Q: Which of the following equations correctly represents the calculation of plaque-forming units (PFU) in a viral plaque assay?

$$\text{PFU} = \frac{\text{Number of plaques}}{\text{Dilution factor} \times \text{Volume plated (mL)}}$$

Question 1

Which of the following statements best explains why viruses are considered obligatory intracellular parasites?

Accepted Answer

They require living host cells to multiply because they lack ribosomes and an ATP-generating mechanism.

Question 2

A scientist is studying a virus with a helical capsid structure. Which of the following viruses is most likely being studied?

Accepted Answer

Rabies virus

Question 3

Which of the following best describes the function of spikes on some enveloped viruses?

Accepted Answer

They are projections made of carbohydrate and protein that may be used for attachment to host cells.

Question 4

A patient is diagnosed with a persistent viral infection that gradually worsens over years and is ultimately fatal. Which virus is most likely responsible?

Accepted Answer

Human immunodeficiency virus (HIV)

Question 5

Which of the following is a unique feature of the lysogenic cycle of bacteriophage lambda (λ) compared to the lytic cycle?

Accepted Answer

The phage DNA is incorporated into the host cell DNA as a prophage.

Question 6

Which of the following best describes the process of specialized transduction?

Accepted Answer

A phage transfers specific bacterial genes adjacent to the prophage to another bacterium.

Question 7

A researcher is using a continuous cell line to grow animal viruses in the laboratory. Which of the following statements is true about continuous cell lines?

Accepted Answer

They are derived from transformed (cancerous) cells and can be maintained indefinitely.

Question 8

Which of the following equations correctly represents the calculation of plaque-forming units (PFU) in a viral plaque assay?

Accepted Answer

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

Question 9

Which of the following is a correct sequence of events in the multiplication of a DNA-containing animal virus?

Accepted Answer

Attachment → Penetration → Uncoating → Early transcription/translation → DNA replication → Late transcription/translation → Maturation → Release

Question 10

Which of the following best explains why prion diseases are difficult to treat?

Accepted Answer

Prions are infectious proteins that lack nucleic acids and are resistant to standard sterilization methods.

Question 11

A virus is classified in the Baltimore system as a retrovirus. Which enzyme is essential for its replication?

Accepted Answer

Reverse transcriptase

Question 12

Which of the following is a real-world example of phage conversion resulting from lysogeny?

Accepted Answer

Production of diphtheria toxin by Corynebacterium diphtheriae

Question 13

Which of the following best describes the difference between a naked virus and an enveloped virus?

Accepted Answer

A naked virus lacks an envelope, while an enveloped virus has a lipid, protein, and carbohydrate envelope derived from the host cell membrane.

Question 14

Which of the following is a characteristic of transformed cells resulting from oncogenic viral infection?

Accepted Answer

They express a tumor-specific transplantation antigen (TSTA) on their surface.

Question 15

A patient presents with cold sores that recur during periods of stress. Which type of viral infection is this an example of?

Accepted Answer

Latent viral infection

Question 16

Which of the following best explains why viruses are not classified as living organisms?

Accepted Answer

They lack cellular structure and cannot carry out metabolism independently.

Question 17

Which of the following is a correct statement about the host range of viruses?

Accepted Answer

The host range is determined by specific host attachment sites and cellular factors.

Question 18

Which of the following best describes the role of reverse transcriptase in the replication of retroviruses?

Accepted Answer

It synthesizes DNA from an RNA template.

Question 19

Which of the following is a real-world application of growing viruses in embryonated eggs?

Accepted Answer

Production of vaccines, such as the influenza vaccine

(Chapter 13) Viruses and Prions: Structure, Classification, and Pathogenic Mechanisms

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