Questions

Virus Replication — Questions

Study questions for Virus Replication.

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Sit this set one question at a time. Multiple-choice questions mark themselves; written questions reveal a tickable mark scheme so you can score your own answer. You get a combined score at the end.

34 questions: 34 MCQ, 0 written.

High-priority (past-paper) questions only

Most particles never form a plaque. This is also why viral load (genome copies measured by PCR) is not the same as infectious titre: PCR cannot distinguish a whole virion from an empty shell.

Positive-sense RNA has the same sequence as mRNA, so a ribosome translates it immediately, producing the polymerase the virus then uses to replicate.

Dolutegravir (and the long-acting cabotegravir) are integrase strand-transfer inhibitors that block insertion of the reverse-transcribed HIV DNA into the host genome.

Naked viruses generally accumulate and are freed when the cell bursts; enveloped viruses wrap themselves in host membrane as they bud out.

Haemagglutinin tethers budding virions to surface sialic acid; neuraminidase cleaves it to free them. Blocking neuraminidase traps progeny at the surface and halts spread.

The RNA polymerases of RNA viruses do not proofread and make errors roughly ten thousand times more often than DNA polymerases, generating the quasispecies swarm that escapes immunity and drugs.

They are chain-terminating nucleos(t)ide reverse-transcriptase inhibitors, active against HIV reverse transcriptase (and the HBV polymerase). Nevirapine, by contrast, is a non-nucleoside RT inhibitor that binds the enzyme allosterically.

Reassortment of segments between two influenza viruses in a single cell produces an abrupt, large antigenic change (shift), distinct from the gradual point-mutation drift.

These are protease inhibitors. A virus that translates its genome as one polyprotein must cut it into functional units with a viral protease; blocking that protease leaves the polyprotein uncut and the virions immature.

Because no host enzyme copies RNA from RNA, an RdRp inhibitor can be highly selective for the virus. Sofosbuvir (hepatitis C) and remdesivir (SARS-CoV-2) act here.

A virus can only infect cells that display the receptor its ligand binds, so receptor distribution largely dictates which tissues are infected and what disease results.

No host enzyme can copy RNA from an RNA template or translate a negative strand, so the negative-sense (class V) and double-stranded RNA (class III) viruses must bring their own polymerase. A positive-sense genome is translated directly on entry, so its polymerase is made on the spot.

Eukaryotic ribosomes do not reinitiate at downstream genes, so viruses reach their other proteins by polyprotein cleavage, splicing, segmentation, ribosomal frameshifting, or an internal ribosome entry site (IRES).

Chloroquine raises endosomal pH and so blocks the cathepsin-L endosomal entry route used by the kidney cells; airway cells expressing TMPRSS2 cleave the spike at the plasma membrane and fuse there, never depending on the endosome.

RNA genomes sit just below an error threshold that caps them near 30 kilobases; coronaviruses are the exception, with an nsp14 proofreading exonuclease. Pushing the rate past the threshold drives the population to extinction.

Poliovirus, coxsackievirus, hepatitis A and hepatitis E can exit cloaked in host vesicles; circulating hepatitis A is entirely quasi-enveloped, which shields it from antibody.

Aciclovir is monophosphorylated only by viral thymidine kinase (HSV/VZV); the activated drug then chain-terminates the viral DNA polymerase, so it is inert in uninfected cells. Ganciclovir works analogously for CMV, activated by the CMV UL97 kinase.

M2 is a proton channel that acidifies the virion interior to allow uncoating; amantadine and rimantadine block it. Near-universal resistance has retired them clinically, but they remain the classic uncoating inhibitor.

An IRES is a structured RNA element that recruits the ribosome internally, one of several ways viruses bypass the rule that a eukaryotic ribosome reads only the first gene of a message.

Diploid strains senesce at the Hayflick limit as telomeres shorten; continuous lines such as HeLa and BHK-21 are immortalised and divide indefinitely but have drifted from normal tissue.

Eggs, used since the 1930s, remain the workhorse for influenza virus stocks and much influenza vaccine manufacture.

HBV (class VII) is a DNA virus that copies a pregenomic RNA intermediate back into DNA by reverse transcriptase, which is why reverse-transcriptase inhibitors such as tenofovir treat it.

The protease normally cleaves the Gag-Pol polyprotein during maturation; inhibiting it leaves the particle assembled but immature and non-infectious.

Human strains prefer alpha-2,6-linked sialic acid (upper airway); avian strains prefer alpha-2,3 (avian gut and deep human lung). A shift in this preference is one of the features watched in pandemic surveillance.

Lenacapavir binds the HIV capsid protein, disrupting both proper capsid assembly/maturation and the incoming capsid’s uncoating and nuclear import.

Letermovir blocks the terminase complex that cuts concatemeric DNA and packages it into preformed capsids, an assembly/packaging step distinct from the DNA polymerase that ganciclovir inhibits.

Maraviroc is a CCR5 antagonist: it occupies the co-receptor so gp120/gp41 cannot complete entry. People homozygous for the CCR5-Δ32 deletion are naturally resistant to CCR5-tropic HIV for the same reason.

Most DNA viruses replicate in the nucleus (poxviruses are the cytoplasmic exception); most RNA viruses replicate in the cytoplasm (influenza is a nuclear exception).

Some enveloped viruses fuse directly at the cell surface at neutral pH; others such as influenza require the low pH of the endosome to trigger fusion.

After uncoating and before the first progeny are assembled, no infectious particle exists, so none can be recovered even by breaking the cells open.

HIV is a retrovirus, class VI: a positive-sense ssRNA genome reverse-transcribed to DNA and integrated. Hepatitis B is class VII (dsDNA, reverse-transcribing).

Sequestering replication concentrates the enzymes and conceals double-stranded RNA, the danger signal that RIG-I and MDA-5 detect to trigger interferon.

Viral “late-domain” motifs recruit the host ESCRT machinery to perform the final membrane scission. Influenza is the exception, severing itself with its M2 protein.

Influenza A (an orthomyxovirus, class V) has a segmented negative-sense RNA genome and packages its RdRp. Rotavirus is also segmented and carries a polymerase, but its genome is double-stranded (class III), not negative-sense.

High priorityMCQ
A particle-to-infectivity ratio greater than 1000:1 for a virus stock means that:
- A. Most particles in the stock are infectious
- B. The plaque assay is overcounting infectivity
- C. Viral load exactly equals the infectious titre here
- D. The stock is contaminated
- E. Physical particles far outnumber those that form plaques
Show answer
Correct answer: E
Most particles never form a plaque. This is also why viral load (genome copies measured by PCR) is not the same as infectious titre: PCR cannot distinguish a whole virion from an empty shell.
High priorityMCQ
A positive-sense single-stranded RNA genome (such as a picornavirus) is infectious as naked RNA because:
- A. It carries a polymerase in the virion
- B. It acts directly as messenger RNA
- C. It is double-stranded
- D. It integrates into the host genome first
- E. It must be reverse-transcribed first
Show answer
Correct answer: B
Positive-sense RNA has the same sequence as mRNA, so a ribosome translates it immediately, producing the polymerase the virus then uses to replicate.
High priorityMCQ
Dolutegravir, the third drug of the standard South African first-line regimen, inhibits which enzyme?
- A. Reverse transcriptase
- B. Protease
- C. Integrase
- D. RNA-dependent RNA polymerase
- E. Neuraminidase
Show answer
Correct answer: C
Dolutegravir (and the long-acting cabotegravir) are integrase strand-transfer inhibitors that block insertion of the reverse-transcribed HIV DNA into the host genome.
High priorityMCQ
Non-enveloped viruses are classically released by cell lysis, whereas enveloped viruses:
- A. also lyse the cell to escape
- B. exit only by exocytosis of intact vesicles
- C. integrate into the host genome
- D. bud through a host membrane
- E. are released by neuraminidase alone
Show answer
Correct answer: D
Naked viruses generally accumulate and are freed when the cell bursts; enveloped viruses wrap themselves in host membrane as they bud out.
High priorityMCQ
Oseltamivir and zanamivir stop influenza spread by inhibiting neuraminidase, an enzyme that normally:
- A. Binds the host-cell sialic acid receptor
- B. Cleaves sialic acid to free progeny
- C. Copies the genome
- D. Cleaves the polyprotein
- E. Forms the M2 ion channel
Show answer
Correct answer: B
Haemagglutinin tethers budding virions to surface sialic acid; neuraminidase cleaves it to free them. Blocking neuraminidase traps progeny at the surface and halts spread.
High priorityMCQ
RNA viruses evolve far faster than DNA viruses chiefly because:
- A. They have much larger genomes
- B. They recombine far more readily than DNA viruses
- C. Their polymerases lack proofreading
- D. They all replicate in the host nucleus
- E. They all have segmented genomes
Show answer
Correct answer: C
The RNA polymerases of RNA viruses do not proofread and make errors roughly ten thousand times more often than DNA polymerases, generating the quasispecies swarm that escapes immunity and drugs.
High priorityMCQ
Tenofovir and lamivudine, the nucleos(t)ide backbone of South African first-line ART, act by:
- A. Blocking integrase
- B. Inhibiting the viral protease
- C. Blocking the CCR5 co-receptor used for entry
- D. Preventing budding
- E. Chain-terminating the DNA made by reverse transcriptase
Show answer
Correct answer: E
They are chain-terminating nucleos(t)ide reverse-transcriptase inhibitors, active against HIV reverse transcriptase (and the HBV polymerase). Nevirapine, by contrast, is a non-nucleoside RT inhibitor that binds the enzyme allosterically.
High priorityMCQ
The abrupt 'antigenic shift' of influenza is caused by:
- A. Gradual point mutation, otherwise known as drift
- B. Reassortment of whole genome segments
- C. Recombination of DNA
- D. Proviral integration
- E. Error catastrophe
Show answer
Correct answer: B
Reassortment of segments between two influenza viruses in a single cell produces an abrupt, large antigenic change (shift), distinct from the gradual point-mutation drift.
High priorityMCQ
The HIV '-navir' drugs, the hepatitis C '-previr' drugs, and nirmatrelvir all work by:
- A. Preventing cleavage of the polyprotein
- B. Blocking reverse transcriptase
- C. Blocking viral attachment
- D. Inhibiting the host cell ribosome directly
- E. Degrading the viral genome
Show answer
Correct answer: A
These are protease inhibitors. A virus that translates its genome as one polyprotein must cut it into functional units with a viral protease; blocking that protease leaves the polyprotein uncut and the virions immature.
High priorityMCQ
The RNA-dependent RNA polymerase (RdRp) of RNA viruses is a particularly clean antiviral target because:
- A. It is identical to a host enzyme
- B. It is found only in DNA viruses, not RNA ones
- C. It proofreads efficiently
- D. Human cells have no equivalent enzyme, so inhibitors are selective
- E. It is non-essential for replication
Show answer
Correct answer: D
Because no host enzyme copies RNA from RNA, an RdRp inhibitor can be highly selective for the virus. Sofosbuvir (hepatitis C) and remdesivir (SARS-CoV-2) act here.
High priorityMCQ
The tissue tropism of a virus is determined chiefly by:
- A. Distribution of the host receptor it binds
- B. Its genome size
- C. Its Baltimore class
- D. The route by which it exits the cell
- E. Whether it is enveloped
Show answer
Correct answer: A
A virus can only infect cells that display the receptor its ligand binds, so receptor distribution largely dictates which tissues are infected and what disease results.
High priorityMCQ
Which viruses must package their own polymerase inside the virion and carry it into the next cell?
- A. Negative-sense RNA and double-stranded RNA viruses
- B. Positive-sense RNA and single-stranded DNA viruses
- C. Double-stranded DNA viruses only
- D. Single-stranded DNA viruses only
- E. Reverse-transcribing DNA viruses only
Show answer
Correct answer: A
No host enzyme can copy RNA from an RNA template or translate a negative strand, so the negative-sense (class V) and double-stranded RNA (class III) viruses must bring their own polymerase. A positive-sense genome is translated directly on entry, so its polymerase is made on the spot.
High priorityMCQ
Why can a virus not simply present a polycistronic genome to be translated in a human cell?
- A. The genome is too large
- B. Host ribosomes reject all viral RNA
- C. The message must first be converted to double-stranded form
- D. Polycistronic RNA cannot be capped
- E. A ribosome reads only the first frame, then stops
Show answer
Correct answer: E
Eukaryotic ribosomes do not reinitiate at downstream genes, so viruses reach their other proteins by polyprotein cleavage, splicing, segmentation, ribosomal frameshifting, or an internal ribosome entry site (IRES).
High priorityMCQ
Why did chloroquine block SARS-CoV-2 in cultured kidney cells yet fail in patients?
- A. The dose achievable in patients was too low
- B. The virus is not pH-sensitive
- C. It blocks neuraminidase, an enzyme the virus does not have
- D. Airway cells enter by a surface route, not the endosome
- E. Resistance emerged rapidly
Show answer
Correct answer: D
Chloroquine raises endosomal pH and so blocks the cathepsin-L endosomal entry route used by the kidney cells; airway cells expressing TMPRSS2 cleave the spike at the plasma membrane and fuse there, never depending on the endosome.
MCQ
'Error catastrophe', exploited by mutagens such as ribavirin, occurs when:
- A. The virus runs out of nucleotide building blocks
- B. The genome becomes too small
- C. Proofreading is enhanced
- D. The mutation rate exceeds the error threshold
- E. The host clears the virus by antibody
Show answer
Correct answer: D
RNA genomes sit just below an error threshold that caps them near 30 kilobases; coronaviruses are the exception, with an nsp14 proofreading exonuclease. Pushing the rate past the threshold drives the population to extinction.
MCQ
'Quasi-enveloped' release refers to:
- A. Naked viruses leaving inside host membrane vesicles
- B. Enveloped viruses shedding their envelope before exit
- C. A defective interfering particle
- D. Budding through the nuclear membrane
- E. Lysis of enveloped viruses
Show answer
Correct answer: A
Poliovirus, coxsackievirus, hepatitis A and hepatitis E can exit cloaked in host vesicles; circulating hepatitis A is entirely quasi-enveloped, which shields it from antibody.
MCQ
Aciclovir is selective for herpes-infected cells because:
- A. A viral kinase must activate it first
- B. It can only enter infected cells
- C. It preferentially inhibits the host DNA polymerase
- D. It is an integrase inhibitor
- E. It is a protease inhibitor
Show answer
Correct answer: A
Aciclovir is monophosphorylated only by viral thymidine kinase (HSV/VZV); the activated drug then chain-terminates the viral DNA polymerase, so it is inert in uninfected cells. Ganciclovir works analogously for CMV, activated by the CMV UL97 kinase.
MCQ
Amantadine inhibits influenza A by:
- A. Blocking neuraminidase
- B. Inhibiting the viral polymerase that copies the genome
- C. Blocking the M2 channel that uncoating needs
- D. Preventing viral attachment to sialic acid receptors
- E. Inhibiting integrase
Show answer
Correct answer: C
M2 is a proton channel that acidifies the virion interior to allow uncoating; amantadine and rimantadine block it. Near-universal resistance has retired them clinically, but they remain the classic uncoating inhibitor.
MCQ
An internal ribosome entry site (IRES) lets a virus:
- A. Integrate its genome directly into the host cell chromosome
- B. Start translation inside a message, not at its start
- C. Carry its own ribosome
- D. Reverse-transcribe its genome
- E. Evade neutralising antibody
Show answer
Correct answer: B
An IRES is a structured RNA element that recruits the ribosome internally, one of several ways viruses bypass the rule that a eukaryotic ribosome reads only the first gene of a message.
MCQ
Diploid cell strains such as WI-38 and MRC-5 stop dividing after roughly fifty divisions because of:
- A. Viral cytopathic effect
- B. Spontaneous transformation
- C. Bacterial contamination
- D. Telomere shortening (the Hayflick limit)
- E. Gradual loss of their surface receptors
Show answer
Correct answer: D
Diploid strains senesce at the Hayflick limit as telomeres shorten; continuous lines such as HeLa and BHK-21 are immortalised and divide indefinitely but have drifted from normal tissue.
MCQ
Embryonated hens' eggs remain in routine use chiefly for:
- A. Routinely growing herpesviruses in the laboratory
- B. Producing influenza virus and influenza vaccine
- C. Performing plaque assays
- D. Measuring viral load
- E. Reverse transcription
Show answer
Correct answer: B
Eggs, used since the 1930s, remain the workhorse for influenza virus stocks and much influenza vaccine manufacture.
MCQ
Hepatitis B virus is unusual among DNA viruses because it:
- A. Its genome is RNA rather than DNA
- B. It is single-stranded throughout
- C. It copies an RNA intermediate by reverse transcriptase
- D. It carries an RNA-dependent RNA polymerase like other RNA viruses
- E. It cannot be transcribed by host enzymes
Show answer
Correct answer: C
HBV (class VII) is a DNA virus that copies a pregenomic RNA intermediate back into DNA by reverse transcriptase, which is why reverse-transcriptase inhibitors such as tenofovir treat it.
MCQ
HIV protease inhibitors leave newly budded virions:
- A. Assembled but immature and non-infectious
- B. Unable to attach to any host cell
- C. Unable to bud from the cell
- D. Lacking an envelope
- E. Hypermutated
Show answer
Correct answer: A
The protease normally cleaves the Gag-Pol polyprotein during maturation; inhibiting it leaves the particle assembled but immature and non-infectious.
MCQ
Human influenza viruses preferentially bind which form of sialic acid?
- A. The alpha-2,3-linked form (avian and deep-lung type)
- B. Heparan sulfate
- C. They do not bind sialic acid
- D. The CD4 receptor
- E. The alpha-2,6-linked form (human upper-airway type)
Show answer
Correct answer: E
Human strains prefer alpha-2,6-linked sialic acid (upper airway); avian strains prefer alpha-2,3 (avian gut and deep human lung). A shift in this preference is one of the features watched in pandemic surveillance.
MCQ
Lenacapavir, a long-acting HIV agent, targets the viral:
- A. Integrase
- B. Reverse transcriptase
- C. Capsid
- D. Protease
- E. gp41 fusion protein
Show answer
Correct answer: C
Lenacapavir binds the HIV capsid protein, disrupting both proper capsid assembly/maturation and the incoming capsid’s uncoating and nuclear import.
MCQ
Letermovir prevents CMV disease in transplant recipients by inhibiting the:
- A. CMV DNA polymerase (the ganciclovir target)
- B. Reverse transcriptase
- C. The viral maturation protease
- D. Neuraminidase
- E. CMV terminase that packages the genome
Show answer
Correct answer: E
Letermovir blocks the terminase complex that cuts concatemeric DNA and packages it into preformed capsids, an assembly/packaging step distinct from the DNA polymerase that ganciclovir inhibits.
MCQ
Maraviroc prevents HIV entry by:
- A. Inhibiting the reverse transcriptase enzyme
- B. Binding CD4
- C. Blocking the CCR5 co-receptor
- D. Inhibiting integrase
- E. Blocking neuraminidase
Show answer
Correct answer: C
Maraviroc is a CCR5 antagonist: it occupies the co-receptor so gp120/gp41 cannot complete entry. People homozygous for the CCR5-Δ32 deletion are naturally resistant to CCR5-tropic HIV for the same reason.
MCQ
Most DNA viruses replicate their genomes in the nucleus, whereas most RNA viruses replicate in the:
- A. nucleus
- B. cytoplasm
- C. mitochondrion
- D. endosome
- E. extracellular space
Show answer
Correct answer: B
Most DNA viruses replicate in the nucleus (poxviruses are the cytoplasmic exception); most RNA viruses replicate in the cytoplasm (influenza is a nuclear exception).
MCQ
Paramyxoviruses and HIV differ from influenza in that they can penetrate by:
- A. Fusing at the plasma membrane at neutral pH
- B. Forming a pore in the membrane without any fusion
- C. Lysing the plasma membrane
- D. Reverse transcription at the surface
- E. Using neuraminidase to enter
Show answer
Correct answer: A
Some enveloped viruses fuse directly at the cell surface at neutral pH; others such as influenza require the low pH of the endosome to trigger fusion.
MCQ
The 'eclipse period' of the one-step growth curve is the interval during which:
- A. The virus is at its most infectious inside the cell
- B. Progeny are being released
- C. No infectious virus can be recovered from the cells
- D. The host cell divides
- E. Antibody first appears
Show answer
Correct answer: C
After uncoating and before the first progeny are assembled, no infectious particle exists, so none can be recovered even by breaking the cells open.
MCQ
To which Baltimore class does HIV belong?
- A. I (double-stranded DNA)
- B. IV (positive-sense ssRNA, non-reverse-transcribing)
- C. V (negative-sense ssRNA)
- D. VI (positive-sense ssRNA, reverse-transcribing)
- E. VII (double-stranded DNA, reverse-transcribing)
Show answer
Correct answer: D
HIV is a retrovirus, class VI: a positive-sense ssRNA genome reverse-transcribed to DNA and integrated. Hepatitis B is class VII (dsDNA, reverse-transcribing).
MCQ
Viruses replicate inside membranous 'replication organelles' or factories partly in order to:
- A. Increase their genome size
- B. Allow the ribosome to translate polycistronic mRNA
- C. Acquire a lipid envelope
- D. Hide double-stranded RNA from innate sensors
- E. Avoid needing a polymerase
Show answer
Correct answer: D
Sequestering replication concentrates the enzymes and conceals double-stranded RNA, the danger signal that RIG-I and MDA-5 detect to trigger interferon.
MCQ
Which host machinery do most enveloped viruses hijack to pinch off a budding particle?
- A. The proteasome
- B. The host ribosome
- C. The Golgi apparatus acting on its own
- D. Neuraminidase
- E. The ESCRT membrane-scission complex
Show answer
Correct answer: E
Viral “late-domain” motifs recruit the host ESCRT machinery to perform the final membrane scission. Influenza is the exception, severing itself with its M2 protein.
MCQ
Which virus has a segmented, negative-sense RNA genome and carries its own polymerase in the virion?
- A. Poliovirus
- B. Influenza A virus
- C. Hepatitis B virus
- D. Herpes simplex virus
- E. Coronavirus
Show answer
Correct answer: B
Influenza A (an orthomyxovirus, class V) has a segmented negative-sense RNA genome and packages its RdRp. Rotavirus is also segmented and carries a polymerase, but its genome is double-stranded (class III), not negative-sense.