- All viruses are obligate intracellular parasites, profoundly dependent on their host cell. For the biosynthesis of new viral nucleic acids and proteins, which of the following is exclusively supplied by the host cell and never encoded by the viral genome?
Answer: Precursor nucleoside triphosphates (e.g., ATP, GTP).
Viruses hijack host cell metabolism for fundamental building blocks. This includes using the host's pool of amino acids for protein synthesis and the host's pool of nucleoside triphosphates for nucleic acid synthesis. No known virus has the genetic capacity to synthesize these precursors de novo. In contrast, many viruses encode their own polymerases and proteases.
- Uncoating is a critical step where the viral genome is released from its protective capsid. In which of the following scenarios would viral uncoating be most likely to fail, thus leading to an abortive infection?
Answer: An adenovirus particle is successfully endocytosed but fails to lyse the endosomal membrane.
Adenovirus enters via endocytosis and must escape the endosome to deliver its genome to the nucleus. This escape is an active process mediated by viral proteins that lyse the endosomal membrane, often triggered by acidification. If this step fails, the virion remains trapped and will be degraded in a phagolysosome. This represents a failure of uncoating.
- A researcher is studying an HIV-infected T-cell. After reverse transcription and integration of the viral genome into the host chromosome, the cell begins to produce viral mRNA molecules. Which enzyme is directly responsible for the transcription of this integrated proviral DNA into viral mRNA?
Answer: Host DNA-dependent RNA polymerase II
Once the retroviral DNA is integrated into the host genome (as a provirus), it is treated by the host cell as one of its own genes. Therefore, the host cell's machinery, specifically DNA-dependent RNA polymerase II, is responsible for transcribing the proviral DNA into viral mRNA and full-length genomic RNA for new virions.
- A researcher infects two sets of cultured human cells with a newly discovered DNA virus. Set 1 consists of normal, dividing cells. Set 2 consists of cells treated with aphidicolin, a specific inhibitor of host DNA polymerases α, δ, and ε, which does not affect most viral DNA polymerases. Viral replication is successful in Set 1 but is completely blocked in Set 2. What is the most likely conclusion about this virus?
Answer: The virus belongs to the Parvoviridae family, which depends on host S-phase enzymes.
The experiment shows a dependence on host DNA polymerases for replication. Parvoviruses are small ssDNA viruses that lack their own DNA polymerase. They are thus entirely dependent on the host cell's DNA replication machinery, which is active during the S-phase of the cell cycle. Aphidicolin would block the required host polymerases, preventing viral replication.
- A researcher attempts to infect Escherichia coli strain B with T4 bacteriophage, which results in a productive lytic infection. However, when the same high-titer stock of T4 phage is used to infect Pseudomonas aeruginosa, no infection occurs, and the phage titer in the medium remains constant. Which stage of viral replication is the most likely barrier in this scenario?
Answer: Attachment, due to the lack of a specific receptor on the bacterial surface.
Viral host range is often determined by the first step of infection: attachment to a specific cell surface receptor. T4 phage recognizes specific molecules on the surface of E. coli that are absent on P. aeruginosa. Without this initial binding, no subsequent steps of infection can occur. The constant phage titer confirms the virions are not being removed from the medium by binding to the cells.
- A novel negative-sense single-stranded RNA (-ssRNA) virus is isolated. Following entry and uncoating of its genome into the cytoplasm of a host cell, what is the immediate next step required for the synthesis of viral proteins?
Answer: Synthesis of a complementary positive-sense RNA strand using a viral polymerase.
The genome of a -ssRNA virus is not recognized as mRNA by host ribosomes. The virus must carry its own RNA-dependent RNA polymerase (RdRp) into the host cell. The first action of this enzyme is to transcribe the negative-sense genome into a complementary positive-sense RNA strand, which can then serve as mRNA for protein synthesis.
- Poliovirus, a positive-sense single-stranded RNA virus, translates its genome into a single large polyprotein, which is subsequently cleaved by viral proteases into individual functional proteins. A mutation occurs in the viral genome that inactivates the 2A protease. Which stage of the viral life cycle would be most directly and immediately inhibited?
Answer: Synthesis of the negative-sense RNA antigenome.
The viral RNA-dependent RNA polymerase (RdRp) is one of the functional proteins that must be cleaved from the polyprotein. If the 2A protease is inactive, the polyprotein cannot be properly processed, and a functional RdRp will not be produced. Without RdRp, the virus cannot synthesize the complementary negative-sense RNA strand, which is the required template for replicating the positive-sense genome. This critical biosynthesis step would be blocked.
- In many complex DNA viruses, gene expression is temporally regulated into early and late phases. Early genes often encode enzymes and regulatory proteins, while late genes typically encode structural components. If a drug were to specifically block the expression of all viral late genes, what would be the terminal stage of the replication cycle?
Answer: Replication of the viral nucleic acid within the host cell.
When you encounter questions about viral replication cycles, focus on the sequential nature of the process and how disrupting one stage affects all subsequent stages.
Complex DNA viruses follow a carefully orchestrated replication program. Early genes produce the molecular machinery needed for replication—DNA polymerases, helicases, and regulatory proteins that hijack the host cell's resources. Only after successful genome replication do late genes activate to produce structural proteins like capsid components and envelope proteins. This temporal regulation ensures the virus doesn't waste energy making protein shells before it has genetic material to package.
If a drug blocks all late gene expression, the virus can still complete early functions including DNA replication, making D correct. The replication machinery from early genes remains functional, so viral genomes multiply normally within the host cell.
Option A is incorrect because genome attachment and entry occur before any gene expression begins—this stage wouldn't be affected by blocking late genes. Option B (virion release) requires fully assembled particles, which depends on late gene products, so this stage would be completely prevented. Option C (nucleocapsid assembly) directly requires structural proteins encoded by late genes, making assembly impossible without these components.
The key insight is that blocking late genes creates a "dead-end" infection where genomes replicate but cannot be packaged into infectious particles.
Study tip: Remember the dependency chain in viral replication: entry → early gene expression → genome replication → late gene expression → assembly → release. Blocking any step prevents all subsequent steps but doesn't affect earlier ones.
- A patient is infected with a virus that causes a persistent, chronic infection with a continuous, low-level release of virions from infected cells over a long period. The infected cells remain alive and metabolically active. This observation strongly suggests that the virus is released from host cells by which of the following mechanisms?
Answer: Budding through a host cell membrane.
When you encounter questions about viral release mechanisms, focus on connecting the cellular outcome with the specific release pathway. The key clue here is that infected cells remain "alive and metabolically active" while continuously releasing virions over a long period.
Budding through a host cell membrane (D) perfectly explains this scenario. During budding, viruses acquire an envelope from the host cell's membrane (plasma membrane, nuclear envelope, or ER) as they exit. This process is relatively gentle and doesn't immediately kill the host cell, allowing for the continuous, low-level viral production described. The cell can repair its membrane and continue functioning while serving as a viral factory.
Option A describes bacterial phage lysis mechanisms where holin proteins create pores leading to cell death - this contradicts the "cells remain alive" observation. Option B is problematic because non-enveloped viruses typically exit through lysis, not gentle exocytosis, and this mechanism wouldn't explain the persistent infection pattern. Option C involves programmed cell death, which directly contradicts the stem's emphasis that cells stay alive and metabolically active.
The chronic, persistent nature with continuous viral shedding is characteristic of enveloped viruses like HIV, hepatitis B, and herpesviruses, all of which use budding mechanisms.
Study tip: Remember the release-outcome connection: lysis = cell death = burst of virions, while budding = cell survival = steady viral production. When you see "persistent infection" with "living cells," think enveloped virus budding.
- A key difference between viral entry via direct fusion at the plasma membrane and entry via receptor-mediated endocytosis is the location where the viral nucleocapsid is ultimately released. Where is the nucleocapsid of a virus that enters by direct fusion released?
Answer: Directly into the cytoplasm from the cell surface.
In direct fusion, the viral envelope merges with the host cell's plasma membrane at the cell surface. This process releases the internal contents of the virion, the nucleocapsid, directly into the host cell's cytoplasm. This is in contrast to endocytosis, where the virion is first enclosed within an endosome.
- Poxviruses are large dsDNA viruses that are unusual because their entire replication cycle occurs within the host cell's cytoplasm. This cytoplasmic replication strategy necessitates that the poxvirus genome must encode for which of the following?
Answer: Its own DNA-dependent DNA polymerase and DNA-dependent RNA polymerase.
Most DNA viruses replicate in the nucleus to use the host's DNA and RNA polymerases. Because poxviruses replicate entirely in the cytoplasm, they cannot access these nuclear enzymes. Therefore, their large genome must encode their own machinery for both replicating their DNA genome (DNA-dependent DNA polymerase) and transcribing it into mRNA (DNA-dependent RNA polymerase).
- While most RNA viruses replicate entirely in the cytoplasm, some require entry into the host cell nucleus for a critical part of their replication cycle. Which of the following provides a correct reason for the nuclear localization of a specific RNA virus group?
Answer: Retroviruses must enter the nucleus for proviral integration into host chromosomes.
When you encounter questions about RNA virus replication sites, remember that while most RNA viruses replicate entirely in the cytoplasm, certain virus families have evolved specific nuclear requirements that are essential to their life cycles.
Retroviruses represent a unique group of RNA viruses that must enter the nucleus to complete their replication cycle. After reverse transcription converts their RNA genome into DNA in the cytoplasm, this newly synthesized proviral DNA must integrate into the host cell's chromosomal DNA. This integration step is absolutely crucial for retroviral replication and can only occur in the nucleus where the host chromosomes are located. The integrated provirus then serves as a template for producing new viral RNA and proteins. This is why option D is correct.
Let's examine why the other options are incorrect: Option A is wrong because picornaviruses (like poliovirus) replicate entirely in the cytoplasm using their own RNA-dependent RNA polymerase, never requiring nuclear access. Option B incorrectly describes orthomyxoviruses (like influenza) - while these viruses do replicate in the nucleus, they actually use the host's transcription machinery for mRNA capping and splicing, not genome replication itself. Option C is factually incorrect because ribosomes are located in the cytoplasm, not the nucleus.
For microbiology exams, remember this pattern: when you see questions about nuclear localization in RNA viruses, focus on which viruses have DNA intermediates (retroviruses) or require host nuclear machinery for specific processing steps (orthomyxoviruses for mRNA processing, not genome replication).
- During a one-step growth curve experiment with a lytic bacteriophage, a sample of the culture is taken during the eclipse period. This sample is treated to lyse all bacterial cells and then plated on a lawn of susceptible bacteria. What is the most likely outcome?
Answer: A low to zero titer of plaques, because infectious virions have not yet been assembled.
The eclipse period is the time from viral entry until the assembly of the first new infectious progeny virions. During this phase, the virus has uncoated and its components are being synthesized, but intact, infectious virions do not yet exist inside the cell. Artificially lysing the cells at this stage will not release functional phages, resulting in a very low or zero plaque count.
- An antiviral drug is developed that functions by neutralizing the acidic environment of endosomes. This drug would be most effective against which of the following viruses?
Answer: An enveloped virus that enters the host cell via receptor-mediated endocytosis.
Many enveloped viruses (e.g., influenza) enter host cells via receptor-mediated endocytosis. The fusion of the viral envelope with the endosomal membrane, which releases the viral nucleocapsid into the cytoplasm, is often triggered by the low pH within the late endosome. A drug that neutralizes this acidic environment would prevent this pH-dependent fusion and trap the virus inside the endosome, inhibiting replication.
- During high-multiplicity infection with an RNA virus, defective interfering (DI) particles are often generated. These particles typically have a genome with a large internal deletion but retain the signals for replication and packaging. How do these DI particles interfere with the replication of the standard, wild-type virus?
Answer: They outcompete the wild-type virus for essential resources like polymerases and structural proteins.
DI particles are replication-competent but lack genes for a productive infection. Because their genomes are smaller, they often replicate faster than the full-length wild-type genome. They compete for the limited pool of viral polymerase, replication factors, and capsid proteins, thereby reducing the yield of infectious wild-type virions.
- The assembly of many simple icosahedral viruses from capsomeres and nucleic acid is a spontaneous process within the host cell. This suggests that the formation of a complete, infectious virion is:
Answer: primarily driven by the high concentration of viral components and is thermodynamically favorable.
The principle of self-assembly implies that the final structure is encoded in the properties of the subunits. For many simple viruses, assembly is spontaneous because the final state represents a minimum free energy configuration, driven by favorable non-covalent interactions. High local concentrations of these components further drive the equilibrium towards the assembled virion.
- Herpesviruses, a family of large dsDNA viruses, assemble their nucleocapsids in the nucleus of the host cell. They are then released through a complex, multi-step process. From which host cell structure is the final, mature viral envelope primarily derived?
Answer: The endoplasmic reticulum or Golgi-derived vesicles.
The maturation of herpesviruses involves a primary envelopment at the inner nuclear membrane, followed by de-envelopment in the cytoplasm. The naked nucleocapsid then acquires its final, mature envelope by budding into vesicles derived from the trans-Golgi network or endosomes before being released via exocytosis. Thus, the final envelope is derived from cytoplasmic organelle membranes.
- Hepatitis B virus (a hepadnavirus) has a partially double-stranded circular DNA genome. Its replication cycle is unique among DNA viruses because it involves reverse transcription of an RNA intermediate. Which of the following steps is essential for its genome replication strategy?
Answer: Transcription of the viral DNA into a pre-genomic RNA by host RNA polymerase.
The hepadnavirus genome is converted to covalently closed circular DNA (cccDNA) in the nucleus. This cccDNA is transcribed by host RNA polymerase II into a pre-genomic RNA (pgRNA). This pgRNA is then packaged into a new core particle in the cytoplasm where it serves as the template for the viral reverse transcriptase to synthesize the new DNA genome. Therefore, the creation of this RNA intermediate is essential.
- In bacteriophage lambda, the decision to enter a lysogenic cycle rather than a lytic cycle is dependent on the accumulation of the cI repressor protein. Which of the following events is a direct consequence of this repressor protein's function in establishing lysogeny?
Answer: Inhibition of transcription of the cro gene and other early lytic genes.
When you encounter questions about bacteriophage lambda lysogeny, focus on the central role of the cI repressor protein as the "master switch" that maintains the dormant prophage state. This protein's primary function is to prevent lytic gene expression, keeping the virus integrated and inactive in the host chromosome.
The cI repressor directly binds to operator sequences that control transcription of lytic genes, most importantly blocking the cro gene and other early lytic genes from being expressed. This creates a stable lysogenic state where the prophage remains dormant. Choice A correctly identifies this fundamental mechanism - the repressor's direct inhibition of lytic gene transcription is what establishes and maintains lysogeny.
Choice B is incorrect because cI repressor doesn't activate host DNA repair mechanisms. The lambda DNA integrates through site-specific recombination involving the phage integrase protein, not through host repair systems. Choice C describes the opposite of what happens during lysogeny establishment - autocatalytic cleavage of cI repressor actually ends lysogeny by allowing lytic genes to be expressed, leading to prophage excision and lytic cycle initiation. Choice D is wrong because late structural proteins are synthesized during the lytic cycle, not during lysogeny establishment. The cI repressor specifically prevents this by blocking the transcriptional cascade that would lead to late gene expression.
Remember: cI repressor = lysogeny maintenance. When studying phage lambda, always connect cI repressor function to its role as the "brake" that prevents lytic development, primarily through transcriptional repression of key lytic genes.
