Topic 4 Flashcards

1. The term filterable viruses was coined by
A. Beijerinck.
B. Iwanowsky.
C. Twort.
D. d'Herelle.

2. Crystallization of tobacco mosaic virus was accomplished by

A. Berkley.
B. Stanley.
C. Iwanowsky.
D. Twort.

3. Viruses that infect bacteria are referred to as
A. viralcidens.
B. bacteriocidins.
C. bacterialogens.
D. bacteriophages.

4. A virion is a(n)
A. pathogenic virus.
B. subviral particle.
C. complete, extracellular virus particle.
D. enveloped virus particle.

5. A virion is composed of
A. lipid, protein, and either RNA or DNA.

B. protein and either RNA or DNA, but no lipid.

C. protein and both RNA and DNA, but no lipid.

D. protein, either RNA or DNA, and possibly lipid.

6. The protein coat of a virus
A. is called a capsomere.
B. is called a capsid.
C. protects the nucleic acid.
D. is involved in the recognition of host cell receptors by non-enveloped viruses.

E. is called a capsid, protects the nucleic acid, AND is involved in the recognition of host cell receptors by non-enveloped viruses.

7. Which does not refer to the shape of a virus?
A. Icosahedral (isometric)

B. Helical

C. Complex

D. Bacillus

8. The shape of the virus is determined by its
A. nucleic acid.
B. capsid.
C. envelope.
D. tail.

9. The viral envelope closely resembles the

A. prokaryotic cell wall.
B. capsomere.
C. eukaryotic cellular membrane.

D. cytoplasm.

10. The protein projections on the surface of a virus that are involved in attachment to the host cell are called
A. suckers.
B. pili.
C. cilia.
D. spikes.
E. hooks.

11. Outside of living cells, viruses are
A. scavenging glucose.
B. slowly stockpiling ATP from the mitochondria.
C. using cilia to move to the next host.
D. metabolically inert.

12. Viruses
A. probably keep the numbers of bacteria in check.
B. have no effect on the number of bacteria.
C. increase the number of bacteria.
D. are active in passing DNA from one bacterium to another.
E. probably keep the numbers of bacteria in check AND are active in passing DNA from one bacterium to another.

13. What part of the attached bacteriophage enters through the host cell wall?
A. The entire virus

B. Only the enzymes necessary for replication

C. The nucleic acid

D. The nucleic acid and capsid

E. The capsid only

14. A phage that replicates inside the host cell and then lyses its host during its release is a
A. virulent or lytic phage.
B. latent phage.
C. lysogenic phage.
D. dormant phage.

15. The correct order for the stages of a phage infection is:
A. penetration, transcription, attachment, replication of nucleic acid and protein, assembly, release.

B. attachment, penetration, transcription, replication of nucleic acid and protein, assembly, release.

C. attachment, replication of nucleic acid and protein, penetration, transcription, assembly, release.

D. transcription, attachment replication of nucleic acid and protein, assembly, penetration, release.

16. Phages that can either replicate and cause cell lysis or can integrate their DNA into the host DNA are called
A. lysogenic phages.
B. lytic phages.
C. virulent phages.
D. segmented phages.

17. One of the most intensively studied virulent phages which infects E. coli is
A. T9.
B. T4.
C. beta.
D. gamma.

18. During attachment of phage to E. coli, the phage
A. actively seek out the bacteria.
B. randomly bump into the bacteria.
C. attach to proteins or carbohydrates on the bacterial surface.
D. attach to the bacterial RNA.
E. randomly bump into the bacteria AND attach to proteins or carbohydrates on the bacterial surface.

19. What part of the E. coli T4 phage attaches to the host cell receptors?
A. Capsid fragments around the nucleic acid

B. Protein fibers at the end of the phage tail

C. Pili of the envelope

D. Spikes of the envelope

20. During penetration of E. coli by the T4 phage
A. lysozyme is used to allow entry of the phage capsid.
B. the tail acts as a "hypodermic needle," injecting the phage DNA into the cell.

C. the protein fibers digest a hole in the cell wall.
D. the bacterial receptor molecules open a hole through the cell wall.

21. Phage-encoded proteins are
A. coded for by host DNA.
B. coded for by phage DNA.
C. proteins normally present in the uninfected cell.
D. early proteins.
E. coded for by phage DNA AND early proteins.

22. Phage-encoded enzymes are
A. all produced simultaneously.
B. produced in a sequential manner.
C. strictly host enzymes.
D. used to customize the cell for viral production.
E. produced in a sequential manner AND used to customize the cell for viral production.

23. Assembly of the T4 phage
A. may involve some self-assembly.
B. may involve the use of scaffolds.
C. is completely self-assembly.
D. is completely dependent on scaffolds.
E. may involve some self-assembly AND may involve the use of scaffolds.

24. In the case of T-even phages, the burst size is about
A. 1 per host cell.
B. 5 per host cell.
C. 200 per host cell.
D. 1,000 per host cell.

25. The time from absorption to release for T-even phage is about
A. 1 minute.
B. 10 minutes.
C. 30 minutes.
D. 1 day.

26. The replicative form of nucleic acid in filamentous phages is
A. dsDNA.
B. dsRNA.
C. positive ssRNA.
D. negative ssDNA.

27. Filamentous phage
A. only infect E. coli that have pili.
B. only infect E. coli lacking pili.
C. infect E. coli regardless of the presence of pili.
D. do not infect E. coli.

28. Which is a filamentous phage?
A. M13.

B. T4.

C. Lambda.

D. TMV.

29. The bacterial viruses that are released by a process termed extrusion are called
A. lysogenic viruses.
B. temperate phages.
C. filamentous phages.
D. lambda viruses.

30. The filamentous phages all contain
A. single-stranded DNA.
B. double-stranded DNA.
C. single-stranded RNA.
D. double-stranded RNA.

31. An exit method used by viruses that does not immediately destroy the host bacterium is

A. lysis.
B. inversion.
C. extrusion.
D. excising.

32. In the replication of phage containing positive-sense DNA,
A. the host's enzymes are used to make dsDNA.
B. the host's DNA polymerase uses the phage RNA as a template to make negative-sense DNA.
C. a phage-encoded DNA polymerase is used to make negative-sense RNA using the phage positive-sense RNA as a template.
D. a phage-encoded DNA polymerase is used to make DNA using the phage positive-sense RNA as a template.

33. Most phages that contain single-stranded DNA
A. are extruded.
B. contain a positive-sense DNA strand.
C. have their DNA transformed to double-stranded DNA before replication and transcription occur.
D. All of the choices are correct.

34. Regarding phage replication,
A. the majority of phages are temperate.
B. when integrated into host DNA, the phage DNA is called a prophage.
C. lambda is a good example of a temperate phage.
D. All of the choices are correct.

35. A temperate phage
A. may be lysogenic.
B. may be lytic.
C. enters a lysogenic or lytic life cycle shortly after entering the host cell.
D. are all RNA viruses.
E. may be lysogenic AND enters a lysogenic or lytic life cycle shortly after entering the host cell.

36. The integration of phage DNA into the bacterial chromosome occurs because of
A. the phage's ability to synthesize an enzyme that integrates its DNA into the host's chromosome.

B. the phage's ability to synthesize enzymes to enter the bacterium.

C. similar RNA nucleotides in both.
D. the similarity in enzyme metabolism.
E. the phage's ability to synthesize enzymes to enter the bacterium AND similar RNA nucleotides in both.

37. Once integrated, phage DNA can remain in the prophage state as long as
A. the bacteria is frequently plated on new media.
B. certain phage genes are excised.
C. certain phage genes are repressed.
D. bacterial repressor genes are activated.

38. The activation of the SOS system in a bacterium infected with a prophage results in
A. destruction of the viral genes.
B. complete lysis of the bacterial culture.
C. mutation of the DNA.
D. destruction of the viral repressor through host protease activity.
E. complete lysis of the bacterial culture AND destruction of the viral repressor through host protease activity.

39. Lysogenized cells
A. are immune to any further infection by any virus.
B. are immune to infection by the same virus.
C. may have new properties.
D. respond to infection with the SOS response.
E. are immune to infection by the same virus AND may have new properties.

40. The phenomenon responsible for the ability of Corynebacterium diphtherium to produce the virulent toxin responsible for the devastating effects of diphtheria is called
A. self-assembly.
B. matrix conversion.
C. prion protein.
D. lysogenic conversion.

41. Transducing virulent phages do not lyse the cells they invade because
A. transformation is taking place in the phage and this is transferred to the bacterium.
B. bacterial DNA has replaced critical viral DNA in the phage.
C. their virulence is dependent on the bacteria and virus replicating together.
D. the lytic genes are unable to enter during penetration and are shed outside the host.

42. DNA is protected from restriction enzymes by being
A. sequestered in a lysosome.
B. turned into RNA.
C. methylated.
D. made into double-stranded RNA.

43. A limiting factor for viral infection is
A. internal metabolic temperature of the host cell.
B. nutrition of the host cell.
C. stage of cell cycle of the host cell.
D. presence of specific receptor molecules on the host cell.

If the infecting phage lacks some critical pieces of DNA necessary for replication, it is called

A. incomplete.
B. mutated.
C. defective.
D. vegetative.

45. Specialized transduction
A. involves the random transmission of any gene.
B. involves the transfer of a few specific genes.
C. utilizes a defective virus.
D. only involves genes near the viral DNA integration site.
E. involves the transfer of a few specific genes, utilizes a defective virus AND only involves genes near the viral DNA integration site.

46. Once inside the host cell, phage DNA
A. is replicated.
B. is transcribed.
C. may get degraded by bacterial nucleases.
D. All of the choices are correct.

47. Using phages to treat a bacterial infection is an interesting idea because
A. a single type of phage can destroy a wide range of strains of the same pathogen.
B. of the increasing problem of antibiotic resistance.
C. lysed bacteria pose no threat.
D. a single phage can be genetically engineered to infect many different species of bacteria.

50. What are two ways that phage can replicate in harmony (not directly lysing) their host cell?
A. Conjugation and lysogeny

B. Lysogeny and transduction

C. Extrusion and transformation

D. Extrusion and lysogeny

51. What is a defective phage?
A. A virus that has lost some of its genetic material, and therefore cannot infect a new target cell

B. A virus that has lost some of its genetic material, and therefore cannot replicate within a new target cell

C. A virus that lacks the ability to replicate independently of its host cell

D. A virus that cannot attach to its host cell

58. The nucleocapsid is composed of
A. DNA and RNA and protein.
B. DNA or RNA and protein.
C. protein located in the nucleus.
D. nucleic acid in the ribosome.

59. Enveloped viruses
A. just require a stamp.
B. have an outer lipid bilayer membrane containing various proteins.
C. are surrounded by an additional layer of carbohydrate.
D. envelope a cell.

60. The term "segmented" refers to viruses that
A. may contain several pieces of RNA.
B. have an icosahedral-shaped capsid.
C. are linked together before budding out.
D. have an envelope.

The terms helical, icosahedral, and complex refer to

A. viral life cycles.
B. forms of nucleic acid.
C. types of viral envelopes.
D. shapes of viruses.

62. Animal viruses are divided into a number of families whose names end in
A. -virus.
B. -viridae.
C. -viscous.
D. -eieio.

There are _______ major families of DNA containing viruses that infect vertebrates.

A. 2

B. 4

C. 5

D. 7

There are _______ major families of RNA containing viruses that infect vertebrates.

A. 2

B. 5

C. 7

D. 13

65. Viruses are commonly referred to by their _________ name.
A. locale
B. genus
C. disease
D. species
E. disease AND species

66. The common species name of the virus is based on the
A. presence or absence of a nuclear membrane.
B. type of nucleic acid it contains.
C. disease the virus causes.
D. geographic area it is found.

67. The family to which the Rhinovirus belongs is the
A. Picornaviridae.
B. Enterovirus.
C. Enteroviridae.
D. Picornavirus.

68. A key feature of all viral infections is the
A. integration of viral DNA into host DNA.
B. disintegration of host DNA.
C. addition of a lipid membrane to the virus.
D. separation of viral nucleic acid from the capsid.

69. An infection in which the virus is continually present in the body is referred to as
A. acute.
B. balanced.
C. determinant.
D. persistent.

70. Attachment of animal viruses to the host cell may be by means of
A. a tail.
B. the envelope.
C. a capsid.
D. spikes.

71. The receptors to which animal virus attachment proteins usually bind are
A. proteins.
B. carbohydrates.
C. nucleic acid.
D. lipids.
E. glycoproteins.

72. Resistance of some animals to certain viral diseases is based on
A. lack of spikes for attachment.
B. phagocytosis of the virus by the host cell.
C. the presence of the viral envelope.
D. lack of specific receptors on the host cell.

73. When an enveloped virus adsorbs to the host cell with its protein spikes, the virions are taken into the cell by the process of
A. penetration.
B. production.
C. fusion.
D. endocytosis.

74. Bacteriophages and animal viruses
A. both may enter a host cell by endocytosis.
B. both may enter a host cell by fusion.
C. both involve entry of the entire nucleocapsid.
D. differ because bacteriophages leave the capsid outside the cell, while animal virus entry involves the entry of the whole nucleocapsid.

75. For which of the following processes are enzymes not required?
A. Replication

B. Translation

C. Maturation

D. Self-assembly

76. Cells infected with animal viruses lyse because
A. the release of the virions depletes the cell of energy.
B. the virus releases enzymes that lyse the cell.
C. functions necessary for cell survival are not carried out and the cell dies.
D. the virus RNA and cellular protein interact to kill the cell.

77. In addition to lysis, animal viruses may exit the host cell by
A. extrusion.
B. budding.
C. fission.
D. fusion.

78. In the region of budding, the inside of the plasma membrane becomes coated with
A. enzymes.
B. carbohydrates.
C. steroids.
D. matrix proteins.

A. carbohydrates.
B. spike proteins.
C. matrix proteins.
D. enzymes.
E. spike proteins AND matrix proteins.

80. The enveloped viruses typically obtain their envelope
A. from the host plasma membrane.
B. as they exit the host.
C. from a newly constructed viral-derived membrane.
D. from the nuclear membrane.
E. from the host plasma membrane AND as they exit the host.

81. Carriers
A. may have a persistent infection.
B. may be a source of infection.
C. usually show symptoms of the disease.
D. have been cured of the infection.
E. may have a persistent infection AND may be a source of infection.

82. In latent infections, the virions are
A. constantly produced.
B. only produced during reactivation.
C. produced slowly.
D. continually being slowly budded out.

83. The best known chronic infection involves
A. chickenpox.
B. herpes.
C. hepatitis A.
D. hepatitis B.

84. The genome of retroviruses is made of
A. ssDNA.
B. dsDNA.
C. ssRNA.
D. dsRNA.

85. The best-known examples of viruses that cause latent infections are
A. polio.
B. herpes.
C. measles.
D. chickenpox.
E. herpes AND chickenpox.

86. Diseases of short duration frequently followed by long-term immunity are referred to as
A. intermittent infections.
B. chronic infections.
C. acute infections.
D. persistent infections.

87. Retroviruses are unique in that they
A. replicate in nervous system cells.
B. do not have a capsid.
C. use RNA as a template to make DNA.
D. use DNA as a template to make RNA.

88. Genetic exchange in segmented viruses that allows a zoonotic virus to infect humans is termed
A. antigenic shift.
B. hemagglutination.
C. genetic reassortment.
D. antigenic drift.

89. Cells taken from a tumor
A. may be used to grow viruses.
B. can be cultivated in vitro indefinitely.
C. may be used to grow bacteriophages.
D. divide 50 times and then die.
E. may be used to grow viruses AND can be cultivated in vitro indefinitely.

90. Viruses may not be cultivated in
A. live organisms.
B. embryonated chicken eggs.
C. tissue culture.
D. blood agar.

The changes that occur in virally infected cells are characteristic for a particular virus and are referred to as the

A. cytopathic effect.
B. phenotypic effect.
C. genotypic expression.
D. cytology.

92. After growth in tissue culture, the infected cells lyse and the virus may be harvested from
A. the lysate, the liquid supernatant after centrifugation.

B. the remainder.
C. the quantal layer.
D. the monolayer.

93. Normal tissue taken from animals and prepared immediately as media for viral growth is termed a(n)
A. advantageous group.
B. monolayer culture.
C. plaque.
D. primary culture.

94. If reasonably pure preparations of virus are available, the number of virus present may be determined by
A. photocolorimetry.
B. gas chromatography.
C. light microscopy.
D. electron microscopy.

95. The approximate viral concentration of a sample may be determined by
A. quantal assay.
B. endpoint assay.
C. the titer.
D. the lysate assay.

96. The concentration of virus that infects or kills 50% of the host cells is referred to as the
A. LD₅₀.
B. ID₅₀.
C. ID₁₀₀.
D. LD₁₀₀.
E. LD₅₀ AND ID₅₀.

97. One group of animal viruses that are able to agglutinate red blood cells are the
A. coronavirus.
B. retrovirus.
C. reovirus.
D. orthomyxovirus.

The site at which a virus has infected and subsequently lysed the infected cell, releasing its progeny to infect and lyse surrounding cells, thereby forming a "clear zone," is

A. a burst area.
B. a lyse area.
C. a plaque.
D. a dead zone.

99. Which of the following is not a characteristic of normal cells?
A. They grow as a monolayer.
B. They grow as multiple layers.
C. They undergo a limited number of divisions and then die.
D. They stick tightly to the surface of glass culture dishes.

100. Plant viruses enter the host plant via
A. wound sites.
B. specific receptors.
C. nonspecific receptors.
D. seeds.

101. Plant viruses may be transmitted by
A. worms.
B. contaminated seeds.
C. humans.
D. insects.
E. All of the choices are correct.

102. Prions
A. replicate by converting normal host proteins into prion proteins.
B. responsible for "mad cow disease" can cause a similar disease in humans.

C. can be transmitted by consumption of dried or cooked food.
D. that cause Spongiform Encephalopathy have the same amino acids but different folding properties from PrP^c.
E. All of the above.

103. Prions
A. consist of a special nucleocapsid.
B. are made of protein only.
C. are made of RNA only.
D. are made of dsRNA and protein.

104. Prions affect the
A. respiratory system.
B. gastrointestinal tract.
C. nervous system.
D. lymphatic system.

105. Viroids characteristically are composed of
A. ssRNA.
B. dsRNA.
C. ssDNA.
D. dsDNA.

106. Viroids cause disease in
A. animals.
B. plants.
C. bacteria.
D. fungus.

107. Prions are
A. a form of RNA virus.
B. a form of DNA virus.
C. a viroid.
D. an infectious protein.

108. Spongiform encephalopathy occurs in
A. humans.
B. cattle.
C. sheep.
D. All of the choices are correct.

118. An antibiotic is added to a culture of E. coli, resulting in death of the cells. Bacteriophage are then added. Would the phage replicate in the E. coli cells? Why or why not?
A. Yes, because the machinery inside the cells is most likely still active, even though the cells are no longer living. The virus could use that machinery to replicate new virus particles.
B. No, because the virus would depend too much on having the active machinery of a living cell for its replication. Without the ability of the cell to try to replace what is lost/damaged/used as the virus goes through its life cycle, the virus couldn't reproduce effectively.
C. Yes, because viruses are capable of reanimating dead cells to force them to produce more virus particles.
D. No, because entry of the virus into the target cell is dependent on the cell being alive to conduct endocytosis of the virus.

119. Most temperate phages integrate into the host chromosome, whereas some replicate as plasmids. Which kind of relationship do you think would be more likely to maintain the phage in the host cell, and why?
A. Plasmids-they're smaller, so they'd be easier to replicate by the host cell.
B. Integrated-the host cell would be less likely to view this DNA as "foreign" on subsequent rounds of replication, and would retain it more easily.

C. Plasmids-these structures often carry other genes that may give the host cell a selective advantage over cells that don't have them.
D. Integration-because plasmids are frequently lost during cell division, which could leave a daughter cell without the virus genome.

120. The clustered, regularly interspaced short palindromic repeats (CRISPR) system in bacterial cells has been called the “immune” system of bacteria. CRISPR protect bacteria from a repeat infection from the same phage because bacterial cells

A. recognize proteins on the surface of the phage and secrete enzymes that digest the phage.

B. recognize proteins on the surface of the phage and secrete proteins that block the binding of the phage.

C. integrate fragments from the phage DNA in their own chromosomes and target for destruction any DNA that contains the same fragments in the future.

D. modify the attachment sites for the phages so that new infections cannot take place.

121. You add an unknown phage to a mixture of F⁺ and F^- cells of E. coli and plate out the bacteria. The bacterial colonies that grow are all F^-. How can you explain this phenomenon?
A. The phage bound to a receptor on the sex pilus, and therefore only infected the F⁺ cells (leaving the F^- cells alone).
B. The phage bound to a receptor on the F^- cells, leaving only them alive and allowing the F⁺ cells to die off.
C. The phage integrated (lysogenized) the F^- cells, giving them a selective advantage over the F⁺ cells.
D. F⁺ cells are uniquely susceptible to phage attack for unknown reasons.

122. A mutation in E. coli results in the loss of both restriction endonucleases and modification enzymes. Would you expect any difference in the frequency of gene transfer via transduction FROM Salmonella INTO this E. coli strain?
A. No-since the Salmonella strain is normal, the rate of production of transducing virus particles would still be the same, resulting in the same frequency of gene transfer.
B. Yes-the loss of the restriction endonucleases would leave the recipient E. coli unable to break down "invading"' viral DNA from the transducing phage. This would lead to higher rates of successful transduction.

C. Yes-the loss of the modification enzymes would leave the recipient E. coli unable to tag its own DNA as "self," leaving the viral DNA untagged and recognizable as "foreign," and targeted for destruction. This would lead to higher rates of successful transduction.

D. No-transduction efficiency isn't affected by either restriction endonucleases or modification enzymes, so there'd be no effect on the overall rate.
E. Yes-the loss of the restriction endonucleases would leave the recipient E. coli unable to break down "invading" viral DNA from the transducing phage, AND the loss of the modification enzymes would leave the recipient E. coli unable to tag its own DNA as "self," leaving the viral DNA untagged and recognizable as "foreign," and targeted for destruction. Together, thesewould lead to higher rates of successful transduction.

123. Why do animal viruses have envelopes and phages rarely do?
A. Since bacteria don't have cell membranes, the bacterial viruses (phages) don't pick them up when they leave the target cells.
B. Phages acquire an outer surrounding that is a part of the cell wall of the bacterium they were created in, rather than an outer surrounding of plasma membrane.
C. Animal viruses will often utilize the envelope in order to fuse with the plasma membrane of a new target cell, gaining entry into the cytoplasm.
D. Animal viruses build the envelope inside of the target cell as they are being replicated, but before the cell breaks open and releases them into the extracellular environment. The envelope is a remnant of this building process.

124. Explain why HIV becomes resistant so quickly when a single drug is used therapeutically.
A. The virus is an RNA genome virus-by using host RNA polymerase to directly copy the genome to make more virus particles, there's no proofreading capability. This leads to a high rate of mutation of the viral genome and increased chance for drug resistance.
B. HIV utilizes reverse transcriptase to make a cDNA version of its RNA genome. This polymerase is "'sloppy," with a high rate of error and lack of a proofreading capability. This leads to a high rate of mutation of the viral genome and increased chance for drug resistance.

C. HIV synthesizes a protein enzyme that directly cleaves anti-HIV drugs, giving it a characteristically high rate of resistance to a single drug type.
D. HIV is constantly changing its genetic structure by swapping genetic elements with other virus strains. This leads to a high rate of mutation of the viral genome and increased chance for drug resistance.

125. Why is it not surprising that AIDS patients frequently suffer a viral-induced tumor?
A. HIV genomes integrate into the host cell chromosome. This integration might result in loss of control of the cell cycle (also known as cancer).
B. The immune system's CD4⁺ T cells are directly responsible for elimination of tumor cells. HIV attacks them and eliminates them, making a person more susceptible to cancer.
C. The therapies for HIV are highly mutagenic, which may lead to cancerous states in people taking the drug regimen.
D. Products of an active HIV infection are highly mutagenic-so, as a person's illness progresses, these mutagenic compounds build up and are more capable of inducing a cancerous state.

126. Is antigenic shift alone likely to lead to influenza pandemics?
A. Yes-ONLY antigenic shift can lead to the large-scale mixing of gene elements required to produce a pandemic flu strain.
B. No-ONLY antigenic DRIFT can lead to the large-scale mixing of gene elements required to produce a pandemic flu strain.
C. Perhaps-but it would most likely be a mixture of antigenic shift AND drift that would result in a pandemic strain.
D. No-antigenic SHIFT is responsible for changes in the hemagglutinin protein, while antigenic DRIFT is responsible for changes in the neuraminidase protein. You must have changes in both to lead to a pandemic strain.

127. Would you expect the number of virions to be the same if you measured them by the plaque assay or by counting using the electron microscope? Why?
A. Yes-both methods measure the total number of virus particles in a solution.
B. No-the plaque assay only measures viable virus particles, while the electron microscope cannot distinguish between defective and viable virus.
C. No-you cannot count virus particles by using a plaque assay. You can only get a relative difference in quantity from one preparation of virus particles to another with this method.
D. Yes-only fully functioning viruses will be released from a host cell, so the quantified number of virus particles in a plaque assay should be identical to the number of free virus particles counted by electron microscopy within a given preparation.

128. Why are viroids resistant to nucleases?
A. Having a circular RNA "genome," they are resistant to the digestion of most exonucleases (that nibble/digest the free ends of RNA or DNA).

B. Having a circular RNA "genome," with no protein shell, they are resistant to the protein-degrading activities of nucleases.

C. Nucleases will only digest DNA, not RNA-so viroids are protected.
D. Viroids have only been identified in plants. Plant nucleases cannot digest RNA.

129. Would ID₅₀ and LD₅₀ necessarily be the same for a given virus? Why or why not?
A. Yes, because the number of viruses that infect 50% of a test population should also kill 50% of that test population.
B. No, because a virus may be highly infectious (very low ID₅₀ value) but only marginally lethal (very high LD₅₀ value). A prime example of this is the rhinovirus (common cold virus).
C. No, because very few viruses are lethal, yet many are highly infectious. The two values should ALWAYS be different.

130. Why is it virtually impossible to stamp out a disease caused by a zoonotic virus?
A. You'd have to drive the vector organism extinct to do so.
B. Many vector organisms have multiple stages of their life cycle that can carry a zoonotic virus, which complicates controlling the vector-borne transmission.
C. Many viruses transmitted in this manner may utilize more than one vector organism.
D. Many zoonotic viruses may be able to reside in more than one host organism, complicating control measures.

E. All of the above are correct.