Questions

Virion Structure and Composition — Questions

Study questions for Virion Structure and Composition.

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20 questions: 20 MCQ, 0 written.

High-priority (past-paper) questions only

Positive-sense RNA has the same sequence as mRNA and is translated directly (picornaviruses, flaviviruses, coronaviruses). Negative-sense RNA must first be transcribed by the virion’s own RNA-dependent RNA polymerase (RdRp).

Retroviruses replicate through a DNA intermediate using reverse transcriptase (an RNA-dependent DNA polymerase) rather than an RdRp. The hepadnaviruses (hepatitis B) use the same enzyme, though they are DNA viruses.

Priming cleavage does not switch fusion on; it leaves the fusion protein metastable (“primed but waiting”). The trigger (low endosomal pH for influenza, or receptor binding for others) then releases it into its fusion-active form.

The hepadnavirus (HBV) genome is a partially double-stranded circle repaired to cccDNA in the nucleus; this persistent template is a major reason HBV is so hard to cure.

Lipid solvents and detergents dissolve the envelope and destroy the infectivity of enveloped viruses while leaving naked viruses intact. This is the basis of the ether-sensitivity test for an envelope.

Only icosahedral and helical symmetry occur. Particles that are neither (poxviruses, the HIV-1 cone, tailed bacteriophages) are termed “complex”.

Retroviruses are uniquely diploid, packaging two copies of their positive-sense ssRNA genome; every other viral genome is haploid.

Rabies virus (a rhabdovirus) has a helical, enveloped nucleocapsid; the others are icosahedral. In vertebrate viruses, helical nucleocapsids are always enclosed within an envelope.

The envelope is taken from a host membrane during budding, so its lipids are host-derived and vary with the budding site, while the glycoprotein spikes embedded in it are virus-coded.

The dsRNA viruses (Reoviridae, Birnaviridae, Picobirnaviridae) all carry segmented genomes; rotavirus, a reovirus, has 11 segments.

The receptor is the host-cell molecule; the ligand is the viral molecule that binds it. For example, the SARS-CoV-2 spike is the ligand and ACE2 the receptor.

Vertex capsomeres bonded to five neighbours are pentons (pentamers); those on faces and edges, bonded to six, are hexons. An icosahedron has twelve vertices, hence twelve pentons.

The mature HIV-1 capsid is a fullerene cone: a lattice of CA hexamers closed by exactly twelve pentamers, curved into a cone. Neither icosahedral nor helical, so “complex”.

Nucleocapsid = the capsid plus the nucleic acid it encloses. Where present, an envelope is added around it to form the complete virion.

The non-glycosylated matrix protein (for example influenza M1) lines the inner face of the envelope, conferring rigidity and bridging the envelope and the nucleocapsid.

Naked enteric viruses (rotavirus, enteroviruses) resist stomach acid, fitting their faecal-oral spread, whereas enveloped viruses are inactivated at pH 5–6. Enveloped viruses are also the more heat-labile, −70 °C preserves infectivity for years, and prions resist boiling.

Cryo-EM now routinely achieves near-atomic resolution and has become the dominant technique for virus particles and their protein machines; X-ray crystallography is reserved for small viral enzymes and antibody-fragment complexes.

Symmetry strictly describes the capsid or nucleocapsid; the envelope is not a rigid symmetric shell, so the whole particle varies in size and shape, even though the nucleocapsid inside is ordered.

Sixty is the maximum in strictly identical environments. Caspar and Klug showed that subunits in quasi-equivalent positions allow multiples of 60 (the triangulation number, T), building larger shells from a single protein.

The light microscope resolves to about 300 nm; most viruses are smaller and so were invisible until electron microscopy. Poxviruses, the largest vertebrate viruses, sit right at that limit.

High priorityMCQ
A positive-sense single-stranded RNA genome is defined by the fact that it:
- A. Can act directly as messenger RNA
- B. Is complementary to mRNA and must be transcribed before translation
- C. Is always segmented
- D. Requires reverse transcriptase
- E. Is double-stranded at the 5' end
Show answer
Correct answer: A
Positive-sense RNA has the same sequence as mRNA and is translated directly (picornaviruses, flaviviruses, coronaviruses). Negative-sense RNA must first be transcribed by the virion’s own RNA-dependent RNA polymerase (RdRp).
High priorityMCQ
All RNA viruses must carry their own RNA-dependent RNA polymerase, because host cells lack one. Which viruses are the exception, replicating through a reverse transcriptase instead?
- A. Coronaviruses
- B. Paramyxoviruses
- C. Flaviviruses
- D. Retroviruses
- E. Orthomyxoviruses
Show answer
Correct answer: D
Retroviruses replicate through a DNA intermediate using reverse transcriptase (an RNA-dependent DNA polymerase) rather than an RdRp. The hepadnaviruses (hepatitis B) use the same enzyme, though they are DNA viruses.
High priorityMCQ
Cleavage of the influenza haemagglutinin precursor (HA0) into HA1 and HA2 by a host protease:
- A. Inactivates the protein completely
- B. Immediately triggers membrane fusion
- C. Adds the lipid envelope to the mature particle
- D. Is required for viral genome replication
- E. Leaves it metastable, primed but waiting to fuse
Show answer
Correct answer: E
Priming cleavage does not switch fusion on; it leaves the fusion protein metastable (“primed but waiting”). The trigger (low endosomal pH for influenza, or receptor binding for others) then releases it into its fusion-active form.
High priorityMCQ
Covalently closed circular DNA (cccDNA) is a hallmark of which virus?
- A. Adenovirus
- B. Poliovirus
- C. Hepatitis B virus
- D. Influenza A virus
- E. Rotavirus
Show answer
Correct answer: C
The hepadnavirus (HBV) genome is a partially double-stranded circle repaired to cccDNA in the nucleus; this persistent template is a major reason HBV is so hard to cure.
High priorityMCQ
Treating a virus with a lipid solvent such as ether or chloroform abolishes its infectivity. This indicates that the virus:
- A. Has a double-stranded DNA genome
- B. Is enveloped
- C. Is icosahedral
- D. Is a prion
- E. Replicates in the cytoplasm
Show answer
Correct answer: B
Lipid solvents and detergents dissolve the envelope and destroy the infectivity of enveloped viruses while leaving naked viruses intact. This is the basis of the ether-sensitivity test for an envelope.
High priorityMCQ
Viral capsids display only two true symmetries. They are:
- A. Cubic and spherical
- B. Icosahedral and helical
- C. Helical and pleomorphic
- D. Icosahedral and complex
- E. Tetrahedral and helical
Show answer
Correct answer: B
Only icosahedral and helical symmetry occur. Particles that are neither (poxviruses, the HIV-1 cone, tailed bacteriophages) are termed “complex”.
High priorityMCQ
Viral genomes are haploid, with one important exception. Which viruses carry a diploid genome?
- A. Herpesviruses
- B. Reoviruses
- C. Retroviruses
- D. Poxviruses
- E. Orthomyxoviruses
Show answer
Correct answer: C
Retroviruses are uniquely diploid, packaging two copies of their positive-sense ssRNA genome; every other viral genome is haploid.
High priorityMCQ
Which of the following viruses has a helical nucleocapsid?
- A. Adenovirus
- B. Human papillomavirus
- C. Rabies virus
- D. Poliovirus
- E. Hepatitis B virus
Show answer
Correct answer: C
Rabies virus (a rhabdovirus) has a helical, enveloped nucleocapsid; the others are icosahedral. In vertebrate viruses, helical nucleocapsids are always enclosed within an envelope.
High priorityMCQ
Which statement about the viral envelope is correct?
- A. Both its lipids and its proteins are virus-encoded
- B. Its lipids are host-derived, its embedded proteins virus-encoded
- C. It is a rigid protein shell with icosahedral symmetry
- D. It is synthesised de novo from viral lipid in the cytoplasm
- E. It makes the virus more resistant to lipid solvents
Show answer
Correct answer: B
The envelope is taken from a host membrane during budding, so its lipids are host-derived and vary with the budding site, while the glycoprotein spikes embedded in it are virus-coded.
MCQ
Double-stranded RNA genomes among vertebrate viruses are characteristically:
- A. Monopartite and circular
- B. Positive-sense and capped
- C. Segmented, as in the reoviruses
- D. Diploid, like the retroviruses
- E. Replicated by a virion reverse transcriptase
Show answer
Correct answer: C
The dsRNA viruses (Reoviridae, Birnaviridae, Picobirnaviridae) all carry segmented genomes; rotavirus, a reovirus, has 11 segments.
MCQ
In virus attachment, which use of terms is correct?
- A. Receptor and ligand both refer to viral proteins
- B. The receptor is on the virus; the ligand on the host cell
- C. The ligand is the lipid envelope itself
- D. The receptor is on the host cell; the ligand on the virus
- E. The receptor is the viral glycoprotein spike
Show answer
Correct answer: D
The receptor is the host-cell molecule; the ligand is the viral molecule that binds it. For example, the SARS-CoV-2 spike is the ligand and ACE2 the receptor.
MCQ
On an icosahedral capsid such as adenovirus, the capsomeres at the twelve vertices, each bonded to five neighbours, are called:
- A. Hexons
- B. Peplomers
- C. Protomers
- D. Matrix units
- E. Pentons
Show answer
Correct answer: E
Vertex capsomeres bonded to five neighbours are pentons (pentamers); those on faces and edges, bonded to six, are hexons. An icosahedron has twelve vertices, hence twelve pentons.
MCQ
The mature capsid of HIV-1 is best described as:
- A. Icosahedral, built on a T = 3 lattice
- B. Helical
- C. A rigid brick-shaped shell
- D. A fullerene cone of hexamers and pentamers
- E. Absent; HIV has no capsid
Show answer
Correct answer: D
The mature HIV-1 capsid is a fullerene cone: a lattice of CA hexamers closed by exactly twelve pentamers, curved into a cone. Neither icosahedral nor helical, so “complex”.
MCQ
The term 'nucleocapsid' refers to:
- A. The lipid envelope together with its surface glycoproteins
- B. The genome together with its surrounding capsid
- C. The matrix protein layer alone
- D. The capsid alone, without nucleic acid
- E. The complete enveloped virion
Show answer
Correct answer: B
Nucleocapsid = the capsid plus the nucleic acid it encloses. Where present, an envelope is added around it to form the complete virion.
MCQ
Where is the matrix protein of an enveloped virus located, and what does it do?
- A. On the outer surface, where it mediates receptor binding
- B. Within the genome, priming replication
- C. Forming the icosahedral capsid shell
- D. It is a host protein with no viral function
- E. Lining the inner face of the envelope, giving rigidity
Show answer
Correct answer: E
The non-glycosylated matrix protein (for example influenza M1) lines the inner face of the envelope, conferring rigidity and bridging the envelope and the nucleocapsid.
MCQ
Which statement about virus stability is correct?
- A. Naked enteric viruses (rotavirus, enteroviruses) survive stomach acid
- B. Enveloped viruses are generally more heat-stable than naked enteric viruses
- C. All viruses are inactivated within seconds at 4 °C
- D. Prions are readily destroyed by boiling
- E. Freezing at −70 °C rapidly inactivates most viruses
Show answer
Correct answer: A
Naked enteric viruses (rotavirus, enteroviruses) resist stomach acid, fitting their faecal-oral spread, whereas enveloped viruses are inactivated at pH 5–6. Enveloped viruses are also the more heat-labile, −70 °C preserves infectivity for years, and prions resist boiling.
MCQ
Which statement best reflects the current role of cryo-electron microscopy (cryo-EM) in virus structure?
- A. It remains low-resolution, useful only for imaging whole infected cells
- B. It has been largely superseded by negative staining
- C. It can be used only on crystallised samples
- D. It routinely reaches near-atomic resolution and dominates virus-particle imaging
- E. It works only on viruses larger than 300 nm
Show answer
Correct answer: D
Cryo-EM now routinely achieves near-atomic resolution and has become the dominant technique for virus particles and their protein machines; X-ray crystallography is reserved for small viral enzymes and antibody-fragment complexes.
MCQ
Why are many enveloped viruses, such as influenza and HIV, described as pleomorphic?
- A. The lipid envelope is not a rigid shell
- B. Their capsids switch between icosahedral and helical forms
- C. They have no internal order at all
- D. They change their genome from particle to particle
- E. They are defective, non-infectious particles
Show answer
Correct answer: A
Symmetry strictly describes the capsid or nucleocapsid; the envelope is not a rigid symmetric shell, so the whole particle varies in size and shape, even though the nucleocapsid inside is ordered.
MCQ
Why can an icosahedral capsid contain more than 60 identical subunits?
- A. The subunits are chemically different from one another
- B. The genome forces the extra subunits in
- C. The capsid switches to helical symmetry
- D. The extra subunits are host-derived
- E. Subunits sit in quasi-equivalent positions, flexing slightly
Show answer
Correct answer: E
Sixty is the maximum in strictly identical environments. Caspar and Klug showed that subunits in quasi-equivalent positions allow multiples of 60 (the triangulation number, T), building larger shells from a single protein.
MCQ
Why was the development of electron microscopy essential to the study of viruses?
- A. Most viruses are smaller than the light microscope's ~300 nm resolution limit
- B. Viruses cannot be propagated in cell culture without electron microscopy
- C. Light microscopy cannot detect viral nucleic acid at all
- D. It is the only method that can determine a full genome sequence
- E. Viruses are transparent to visible light only when alive
Show answer
Correct answer: A
The light microscope resolves to about 300 nm; most viruses are smaller and so were invisible until electron microscopy. Poxviruses, the largest vertebrate viruses, sit right at that limit.