Microbiology EXAM #3

Mutations contribute to the vast diversity of viruses. What viral genetic type has the most stable genome and the lowest mutation rate?

double-stranded DNA virus

Viral replication is

independent of the host cell's DNA but dependent on the host cell's enzymes and metabolism.

Based on your knowledge of cellular and viral processes, which of the following would be (an) appropriate target(s) for antiviral drugs?

integrases

Herpes simplex virus causes a __________ infection of animal cells.

latent

Viruses go through phases during which they are actively replicating and destructive to the host cell, or dormant within the host cell. During which part of the bacteriophage life cycle will the viral genome be replicated in synchrony with the host cell?

lysogenic phase

When solutions of host cells and infectious virions are mixed and spread on an agar plate, ________ form where viruses lyse the host cells.

plaques

Viruses infecting ________ are typically the easiest to grow in the laboratory.

prokaryotes

Two human diseases caused by viruses are __________.

smallpox and polio

When describing the viral genome, which terms could be used?

ss DNA ds DNA ds RNA

A virus whose genome is replicated in synchrony with the host cell genome is known as __________.

temperate

The viral metagenome refers to __________.

the sum total of all the viral genes present in an environment

In HIV the function of the integrase protein is __________.

to integrate viral DNA into the host cell genome

What happens to the virus during the eclipse phase?

uncoating of the virus particles

Virus-encoded "early" proteins are required for __________.

viral genome replication

Overlapping genes are found in __________.

viruses with small genomes

List the steps in the bacteriophage replication cycle in the correct order.

1. Attachment 2. Penetration of viral nucleic acid 3. Synthesis of viral nucleic acid and proteins 4. Assembly and packaging of new viruses 5. Cell lysis and release of new virions

The introductory passage mentions that mutations conferred an evolutionary advantage to certain viruses. Mutations arise when viruses replicate themselves inside a host cell. This requires a precise sequence of events where the virus hijacks the host's machinery to use for itself. The general mechanism of cell infection by a virus always involves the same five general steps. What are the steps of the viral replication cycle?

1. The virus attaches to the host cell, which can be mediated by cell surface receptors. 2. Nucleic acids penetrate the host cell and the capsize is discarded. 3. Host cell enzymes and replication machinery are used to synthesize new viral genetic and capsid material. 4. Newly synthesized viral genomes and capsids are assembled into complete viral particles. 5. The host cell lyses, releasing the newly assembled virus particles.

In the bacteriophage T4's genome, the DNA base cytosine is replaced by __________.

5-hydroxymethyl cytosine

What is the benefit, for a virus, to be a temperate or lysogenic virus?

A single infection event can produce millions of new viral particles instead of hundreds of viral particles.

The introductory passage suggests that selection on mutation rate—from rapidly mutating ssRNA viruses to slower dsDNA viruses—may have been a critical step in the evolution of more stable DNA-based genomes, and subsequently first cells. However, if viruses were able to infect the vast majority of host cells, evolution would have stalled. According to the chapter, what is one way that cells have evolved to avoid viral infection?

An attachment receptor on the host cell surface that is recognized by the virus is mutated, making the host resistant to infection.

When determining the number of bacterial virus particles in a suspension, why is the number obtained always less than the actual viral titer?

Because all of the virus particles do not infect with 100% efficiency.

Virus-specified anti-sigma factors are used to _____.

Bind to the host cell sigma factor, inactivating transcription

Bacteriophages can influence bacterial evolution by __________.

Culling bacterial populations

Some viruses, especially animal viruses, have an envelope surrounding their nucleocapsid. This envelope may have viral proteins embedded in it. Why are the viral proteins more readily recognized and targeted by the immune system than the envelope?

Enveloped viruses are often coated with animal cell membrane as they leave the cell. This membrane is a major component of the viral envelope. The viral proteins are more readily recognized by the immune system, as they are more distinct from the materials normally found in and on the cells. However, the envelope can contain several types of viral components as well.

Lysogeny is unique to bacteriophages; similar relationships have not been found among the animal viruses. T/F

FALSE

Penetration requires that the entire virus is inserted within the host. T/F

FALSE

All of the following are negative-strand RNA viruses EXCEPT __________.

HIV

What type of symmetry does the tobacco mosaic virus exhibit?

Helical symmetry

Bacteriophage gene expression uses _____.

Host and virally encoded proteins

What does RNA replicase do?

It replicates viral RNA.

Certain viruses are immune to host restriction systems. Which of the following viral genomes would be affected by this system?

Lambda, a dsDNA bacteriophage

The enzyme lysozyme is used to _____.

Make a hole in the bacterial cell wall for release of the mature virus particles

TRUE STATEMENT

Many viruses contain their own nucleic acid polymerases.

The diagram given below shows a graph that represents viral replication. What phase begins as newly synthesized viral nucleic acid molecules are packaged into their protein capsids?

Maturation phase

An enveloped virus infects a cell and slowly releases mature virus particles by budding without lysing the host cell. This is termed a __________.

Persistent infection

Why does the structure of plant and bacterial cells illustrate the reason for having few enveloped viruses that infect these cells?

Plant and bacterial cells contain cell walls that limit the virus from budding out and obtaining the envelope.

The following figure shows a method commonly used in the study of viruses. This method is used to __________.

Quantify bacterial viruses

Viruses rely on the host cell machinery to make new viruses, but they sometimes provide their own enzymes (such as reverse transcriptase, which is an RNA-dependent DNA polymerase). Which of the following types of enzymes would a virus need to provide in order to replicate within a cell? In other words, which of these enzymes would not normally be found in a cell unless a virus provided it?

RNA replicases (RNA-dependent RNA polymerases) for RNA viruses

Which of the following groups contains some representatives with genomes that are so small that they may have as few as two genes?

RNA viruses

Reverse transcriptase is a(n)

RNA-dependent DNA polymerase.

These RNA viruses replicate through a DNA intermediate.

Retroviruses

Retroviruses mutate relatively rapidly. What is one of the reasons?

Reverse transcriptase lacks the proofreading function found in many DNA polymerases, so mutations accumulate more easily.

Neuraminidases are found in _____.

Some animal viruses

A lytic infection results in death of the host cell. T/F

TRUE

Viruses that can infect their host and establish a stable, long-term relationship are known as __________.

Temperate

Plaque assays are often used to estimate the number of virions in a sample of a particular volume (the titer). The count is given as plaque-forming units. Which of the following is NOT generally a concern that must be considered in evaluating the results of plaque assays?

The largest problem is that single viruses may create multiple plaques, resulting in overestimates of the number of viruses present.

FALSE STATEMENTS

The origins of the nucleic acid polymerases used by viruses are eukaryotic. All viruses contain their own nucleic acid polymerases. Viruses do not contain their own nucleic acid polymerases.

What event has to happen for viral replication to initiate?

The viral genome enters a living host cell.

Which of the following examples is an example of lysogenic conversion?

Vibrio cholerae bacteria produce cholera toxin when infected with a phage.

For an enveloped animal virus, what would be be a good target for antiviral drugs?

Viral-encoded proteins in the envelope

A virus in its extracellular state is known as a _____.

Virion

Viruses are non-cellular infectious agents. The genetic information of a virus is surrounded by a protective coat made up of many protein monomers, called the capsid. The genetic component of a virus plus its capsid layer are collectively called the nucleocapsid. Which of the following accurately describe viruses?

Viruses are as old as Bacteria and Archaea. Viruses are smaller than Bacteria and Archaea. Viruses are unable to perform replication on their own. The genetic information of a virus can be in the form of RNA or DNA. Viruses cannot replicate on their own. Viruses do not belong to any of the three domains: Bacteria, Archaea, and Eukarya.

Which of the following is NOT a way in which viruses are different from living organisms?

Viruses are obligate parasites and no living organisms are obligate parasites.

Which of the following is true about viral infection?

Viruses are very specific in which cells they infect. For a virus to be taken up by a cell, it must find a cell with appropriate receptors on the surface. It cannot be taken up by cells without the appropriate receptors.

Some viruses have plus sense RNA genomes. Which of the following could describe one of those viruses?

a virus that contains RNA with the same nucleotide sequence as the functional mRNA that its host cell will translate to produce viral products

Based on what is currently thought about lytic versus lysogenic life cycles, which of the following viruses would be most likely to be found as a prophage?

a virus that is found in a low-density population of host bacteria deep in the ocean

ALL viral particles

are metabolically inert.

The first step in the infection cycle of a virus is __________.

attachment to the host cell surface

Plaque assays use __________ to cultivate and quantify virus particles.

bacteria

The term "phage" is generally reserved for the viruses that infect

bacteria.

M13, Mu, and T4 are examples of __________.

bacteriophages

Which protein accumulates when phage lambda infects a bacterial cell and leads to the activation of the lysogenic pathway?

cI

What are the minimal components needed for a virion?

capsid and genome