Chapter 6: Acellular Pathogens

Viruses that infect bacterial cells are known as ________.

bacteriophages

Proteinaceous infectious particles are known as

prions

Although there are no current treatments for prion infections, which of the following could be used? A. antibiotics B. nucleases C. proteases D. sulfa drugs

C

An acellular entity composed of a small circular RNA molecule without a capsid and capable of replicating its genome is known as which of the following? A. satellite RNA B. virion C. viroid D. virusoid

C

Chicken pox can reemerge as which disease? A. cowpox B. genital warts C. shingles D. smallpox

C

Transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease, are caused by which of the following? A. bacterium B. fungus C. prion D. virus

C

Which effect does tomato planta macho viroids have on the crop? A. browning of fruit B. crown-gall formation C. loss of chlorophyll D. lysis of root cells

C

Which mechanism of horizontal gene transfer relies on the use of viruses? A. conjugation B. mutation C. transduction D. transformation

C

Which method cannot be used to detect virus in a patient's serum? A. EIA B. hemagglutinin assay C. growth in pure culture D. RT-PCR

C

(−)ssRNA is transcribed into (+)ssRNA using which of the following? A. DNA polymerase encoded by the host cell B. DNA polymerase encoded by the virus C. RNA polymerase encoded by the host cell D. RNA polymerase encoded by the virus

D

An acellular entity composed of a small circular RNA molecule without a capsid and is not capable of replicating its genome is known as which of the following? A. provirus B. virion C. viroid D. virusoid

D

How is kuru transmitted? A. bloodborne transmission B. contaminated neurosurgical instruments C. eating contaminated cattle meat D. eating infected meat through cannibalism

D

The Baltimore Classification system describes viruses according to which characteristic? A. capsid shape B. host-cell susceptibility C. the presence or absence of an envelope D. the viral genome

D

The liquid portion of blood that does not contain clotting factors is known as which of the following? A. erythrocytes B. plasma C. platelets D. serum

D

Which scientist first discovered prions? A. Dimitri Ivanovski B. Edward Jenner C. Henrietta Lacks D. Stanley Prusiner

D

T/F: A zoonosis can only be transmitted from animals to humans.

False

T/F: Any virus can infect any host cell.

False

T/F: Prion diseases cannot be transmitted from one generation to the next.

False

T/F: The hemagglutinin assay can only identify infectious virus particles.

False

T/F: There is a cure for transmissible spongiform encephalopathies.

False

T/F: Viruses are facultative intracellular pathogens.

False

What two methods can an enveloped animal virus use to enter a host cell?

Receptor-mediated endocytosis or membrane fusion

________ is an enzyme used by retroviruses to produce complementary DNA from RNA.

Reverse transcriptase

What is a provirus? How does it compare to a prophage?

Sample Answer: A provirus is the integrated genome from a retrovirus, whereas a prophage is the integrated genome from a bacteriophage.

What is the difference between viroids and virusoids?

Sample Answer: A viroid is a short strand of circular single-stranded RNA without a capsid (protein coat). Like viruses, viroids can replicate their genome in host cells. Virusoids (a type of satellite virus) are circular, single-stranded RNAs that require a helper virus for replication.

Describe three cytopathic effects of viruses on eukaryotic cells.

Sample Answer: Answers can include inclusion bodies, cell swelling, cytoplasmic stranding, rounding, "grape-like" clustering, change in cell shape, cytoplasmic membrane fusion, multinucleated syncytia, cell lysis.

List, in order, the stages of animal virus replication.

Sample Answer: Attachment --> Penetration (entry) --> Uncoating --> Biosynthesis --> Assembly --> Release

The lipid content of an enveloped virus is produced from the ________.

host cell

When a lawn of bacteria is grown on semisolid medium and those cells are lysed, the clear zones produced are known as ________.

plaque

All viruses are composed of ________ and ________.

protein and nucleic acid

The integrated genome of a retrovirus in a host cell is known as ________.

provirus

The protein content of an enveloped virus is produced from the ________.

viral genome

The number of virions per unit volume is known as the ________.

viral titer

Arthropods typically serve as which mechanism of transmission for viruses? A. biological vector B. fomite C. mechanical vector D. both as fomites and as vectors

A

In the lysogenic cycle of bacteriophages, the integrated phage genome is known as which of the following? A. prophage B. provirus C. virioid D. virion

A

Reverse transcriptase is used to generate which nucleic acid from RNA? A. cDNA B. dsRNA C. (+)ssRNA D. (−)ssRNA

A

The excision of the viral genome from the host chromosome is known as which of the following? A. induction B. lysogeny C. phage conversion D. transduction

A

Which of the following is not a property of a virus? A. It is an infectious cellular pathogen. B. It is an obligate intracellular pathogen. C. Its genome consists of DNA or RNA. D. Its genome is surrounded by a protein capsid.

A

Which of the following is the host-cell receptor that recognizes human immunodeficiency virus (HIV)? A. CD4 B. CD8 C. Gp120 D. sialic acid

A

Which of the following is the scientist responsible for first discovering viruses (specifically, tobacco mosaic virus)? A. Dimitri Ivanovski B. Edward Jenner C. Henrietta Lacks D. Stanley Prusiner

A

Which statement regarding the introduction of virus in eggs is true? A. Pox can form on the embryo. B. The virus cannot be introduced into the yolk sac. C. The virus will not kill the embryo if it is introduced early in development. D. Virus introduction will not disrupt embryonic development.

A

You are a nurse and you suspect that you may have been exposed to a particular virus. You and your health care provider decide to perform a hemagglutinin assay. A sample of your serum is taken and added to a sample of virus. Next, the virus and serum sample are added to erythrocytes. Upon examination, you do not see any agglutination occurring. Which can you conclude from these results? A. The test result suggests you may have been exposed to the virus, and your serum contains antibodies against the virus. B. The test result suggests you may have been exposed to the virus, but your serum does not contain antibodies against the virus. C. The test result suggests you may not have been exposed to the virus. D. You cannot make a conclusion from the information given.

A

Which of the following is a novel enzyme found in retroviruses? A. forward transcriptase B. retrotranscriptase C. reverse transcriptase D. Taq polymerase

C

Which unit of length best describes the average size of a virion? A. mm B. µm C. nm D. pm

C

________ occurs when a random piece of bacterial chromosomal DNA is transferred by the phage during the lytic cycle.

Generalized transduction

Name two possible shapes for viral capsids.

Helical and polyhedral (icosahedral)

T/F: (+)ssRNA can be directly used in translation at the ribosome.

True

Define the term "tissue tropism."

Viruses can only infect certain types of cells.

On the axes below, sketch the multiplication curve for a bacteriophage population. Be sure to indicate each of the four stages.

N/A

T/F: Most plant viruses do not kill their host.

True

T/F: RT-PCR relies on an enzyme encoded by retroviruses.

True

T/F: Satellite RNAs can encode for protein.

True

Which of the following viruses does not cause latent infections? A. adenovirus B. Epstein-Barr virus C. herpes simplex virus D. varicella-zoster virus

A

Match the virus detection method to its proper description: A. EIA B. Hemagglutinin C. PCR D. RT-PCR i. using antibodies to detect viral antigens (proteins) ii. using primers to amplify DNA iii. making cDNA from RNA iv. isolating antibodies from a patient, adding them to a virus, and seeing if the virus can clump erythrocytes

Answers: A. i., B. iv., C. ii., D. iii.

Match the virus family to its clinical features: A. Herpesviridae B. Papillomaviridae C. Parvoviridae D. Picornaviridae E. Poxviridae i. penile and/or cervical cancer ii. cold sores and/or genital warts iii. respiratory tract infections iv. skin lesions v. hepatitis

Answers: A. ii., B. i., C. iii., D. v., E. iv.

Put the steps of HIV infection in order by matching the correct events with each step: A. Step 1 B. Step 2 C. Step 3 D. Step 4 E. Step 5 i. New virus particles are released. ii. Provirus appears. iii. HIV RNA and other viral proteins enter the cell. iv. Viral DNA is transcribed into viral RNA. v. Viral DNA is synthesized.

Answers: A. iii., B. v., C. ii., D. iv., E. i.

Match the virus family to its genome (as defined by the Baltimore classification): A. (+)ssRNA naked B. (+)ssRNA enveloped C. (−)ssRNA enveloped D. dsDNA naked E. dsDNA envelope i. reovirus ii. HIV iii. hepatovirus iv. herpes simplex virus v. influenza virus

Answers: A. iv., B. i., C. ii., D. v., E. iii.

A bacteriophage infects a host cell and the genome integrates itself into the host chromosome. Sometime later, the phage is excised along with a short piece of DNA adjacent to the insertion point. Both the phage DNA and the host DNA are packaged into the same capsid. The bacteriophage then infects a new cell, delivering both phage and bacterial DNA. Which process is being described in this scenario? A. The lysogenic cycle, reactivating as the lytic cycle, followed by generalized transduction B. The lysogenic cycle, reactivating as the lytic cycle, followed by specialized transduction C. The lytic cycle, reactivating as the lysogenic cycle, followed by generalized transduction D. The lytic cycle, reactivating as the lysogenic cycle, followed by specialized transduction

B

In retroviruses, the integrated phage genome is known as which of the following? A. prophage B. provirus C. virioid D. virion

B

The protein subunits that surround the nucleic acid of a virus are known as which of the following? A. amino acids B. capsomeres C. chromatins D. telomeres

B

Viruses can be visualized using which method? A. compound light microscope B. electron microscope C. the naked eye D. ultraviolet rays

B

Which of the following properly lists (in order) the steps of viral replication? A. Attachment --> Lysis --> Penetration --> Biosynthesis --> Maturation B. Attachment --> Penetration --> Biosynthesis --> Maturation --> Lysis C. Biosynthesis --> Attachment --> Penetration --> Lysis --> Maturation D. Penetration --> Biosynthesis --> Lysis --> Attachment --> Maturation

B

Chicken pox and shingles are distinctly different diseases but are due to the actions of the same virus. Which virus causes both these diseases? Discuss why different signs and symptoms are observed during the two diseases.

Sample Answer: Both chicken pox and shingles are caused by the varicella-zoster virus. This virus can infect different cells throughout the body. At first, a rash of blisters covers the skin (chicken pox). The virus then becomes dormant and lives within nerve cells. The virus can be reactivated and cause a new disease called shingles. Shingles is specific to nerve cells. Pain, burning, tickling, tingling, and/or numbness occurs in the area around the affected nerves several days or weeks before a rash appears. Next, a rash will appear (typically on one side of the body) along with burning and pain. [Source: WebMD.com].

Compare and contrast bacteriophages and animal viruses. In what ways are they the same and in what ways are they different?

Sample Answer: Both viruses demonstrate host-cell specificity. During penetration, the bacteriophage does not uncoat its nucleic acid. Rather, it injects its genome into the host cell. Animal cells enter host cells either through membrane fusion or receptor-mediated endocytosis. Once in the cytoplasm, viral DNA or RNA can be replicated. Viral components are synthesized. In some cases (lysogenic phages or retroviruses), the viral DNA can be integrated into the host chromosome (known as prophages and proviruses, respectively). During bacteriophage infections, the cell lyses to release new phage particles due to the action of viral enzymes. For animal viruses, release from the host cell can occur either by cell lysis or through budding off of the cell membrane. Therefore, host-cell destruction of bacteriophage-infected cells (if going through the lytic cycle) is immediate, whereas in animal cells, it may be prolonged.

When there was an outbreak of mad cow disease (bovine spongiform encephalopathy) in 2003, everyone was concerned about eating certain beef products but not about eating steak. Why?

Sample Answer: Bovine spongiform encephalopathy is caused by a prion. Prions are misfolded proteins in neural tissue. There was concern about acquiring Creutzfeldt-Jakob disease (a human variant of mad cow disease) by eating food containing neural tissue. Steaks do not contain a high proportion of neural tissue.

From where were HeLa cells first isolated? Why are they used in virology?

Sample Answer: HeLa cells were tumor cells isolated from Henrietta Lacks. They were used to develop the first continuous tissue-culture cell line (cells that can be continually grown indefinitely). Because viruses are obligate intracellular entities, these types of cell lines allow scientists to culture and study viruses.

What are the two spike proteins of influenza virus

Sample Answer: Hemagglutinin and neuraminidase.

Briefly describe the difference between the lytic and lysogenic cycles.

Sample Answer: In the lytic cycle, the bacteriophage causes the death of the bacterium host cell. In the lysogenic cycle, the bacteriophage DNA integrates into the host-cell chromosome (known as a prophage). This prophage can then be excised, and the lytic cycle (the new bacteriophage being manufactured and being released via cell lysis) can be resumed.

Why are naked animal viruses unable to enter host cells through membrane fusion?

Sample Answer: Naked viruses do not have lipid envelopes (only a protein capsid), so it cannot fuse with the host-cell membrane; rather, it enters via receptor-mediated endocytosis.

What is the difference between plasma and serum?

Sample Answer: Plasma is the liquid portion of blood. Serum is the liquid portion of blood with clotting factors removed.

When studying bacterial genomes, it is common to find prophages that have never been reactivated. Some of them are also known as "defective prophages," which can never be reactivated. Genome scientists use these defective prophages to study the ways viruses have shaped the genomes of bacteria. Based on this information, answer the following questions. (a) Define what a prophage is. (b) Explain how prophages normally become activated and what might cause a prophage to become "defective." (c) Discuss other ways viruses have contributed to the evolution of the genomes of bacteria.

Sample Answer: Prophage is the integrated genome of a bacteriophage in the host bacterial cell chromosome (the lysogenic cycle). During induction, the viral genome is excised from the host chromosome. Environmental stresses, cell damage, or the presence of chemicals or ultraviolet light can trigger induction, allowing the lytic cycle to resume. The maintenance of the prophage or the induction of prophage is under the control of viral regulatory genes. Loss of the functions of these regulatory genes (either through mutations or evolutionary stresses) may result in the inability of the prophage to be excised and the lytic cycle to resume. These defective prophages have now become an integral part of the bacterium's chromosome and may even be necessary for survival. These defective prophages have introduced important virulence factors for pathogens, including biofilm production ability and antibiotic resistance. Another way that viruses can contribute to the evolution of bacteria is through the horizontal gene-transfer method of transduction (either generalized, in which a random piece of DNA from one bacterium is carried by a virus to another bacterium, or specialized, in which DNA next to the prophage site is transferred).

Identify which type of virus is shown in the diagram below. Additionally, indicate what the labels A, B, C, and D represent.

Sample Answer: Virus shown is a bacteriophage. A. capsid; B. viral genome; C. sheath; D. tail fibers.