alternative check:T-even phages (e.g., T4) demonstrate orderly replication cycles with assembly of progeny phage—an archetype of self-replication guided by genetic information.Why Other Options Are Wrong:“Duplication,” “self-duplication,” and “multiplication” are vague and nonstandard relative to “self-replication,” the accepted term in genetics and molecular biology.Common Pitfalls:Assuming “self-replication” requires metabolic autonomy; in virology, replication refers to information-driven reproduction within a host system.Final Answer:self-replication

Question 6

In bacteriophage replication (lytic cycle), the virion parts self-assemble into mature phages only after both the phage genome and the structural proteins have been synthesized. Which prerequisite(s) trigger assembly?

Accepted Answer

Introduction / Context:Bacteriophage (phage) replication proceeds through ordered stages: adsorption, injection, replication of nucleic acid, synthesis of capsid and tail proteins, assembly (maturation), and lysis. The question asks what must be synthesized before components begin to assemble into complete virions. Understanding this sequence clarifies how phages coordinate genome packaging with capsid formation.Given Data / Assumptions:The context is a lytic phage infection of a bacterial host.Phage genes drive host machinery to produce viral nucleic acid and structural proteins.Assembly (head, tail, tail fibers) requires both the genome and capsid/tail subunits to be available.Concept / Approach:Phage assembly is a self-organizing process guided by scaffolding proteins and precise protein–protein interactions. Genome packaging motors load the replicated nucleic acid into preformed heads. Thus, assembly requires two categories of inputs: the replicated nucleic acid (DNA or RNA, depending on the phage) and the synthesized structural proteins (capsomers, tail proteins, base plates). Mere availability of amino acids or genome alone is insufficient.Step-by-Step Solution:Recognize that structural proteins form procapsids and tails via ordered pathways.Note that genome replication produces many copies of phage nucleic acid.Assembly initiates once both structural subunits and the nucleic acid are present.Genome is packaged into the head; tails and fibers attach to complete virions.Cells lyse to release mature progeny.Verification / Alternative check:Pulse-chase experiments show a time lag: first nucleic acid and protein synthesis, then sudden appearance of infectious particles (one-step growth curve), confirming assembly occurs after both categories are synthesized.Why Other Options Are Wrong:Structural proteins only: without genomes, empty heads cannot produce infectious phages.Phage nucleic acid only: genome without capsids cannot mature or survive outside the cell.Free amino acids only: building blocks alone do not trigger assembly.Common Pitfalls:Confusing translation of proteins with assembly, or assuming packaging can begin before structural scaffolds exist.Final Answer:Both (a) and (b)

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