$$\text{Plating Efficiency} = \frac{\text{Number of Plaque-Forming Units}}{\text{Total Number of Virions}} \times 100\%$$

Question 1

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

Accepted Answer

A naked virus contains only nucleic acid and capsid, while an enveloped virus has an additional lipoprotein membrane derived from the host cell.

Question 2

Which enzyme found in some virions is responsible for making a hole in the bacterial cell wall to allow viral nucleic acid entry?

Accepted Answer

Lysozyme

Question 3

A virus with a single-stranded RNA genome that is complementary to mRNA is described as:

Accepted Answer

Minus sense

Question 4

During the replication cycle of bacteriophage T4, which step is directly responsible for host cell specificity?

Accepted Answer

Attachment to complementary receptors on the host cell surface

Question 5

Which of the following mechanisms allows prokaryotes to protect themselves from viral infection by cleaving foreign DNA at specific sites?

Accepted Answer

Restriction endonucleases

Question 6

What is the function of the packaging motor during the assembly of bacteriophage T4?

Accepted Answer

It uses ATP to pump the double-stranded linear genome into the prohead.

Question 7

Which of the following best explains why relatively few enveloped plant or bacterial viruses exist?

Accepted Answer

Cell walls surrounding the cell membrane prevent the acquisition of an envelope.

Question 8

A plaque assay is used to:

Accepted Answer

Quantify the number of infectious virus particles by counting clear zones on a lawn of host cells.

Question 9

Which of the following is a feature of circular permutation in viral genomes?

Accepted Answer

The same genes are arranged in different orders in different genome copies.

Question 10

Which protein is responsible for forming a pore in the bacterial cytoplasmic membrane to allow lysozyme to access the cell wall during phage lysis?

Accepted Answer

Holin

Question 11

In the context of phage lambda, what is the role of the integrase gene product?

Accepted Answer

It mediates site-specific recombination to integrate phage DNA into the bacterial chromosome.

Question 12

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

Accepted Answer

It is the sum of the eclipse and maturation phases before virions are released.

Question 13

Which of the following is true regarding the replication of retroviruses?

Accepted Answer

Reverse transcriptase synthesizes a DNA copy from the viral RNA genome.

Question 14

Which of the following equations correctly represents the calculation of plating efficiency in a plaque assay?

Accepted Answer

$$	ext{Plating Efficiency} = \frac{	ext{Number of Plaque-Forming Units}}{	ext{Total Number of Virions}} 	imes 100\%$$

Question 15

Which of the following statements about the lytic and lysogenic cycles of temperate phages is correct?

Accepted Answer

Lysogeny is maintained by a phage-encoded repressor protein that prevents entry into the lytic cycle.

Question 16

Which of the following is a plausible consequence of lysogenic conversion in bacteria?

Accepted Answer

The bacterium may express a pathogenic toxin encoded by the prophage.

Question 17

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

Accepted Answer

Not all virions are infectious or conditions may be inappropriate for infectivity.

Question 18

Which of the following is a correct description of rolling circle replication in phage M13?

Accepted Answer

A nick is made in one strand, and DNA synthesis proceeds around the circle, producing a long concatemer.

Question 19

Which of the following is a function of the Cro protein in phage lambda?

Accepted Answer

It inhibits the synthesis of the CI repressor and promotes the lytic cycle.

Viruses and Phages: Structure, Replication, and Life Cycles

Study Guide - Practice Questions