Q: Which equation correctly represents the calculation of viral titer in a plaque assay?

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

Question 1

Which of the following statements best explains why viruses are not considered living organisms?

Accepted Answer

Viruses possess their own genome but lack the ability to generate energy or synthesize proteins independently.

Question 2

A virus with a single-stranded RNA genome that can be directly translated by host ribosomes is classified as:

Accepted Answer

ssRNA (+) sense virus

Question 3

Which structural feature distinguishes an enveloped virus from a naked virus?

Accepted Answer

Presence of a lipid membrane surrounding the nucleocapsid

Question 4

The helical symmetry of a rod-shaped virus, such as Tobacco Mosaic Virus (TMV), is determined by:

Accepted Answer

The length of the nucleic acid and the size of the capsomere subunits

Question 5

Which of the following best describes the function of tail fibers in bacteriophage T4 infection?

Accepted Answer

They recognize and bind to specific receptors on the bacterial cell surface.

Question 6

During the lytic cycle of a bacteriophage, which step immediately follows the assembly of new virions?

Accepted Answer

Lysis of the host cell and release of new virions

Question 7

A plaque assay is used to determine viral titer. What does each plaque on a bacterial lawn represent?

Accepted Answer

Infection initiated by a single infectious virion

Question 8

Which of the following is a reason why the number of plaque-forming units (PFU) is usually lower than the number of virions observed by electron microscopy?

Accepted Answer

Some virions are inactive or contain defective genomes.

Question 9

Which of the following best describes lysogenic conversion?

Accepted Answer

A temperate phage integrates its genome into the host chromosome, conferring new genetic properties to the host.

Question 10

In bacteriophage lambda, what determines whether the virus enters the lytic or lysogenic pathway?

Accepted Answer

Relative levels of cI and Cro repressor proteins

Question 11

Which equation correctly represents the calculation of viral titer in a plaque assay?

Accepted Answer

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

Question 12

Which of the following is TRUE regarding the eclipse period in the one-step growth curve of a virus?

Accepted Answer

No infectious virions are detected in the medium because they are being assembled inside the host cell.

Question 13

Which of the following best explains why animal viruses often have an envelope, while most bacterial viruses do not?

Accepted Answer

Animal viruses acquire their envelope from the host cell membrane during exit, while bacterial viruses typically lyse the host cell.

Question 14

Which of the following is a plausible consequence if a mutation alters the receptor used by a virus to attach to its host cell?

Accepted Answer

The host cell may become resistant to infection by that virus.

Question 15

Which of the following statements about the diversity of viral genomes is correct?

Accepted Answer

Viral genomes can be composed of DNA or RNA, which may be single- or double-stranded, linear or circular.

Question 16

Which of the following best describes the role of early proteins in the T4 bacteriophage life cycle?

Accepted Answer

They are enzymes needed for DNA replication and modification of host RNA polymerase.

Question 17

A virus is found to have a genome that switches between RNA and DNA forms during its life cycle. Which class of viruses does this describe?

Accepted Answer

Retroviruses

Question 18

Which of the following is a real-world example of lysogenic conversion?

Accepted Answer

A bacterium acquires the ability to produce a toxin after infection by a temperate phage.

Question 19

Which of the following best explains why most of Earth's genetic diversity is thought to reside in viruses?

Accepted Answer

Viruses are extremely abundant and have highly diverse genomes, many of which encode unknown functions.

Viruses and Their Replication: Structure, Diversity, and Life Cycle

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