Question 1

Which of the following best describes the main difference between a virion and an intracellular replication complex?

Accepted Answer

A virion is the extracellular, metabolically inert form of a virus, while an intracellular replication complex is the active form inside a host cell directing viral replication.

Question 2

A researcher is studying a virus with a helical capsid symmetry. Which of the following statements about this virus is most likely true?

Accepted Answer

The length of the virus is determined by the length of its nucleic acid, and the width is determined by the size and packaging of its capsomeres.

Question 3

Which of the following best explains why enveloped viruses are rare among plant and bacterial viruses?

Accepted Answer

Plant and bacterial cells have cell walls that prevent the acquisition or retention of a viral envelope derived from the host membrane.

Question 4

A scientist performs a plaque assay to determine the titer of a viral stock. If 0.1 mL of a $$10^{-6}$$ dilution produces 50 plaques, what is the titer of the original stock in plaque-forming units per mL (PFU/mL)?

$$	ext{Titer} = \frac{	ext{Number of plaques}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}$$

Accepted Answer

$$5 	imes 10$$^{8}$$ PFU/mL

Question 5

Which of the following is a key feature of prion diseases that distinguishes them from viral infections?

Accepted Answer

Prion diseases are caused by misfolded proteins that induce conformational changes in normal host proteins, without involving nucleic acids.

Question 6

A patient is infected with a virus that integrates its genome into the host chromosome and remains dormant for years before reactivating. Which term best describes this type of viral life cycle?

Accepted Answer

Lysogenic cycle

Question 7

Which of the following best explains the mechanism by which restriction endonucleases protect prokaryotes from viral infection?

Accepted Answer

They cleave foreign double-stranded DNA at specific recognition sites, while host DNA is protected by methylation.

Question 8

A new virus is discovered with a segmented, negative-strand RNA genome. Which of the following is a likely consequence of this genome structure?

Accepted Answer

The virus can undergo antigenic shift by reassorting genome segments with other strains, potentially leading to new surface protein combinations.

Question 9

Which of the following statements about retroviruses is correct?

Accepted Answer

Retroviruses use reverse transcriptase to synthesize DNA from their RNA genome, which is then integrated into the host genome.

Question 10

A viroid is most likely to cause disease in which of the following organisms, and what is a key structural feature that contributes to its stability outside the host?

Accepted Answer

Plants; a hairpin-shaped, double-stranded molecule with closed ends

Question 11

Which of the following best describes the function of the cI protein in bacteriophage lambda?

Accepted Answer

It represses lytic genes, promoting lysogeny by inhibiting the expression of genes required for the lytic cycle.

Question 12

A virus is found to encode its own RNA-dependent RNA polymerase. Which class of viruses is this enzyme essential for, and why?

Accepted Answer

RNA viruses, because host cells lack enzymes that can replicate RNA from an RNA template.

Question 13

Which of the following is a plausible reason why DNA viruses may have evolved after RNA viruses?

Accepted Answer

DNA is more stable than RNA and provides protection from ribonucleases, possibly allowing for more complex genomes.

Question 14

A patient is diagnosed with influenza. Which of the following viral features is most responsible for the emergence of new influenza strains that can evade the immune system?

Accepted Answer

Segmented genome allowing for antigenic shift through reassortment of RNA segments

Question 15

Which of the following best describes the process of rolling circle replication as used by certain viruses?

Accepted Answer

A circular DNA molecule is nicked, and new DNA is synthesized continuously around the circle, displacing the old strand.

Question 16

Which of the following is a unique feature of poxviruses compared to most other DNA animal viruses?

Accepted Answer

All replication occurs in the cytoplasm rather than the nucleus.

Question 17

A virus infects E. coli and is found to have a short tail, an icosahedral head, and encodes its own RNA polymerase. Which virus is this most likely to be?

Accepted Answer

Bacteriophage T7

Question 18

Which of the following is a likely ecological role of acute viruses in microbial communities?

Accepted Answer

They act as predators or parasites, limiting host population density and increasing host diversity.

Question 19

Which of the following best explains why the number of plaque-forming units (PFU) is always lower than direct counts by electron microscopy?

Accepted Answer

Not all virus particles are infectious; some may be defective and unable to form plaques.

Viruses, Viroids, and Prions: Structure, Replication, and Ecological Roles

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