Question 1

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

Accepted Answer

Viruses cannot replicate unless inside a host cell and lack independent metabolism.

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 of the following best describes the structure of a helical virus such as Tobacco Mosaic Virus (TMV)?

Accepted Answer

A single type of protein arranged in a helix around the viral RNA

Question 4

During the lytic cycle of a bacteriophage, which of the following events occurs first after the virus attaches to the host cell?

Accepted Answer

Injection of viral nucleic acid into the host cytoplasm

Question 5

Which of the following is a key difference between animal virus replication and bacteriophage replication?

Accepted Answer

The entire virion enters the animal cell, whereas only the nucleic acid enters the bacterial cell.

Question 6

A researcher performs a plaque assay and counts 50 plaques on a plate after adding 0.1 mL of a 10⁻⁶ dilution of a viral stock. What is the viral titer in plaque-forming units per mL (PFU/mL)?

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

Accepted Answer

$$5 	imes 10^7$ PFU/mL

Question 7

Which of the following best describes the function of early proteins synthesized after bacteriophage T4 infection?

Accepted Answer

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

Question 8

A temperate bacteriophage infects a bacterial cell and integrates its genome into the host chromosome. What is the integrated viral DNA called?

Accepted Answer

Prophage

Question 9

Which of the following is a plausible reason why the number of plaque-forming units (PFU) is often lower than the number of virus particles counted by electron microscopy?

Accepted Answer

Some virions are defective and cannot infect host cells.

Question 10

Which of the following best explains lysogenic conversion?

Accepted Answer

A prophage confers new genetic properties to the bacterial host.

Question 11

A bacteriophage lambda infection results in either lysis or lysogeny. Which factor most directly determines the outcome?

Accepted Answer

The relative levels of the cI and Cro repressor proteins

Question 12

Which of the following is true regarding the symmetry of viral capsids?

Accepted Answer

Icosahedral viruses have 20 triangular faces and multiple capsomeres per face.

Question 13

Which of the following best describes the latent period in a one-step viral growth curve?

Accepted Answer

It includes the eclipse and maturation phases before virions are released.

Question 14

Which of the following is NOT a possible host for viruses?

Accepted Answer

Minerals

Question 15

A virus is found to have a genome size of 1,800 base pairs and encodes only two proteins. Which of the following is most likely true about this virus?

Accepted Answer

It is among the smallest known viruses.

Question 16

Which of the following best describes the role of tail fibers in bacteriophage T4 infection of E. coli?

Accepted Answer

They attach to specific receptors on the E. coli cell surface.

Question 17

Which of the following is a real-world consequence of lysogenic conversion in bacteria?

Accepted Answer

Bacteria acquire new traits, such as toxin production, due to prophage genes.

Question 18

Which of the following best explains why enveloped viruses are more sensitive to detergents than non-enveloped (naked) viruses?

Accepted Answer

Detergents dissolve the lipid envelope, inactivating the virus.

Question 19

A virus is found to have both DNA and RNA as genetic material at different stages of its life cycle. Which of the following is most likely true about this virus?

Accepted Answer

It is a retrovirus that uses reverse transcription.

Viruses and Their Replication: Structure, Genomes, and Infection Cycles

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