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Test Bank For Microbiology Evolving Science 3rd Edition Foster

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Test Bank For Microbiology Evolving Science 3rd Edition Foster

Test Bank For Microbiology Evolving Science 3rd Edition Foster

CHAPTER 6: Viruses

 

MULTIPLE CHOICE

 

  1. All of the following are true about the measles virus EXCEPT:
a. It is encased by an envelope of membrane.
b. It fuses with the host cell membrane during infection.
c. It has progeny that bud out of the host cell.
d. It generates a rash of red spots on the skin of infected patients.
e. It is never fatal.

 

 

ANS:  E                    DIF:    Easy               REF:   6.1                 TOP:   I.B

MSC:  Remembering

 

  1. The destruction of cells as a result of viral lysis can be observed as a:
a. tissue culture d. plaque
b. lawn e. host
c. soft agar  

 

 

ANS:  D                    DIF:    Easy               REF:   6.1                 TOP:   I.B.iii

MSC:  Remembering

 

  1. __________ can be counted as representing individual infectious virions from a phage suspension.
a. Plaques d. Proteomes
b. Viruses e. Burst size
c. Genomes  

 

 

ANS:  A                    DIF:    Easy               REF:   6.1                 TOP:   I.B.iii

MSC:  Remembering

 

  1. Which of the following is INCORRECT about the giant mimivirus?
a. It may cause human pneumonia.
b. It is as large as some bacteria.
c. It conducts DNA repair.
d. It conducts protein folding by chaperones.
e. It contains reverse transcriptase.

 

 

ANS:  E                    DIF:    Medium         REF:   6.1                 TOP:   I.C

MSC:  Remembering

 

  1. Infectious agents for which the nucleic acid genome is itself the entire infectious particle are called:
a. prions d. bacteria
b. viroids e. virions
c. viruses  

 

 

ANS:  B                    DIF:    Easy               REF:   6.1                 TOP:   I.D.i

MSC:  Remembering

 

  1. Spontaneous __________ diseases can be inherited genetically.
a. prion d. virion
b. viroid e. bacterial
c. virus  

 

 

ANS:  A                    DIF:    Easy               REF:   6.1                 TOP:   I.E

MSC:  Remembering

 

  1. __________ gained notoriety when they were implicated in brain infections, such as what is popularly known as mad cow disease.
a. Viroids d. Bacteria
b. Viruses e. Virions
c. Prions  

 

 

ANS:  C                    DIF:    Easy               REF:   6.1                 TOP:   I.E

MSC:  Remembering

 

  1. An open reading frame corresponds to a series of codons that are translated to amino acids, producing a(n):
a. RNA d. capsid
b. protein e. envelope
c. DNA  

 

 

ANS:  B                    DIF:    Easy               REF:   6.1                 TOP:   I.C

MSC:  Understanding

 

  1. The RNA genome of the potato spindle tuber viroid requires which type of modified host polymerase for replication?
a. DNA-dependent DNA polymerase d. RNA-dependent RNA polymerase
b. DNA-dependent RNA polymerase e. reverse transcriptase
c. RNA-dependent DNA polymerase  

 

 

ANS:  D                    DIF:    Medium         REF:   6.1                 TOP:   I.D

MSC:  Understanding

 

  1. __________ diseases are unique in that they can be transmitted by an infective protein that propagates conformational change of existing molecules without synthesizing entirely new infective molecules.
a. Viral d. Virion
b. Viroid e. Prion
c. Bacterial  

 

 

ANS:  E                    DIF:    Easy               REF:   6.1                 TOP:   I.E

MSC:  Understanding

 

  1. An icosahedral capsid has how many sides?
a. 4 d. 10
b. 6 e. 20
c. 8  

 

 

ANS:  E                    DIF:    Easy               REF:   6.2                 TOP:   II.A.i

MSC:  Remembering

 

  1. Icosahedral viral capsids take the form of a polyhedron with 20 identical __________ faces.
a. helical d. triangular
b. filamentous e. asymmetrical
c. rectangular  

 

 

ANS:  D                    DIF:    Easy               REF:   6.2                 TOP:   II.A.i.a

MSC:  Remembering

 

  1. Virus particles can be observed by standard transmission electron microscopy (TEM), but details of capsid structure require visualization by:
a. phase-contrast microscopy d. negative stain
b. scanning electron microscopy e. bright field microscopy
c. cryo-electron microscopy  

 

 

ANS:  C                    DIF:    Medium         REF:   6.2                 TOP:   II.A.i.b

MSC:  Remembering

 

  1. Some viral species may derive their __________ from intracellular membranes, such as the nuclear membrane or endoplasmic reticulum.
a. capsid d. neck
b. genome e. tail fibers
c. envelope  

 

 

ANS:  C                    DIF:    Medium         REF:   6.2                 TOP:   II.A.i.c

MSC:  Remembering

 

  1. Tegument proteins may be found:
a. in the capsid d. in the nucleus
b. between the capsid and envelope e. in the spikes
c. in the envelope  

 

 

ANS:  B                    DIF:    Easy               REF:   6.2                 TOP:   II.A.i.e

MSC:  Remembering

 

  1. __________ phages have been used to nucleate the growth of crystalline “nanowires” for electronic devices.
a. Icosahedral d. Filamentous
b. Complex e. Viroid
c. Asymmetrical  

 

 

ANS:  D                    DIF:    Easy               REF:   6.2                 TOP:   II.A.ii

MSC:  Remembering

 

  1. In a filamentous virus, the pattern of capsid monomers forms a __________ tube around the genome.
a. icosahedral d. complex
b. filamentous e. helical
c. asymmetrical  

 

 

ANS:  E                    DIF:    Easy               REF:   6.2                 TOP:   II.A.ii.a

MSC:  Remembering

 

  1. In bacteriophage T4, the DNA genome is contained in the head, and binding to the host is facilitated by attachment of the:
a. capsid d. envelope
b. neck e. tegument
c. tail fibers  

 

 

ANS:  C                    DIF:    Easy               REF:   6.2                 TOP:   II.B.i

MSC:  Remembering

 

  1. Large __________ viruses contain so many enzymes that they appear to have evolved from degenerate cells.
a. icosahedral d. bacteriophage
b. filamentous e. asymmetrical
c. complex  

 

 

ANS:  E                    DIF:    Medium         REF:   6.2                 TOP:   II.B.ii

MSC:  Remembering

 

  1. Genes for the capsid proteins are found in the:
a. plasmid d. ribosome
b. bacterial chromosome e. viral genome
c. envelope  

 

 

ANS:  E                    DIF:    Medium         REF:   6.2                 TOP:   II.A.i.a

MSC:  Understanding

 

  1. If something descended from a common ancestor, it is:
a. monomeric d. monosyllabic
b. monophyletic e. monolithic
c. monogrammed  

 

 

ANS:  B                    DIF:    Medium         REF:   6.3                 TOP:   III

MSC:  Remembering

 

  1. __________ of viruses are designated with the suffix viridae.
a. Genera d. Orders
b. Families e. Classes
c. Species  

 

 

ANS:  B                    DIF:    Medium         REF:   6.3                 TOP:   III.A

MSC:  Remembering

 

  1. Viruses may be classified by the __________ method.
a. Annapolis d. Augusta
b. Baltimore e. Portland
c. Orono  

 

 

ANS:  B                    DIF:    Easy               REF:   6.3                 TOP:   III.B

MSC:  Remembering

 

  1. So far, the known mechanisms of replication and mRNA expression define how many fundamental groups of virus species?
a. 5 d. 8
b. 6 e. 9
c. 7  

 

 

ANS:  C                    DIF:    Medium         REF:   6.3                 TOP:   III.B

MSC:  Remembering

 

  1. David Baltimore proposed that the primary distinction among classes of viruses was the __________ composition and the route used to express messenger RNA.
a. genome d. tegument
b. envelope e. neck
c. capsid  

 

 

ANS:  A                    DIF:    Easy               REF:   6.3                 TOP:   III.B.i

MSC:  Remembering

 

  1. The genome of __________ ssRNA viruses can serve directly as mRNA.
a. positive-sense (+) d. double-stranded
b. negative-sense (–) e. prion
c. all  

 

 

ANS:  A                    DIF:    Easy               REF:   6.3                 TOP:   III.B.i.d

MSC:  Remembering

 

  1. A key factor in the evolution of killer strains of influenza is that they:
a. only have eight genes d. have a segmented genome
b. have an RNA genome e. have a circular chromosome
c. contain reverse transcriptase  

 

 

ANS:  D                    DIF:    Medium         REF:   6.3                 TOP:   III.B.i.e

MSC:  Remembering

 

  1. Which of the following is NOT true of the pararetroviruses?
a. They have an RNA genome.
b. They do not make a DNA intermediate.
c. Some have a viral reverse transcriptase.
d. Some use a host reverse transcriptase.
e. They consist of human and plant pathogens.

 

 

ANS:  A                    DIF:    Medium         REF:   6.3                 TOP:   III.B.i.g

MSC:  Remembering

 

  1. __________ are genes of common ancestry in two genomes that share the same function.
a. Homologs d. Orthologs
b. Open reading frames e. Lysogenic
c. Proteomes  

 

 

ANS:  D                    DIF:    Difficult         REF:   6.3                 TOP:   III.C.i.a

MSC:  Remembering

 

  1. The proteomic tree of bacteriophages is based on comprehensive analysis of viral:
a. DNAs d. capsids
b. RNAs e. host range
c. proteins  

 

 

ANS:  C                    DIF:    Easy               REF:   6.3                 TOP:   III.C.ii

MSC:  Remembering

 

  1. Proteomic analysis predicts seven major evolutionary categories of phage species with subgroups based on:
a. type of genetic material d. transcriptional strategy
b. virion structure e. replicase gene sequence
c. shared hosts  

 

 

ANS:  C                    DIF:    Medium         REF:   6.3                 TOP:   III.C.ii

MSC:  Understanding

 

  1. After the genome is inserted, the phage capsid remains outside, attached to the cell surface, and is referred to as:
a. an envelope d. a ghost
b. a neck e. temperate
c. tail fibers  

 

 

ANS:  D                    DIF:    Easy               REF:   6.4                 TOP:   IV.B

MSC:  Remembering

 

  1. Which of the following steps is NOT part of the life cycle of a lytic phage?
a. Phage DNA is injected into the bacterial cell.
b. The phage DNA integrates into the bacterial chromosome.
c. Many copies of phage DNA are made.
d. The phage DNA is transcribed, and the resulting mRNA is translated to make capsid proteins.
e. All of the above steps are part of the life cycle of a lytic phage.

 

 

ANS:  B                    DIF:    Difficult         REF:   6.4                 TOP:   IV.C.i

MSC:  Remembering

 

  1. The number of virus particles released at lysis is referred to as:
a. lytic number d. temperate number
b. burst size e. release number
c. lysogenic number  

 

 

ANS:  B                    DIF:    Easy               REF:   6.4                 TOP:   IV.C.ii.b

MSC:  Remembering

 

  1. A(n) __________ phage may integrate its genome into that of the host cell.
a. lytic d. asymmetrical
b. temperate e. oncogenic
c. viroid  

 

 

ANS:  B                    DIF:    Easy               REF:   6.4                 TOP:   IV.C.iii.a

MSC:  Remembering

 

  1. The integrated phage genome is called a(n):
a. temperate d. lytic
b. lysogen e. prophage
c. oncogene  

 

 

ANS:  E                    DIF:    Easy               REF:   6.4                 TOP:   IV.C.iii.b

MSC:  Remembering

 

  1. A spacer is a(n):
a. bacterial gene d. inactive viral RNA
b. bacterial enzyme e. cascade protein
c. piece of phage DNA  

 

 

ANS:  C                    DIF:    Easy               REF:   6.4                 TOP:   IV.D.iii

MSC:  Remembering

 

  1. What happens to the virus protein coat when a bacterial cell is infected?
a. It enters the host cell with the viral genome.
b. It remains on the outside of the host cell.
c. It enters the host cell separately from the viral genome.
d. It is released to attach to and inject another host cell.
e. It becomes part of the host cell membrane.

 

 

ANS:  B                    DIF:    Easy               REF:   6.4                 TOP:   IV.B

MSC:  Understanding

 

  1. Which one of the viral gene products is likely to be expressed early in bacteriophage T4 life cycle to facilitate the replication of viral genome?
a. a DNase that digests host genome
b. an RNase that digests viral genome
c. a protease that digests viral capsids
d. the capsids required to produce mature viral particles
e. the ribosomes required for translation of viral mRNA

 

 

ANS:  A                    DIF:    Difficult         REF:   6.4                 TOP:   IV.C.i

MSC:  Understanding

 

  1. Which of the following statements regarding bacterial defenses against phage infections is FALSE?
a. Resistance to phages can occur through mutations.
b. Bacterial restriction endonucleases can cleave viral RNA genome.
c. The clustered, regularly interspaced short palindromic repeats inserted in the bacterial genome facilitate cleavage of specific phage DNA.
d. Bacterial restriction endonuclease system cleaves DNA lacking appropriate methylation patterns.
e. Resistance to phages can occur through altered host receptor proteins.

 

 

ANS:  B                    DIF:    Medium         REF:   6.4                 TOP:   IV.D.i | IV.D.ii | IV.D.iii

MSC:  Understanding

 

  1. The papillomavirus is an example of:
a. latent infection d. transformation
b. lytic infection e. acute infection
c. persistent infection  

 

 

ANS:  A                    DIF:    Difficult         REF:   6.4 | 6.5          TOP:   IV.C.iii | V.B.i.a

MSC:  Understanding

 

  1. Within a host, receptor molecules can also determine the __________, or tendency to infect a particular tissue type.
a. tropism d. burst size
b. host range e. plaque
c. virulence  

 

 

ANS:  A                    DIF:    Easy               REF:   6.5                 TOP:   V.A.i

MSC:  Remembering

 

  1. The primary factor determining the life cycle of an animal virus is the physical form of the:
a. envelope d. receptor
b. capsid e. burst
c. genome  

 

 

ANS:  C                    DIF:    Easy               REF:   6.5                 TOP:   V.B

MSC:  Remembering

 

  1. The flu is an example of:
a. latent infection d. transformation
b. lytic infection e. oncogenic infection
c. persistent infection  

 

 

ANS:  B                    DIF:    Easy               REF:   6.5                 TOP:   V.B

MSC:  Remembering

 

  1. Which of the following is the most common sexually transmitted disease in the United States, and one of the most common worldwide?
a. human papillomavirus (HPV) d. syphilis
b. human immunodeficiency virus (HIV) e. chlamydia
c. gonorrhea  

 

 

ANS:  A                    DIF:    Medium         REF:   6.5                 TOP:   V.B.i

MSC:  Remembering

 

  1. In contrast to animal viruses and bacteriophages, plant viruses infect cells by mechanisms that do NOT involve specific:
a. receptors d. vectors
b. membranes e. hosts
c. envelopes  

 

 

ANS:  A                    DIF:    Medium         REF:   6.5                 TOP:   V.C.i.a

MSC:  Remembering

 

  1. Which of the following is FALSE concerning HIV?
a. It is the causative agent of AIDS.
b. It makes a DNA copy of its RNA genome.
c. Reverse transcriptase is translated from an early gene.
d. The virion contains two copies of the HIV genome.
e. The viral RNA is copied into double-stranded DNA.

 

 

ANS:  C                    DIF:    Medium         REF:   6.5                 TOP:   V.B.iii

MSC:  Understanding

 

  1. The period of time when virions are virtually undetectable inside the infected cell is the:
a. eclipse period d. burst period
b. latent period e. lag period
c. rise period  

 

 

ANS:  A                    DIF:    Easy               REF:   6.6                 TOP:   VI.A.iii

MSC:  Remembering

 

  1. HIV is classified in a category of slowly replicating retroviruses termed:
a. picornaviruses d. lentiviruses
b. papillomaviruses e. pararetroviruses
c. oncogenic viruses  

 

 

ANS:  D                    DIF:    Difficult         REF:   6.6                 TOP:   VI.B

MSC:  Remembering

 

  1. Hosts may benefit from infection by persistent viruses because:
a. hosts never come down with illness when infected by persistent viruses
b. persistent viruses could encode growth factors beneficial to infected hosts
c. persistent viruses shorten hosts’ generation time, resulting in hosts that outcompete their competitors
d. persistent viruses could encode toxins that kill off competing bacteria
e. persistent viruses could improve hosts’ efficiency in uptake of nutrients

 

 

ANS:  D                    DIF:    Difficult         REF:   6.7                 TOP:   VII.B.iii

MSC:  Understanding

 

SHORT ANSWER

 

  1. Why are viruses not affected by antibiotics? Why are there so few antiviral drugs?

 

ANS:

Viruses are not cellular entities. Common antibiotics disrupt cell wall synthesis or interact with ribosomes and disrupt the translation process. Viruses do not have either cell walls or ribosomes, so these antibiotics would be ineffective.

Viruses that infect humans commandeer the host cell to make more viruses. One cannot target the host machinery because then the host would be harmed. There are few viral targets that can be attacked without harming the host cell.

 

DIF:    Difficult         REF:   6.1                 TOP:   I.B.vi              MSC:  Understanding

 

  1. What are prions and how do they cause disease?

 

ANS:

Prions are aberrant forms of naturally occurring proteins that assume an abnormal conformation. They cause disease by binding to normally folded proteins of the same type and altering their conformation. Harmful aggregates of these proteins cause cell death.

 

DIF:    Medium         REF:   6.1                 TOP:   I.E.i                MSC:  Understanding

 

  1. What is the advantage of symmetry in viral particles?

 

ANS:

Symmetry provides a way to form a package out of repeating protein units. This requires a small number of genes, since the same protein unit is repeated. Viral genomes are generally very small, so they have few genes. They can also devote more resources to copying their genome for viral replication if it takes few resources to make the capsid proteins.

 

DIF:    Medium         REF:   6.2                 TOP:   II.A                MSC:  Understanding

 

  1. What is the viral envelope composed of? How does it differ from the bacterial cell envelope?

 

ANS:

The viral envelope is composed of the phospholipid portion of the membrane from the host cell from which it came and proteins encoded by the virus genome. The bacterial cell envelope refers to the layers outside of the cell membrane.

 

DIF:    Medium         REF:   6.2                 TOP:   II.A.i.c           MSC:  Understanding

 

  1. Compare and contrast the Baltimore classification of viruses to the proteomic analysis of bacteriophages. Which more accurately reflects the significant impact of phage evolution?

 

ANS:

The Baltimore classification divides the viruses into seven groups based on the genome of the virus and the route used to get to mRNA. Proteomic analysis is a comparison of the proteins encoded by the genomes. The proteomic tree also yields seven categories of phages. These categories, however, group the phages by host bacteria. Phages with different types of genomes may be grouped together. The proteomic tree analogy more truly reflects the evolutionary process. Viruses that share a host have the ability to exchange genetic information.

 

DIF:    Difficult         REF:   6.3                 TOP:   III.B.i | III.C.ii

MSC:  Understanding

 

  1. Mosaic genomes are genomes that are derived from different sources. How does this occur in viruses?

 

ANS:

When different viruses coinfect a host, they have the opportunity to exchange genetic information through recombination or reassortment of segmented genomes. Viruses may also pick up genes from the host cell, as seen in the oncogenic viruses. The genomic content of a virus is truly influenced by its host range.

 

DIF:    Difficult         REF:   6.3                 TOP:   III.C               MSC:  Understanding

 

  1. Some viruses require an RNA-dependent RNA polymerase. What does that mean? Using the same vernacular, what would you call the host cell RNA polymerase (RNA pol)? What would you call reverse transcriptase (RTase)?

 

ANS:

An RNA-dependent RNA polymerase means that the polymerase requires an RNA template (RNA-dep) to synthesize RNA (RNA pol). The host cell RNA pol would be called a DNA-dependent RNA polymerase, since it requires a DNA template to synthesize RNA. RTase would be referred to as an RNA-dependent DNA polymerase since it utilizes an RNA template to synthesize DNA.

 

DIF:    Difficult         REF:   6.3                 TOP:   III.B.i             MSC:  Applying

 

  1. Why do many RNA viruses encode their own RNA-dependent RNA polymerase and package them in viral particles? How do we take advantage of these viral-specific polymerases?

 

ANS:

Animal or bacterial cells lack RNA-dependent RNA polymerase and thus cannot replicate RNA regardless of whether they are single- or double-stranded RNA. The initial replication of viral RNA genomes have to be accomplished by existing RNA-dependent RNA polymerase. Subsequent replication and/or transcription of viral RNA genome and mRNA could be accomplished by viral RNA-dependent RNA polymerase produced in the hosts.

These viral-specific polymerases could be the targets for antiviral therapies with lowered side effects. They could also be utilized for in vitro replication/amplification of RNA molecules.

 

DIF:    Difficult         REF:   6.3                 TOP:   III.B.i             MSC:  Applying

 

  1. What determines whether a temperate phage will go through the lysogenic cycle or switch to the lytic cycle?

 

ANS:

Proteins bind DNA to repress viral replication genes during lysogeny. The switch to the lytic cycle can occur randomly or can be triggered by environmental stresses that damage the cell’s DNA. Events that threaten a cell’s survival can trigger the lytic cycle. If the host cells are growing well, the virus can continue to survive and be propagated as a prophage.

 

DIF:    Difficult         REF:   6.4                 TOP:   IV.C.iii           MSC:  Understanding

 

  1. How can a virus be used in biotechnology to transfer genes from one bacterial cell to another?

 

ANS:

In the process of transduction, a lysogenic phage is triggered to begin the lytic cycle. When this occurs, the viral DNA is excised from the host chromosome, and sometimes some of the host chromosome is excised with the viral DNA. A combination of viral DNA and host DNA can be packaged into a viral capsid, which then infects a new host and brings both viral and host DNA to the new host. In other cases, a viral infection causes host DNA to be cut into small pieces, and some of those pieces are accidentally packaged into viral capsids. This host DNA in the viral capsid can be transferred to a new host when the virus infects the new host.

 

DIF:    Difficult         REF:   6.4                 TOP:   IV.C.iv.a        MSC:  Applying

 

  1. What determines the host range and tropism of a virus?

 

ANS:

Viruses must attach to specific host cell receptors to cause infection. Only host cells with the specific receptors that a particular virus recognizes can be infected with that viral species. Receptors may also only be found on specific tissues, meaning that the receptors also determine tropism, or which tissues can be infected.

 

DIF:    Medium         REF:   6.4 | 6.5         TOP:   IV.A | V.A.i   MSC:  Understanding

 

  1. How does genome uncoating occur with animal viruses?

 

ANS:

In extracellular uncoating, the virus attaches to the host cell, then inserts its DNA into the host, similar to bacteriophage. Intracellular uncoating can occur in several different ways. The virus can bind host receptors that cause the viral envelope to fuse with the host cell membrane. The genome is then uncoated and released into the host cell. Some viruses are taken up by endocytosis. The host forms an endocytic vesicle around the virus and brings the virus into the cell. The vesicle fuses with a lysosome whose acidic environment activates entry of the capsid into the cytoplasm. Some viruses enter the host by endocytosis and are then transported to the nucleus, where the viral genome is injected into the nucleus.

 

DIF:    Difficult         REF:   6.5                 TOP:   V.A.ii             MSC:  Remembering

 

  1. Define oncogenes and oncogenic viruses, and explain how they can be used in gene therapy.

 

ANS:

Oncogenes are genes that, when expressed, cause uncontrolled proliferation of the infected cell and ultimately cancer. These oncogenes may be activated by viral transfer of host genes to abnormal chromosome locations. Alternatively, viruses may carry these oncogenes in their genome. Viruses that lead to oncogene expression are referred to as oncogenic viruses.

In gene therapy, a virus is used as a vehicle to transfer a functional copy of a gene to an individual to “correct” a genetic defect.

 

DIF:    Difficult         REF:   6.5                 TOP:   V.B.iv.a         MSC:  Understanding

 

  1. What is necessary for bacteriophage and animal virus entry into a cell? How does that differ from plant viral entry?

 

ANS:

There is a specific receptor on the host cell to which a bacteriophage or animal virus must bind to initiate viral entry into the host. There is no receptor involved in plant viral entry. It is instead gained by either contact with damaged tissues, transmission by an animal vector, or transmission through seed.

 

DIF:    Medium         REF:   6.5                 TOP:   V.B | V.C       MSC:  Understanding

 

  1. How is a plaque assay used to count virions in a solution?

 

ANS:

Dilutions of a solution containing an unknown number of viruses is diluted, then mixed with host cells. The mixture is added to soft agar, then poured onto a plate of solid medium. When the virions infect host cells, the host cells will lyse and release more virions to infect neighboring host cells. This cycle continues until a visible clearing, a plaque, can be seen within the lawn of bacteria. The plaques can be counted and multiplied by the dilution factor to determine plaque-forming units/ml.

 

DIF:    Medium         REF:   6.6                 TOP:   VI.C               MSC:  Remembering

 

  1. During the eclipse period of a viral infection, why are the virions undetectable in the growth medium?

 

ANS:

The virions attach to cell-surface receptors and inject their DNA. Since all or most of the virions are attached to the host-cell receptors, there are few present in the growth medium.

 

DIF:    Medium         REF:   6.6                 TOP:   VI.A.iii           MSC:  Understanding

 

  1. Why was the advent of tissue culture heralded as the end of the “monkey era”? What types of things are we able to do with it?

 

ANS:

Monkeys were being used to culture polio virus. This was very expensive and labor-intensive, and involved the use of many animals. Tissue culture affords us the option to avoid animal use. It has become the method of choice to test chemical and antibiotic reagents. It is also used to study animal and plant viruses and to develop vaccines and antiviral chemicals.

 

DIF:    Medium         REF:   6.6                 TOP:   VI.B.iii           MSC:  Understanding

 

  1. To count the number of viruses in a solution, one can perform a plaque assay. Describe how a plaque is generated in the case of a lytic virus and how a virus that does not lyse its host can be detected.

 

ANS:

A plaque represents the area where one virus initially infected one cell. It looks like a clear zone on a lawn of uninfected cells. In the case of a lytic virus, the plaque appears because the cells lyse when the viruses are released. In the case of a nonlytic virus, the cells are not lysed, but when infected they contain components of the replicating virus. Fluorescent antibodies targeting specific viral proteins can be reacted with the cells.

 

DIF:    Medium         REF:   6.6                 TOP:   VI.C | VI.D    MSC:  Understanding

 

  1. The myxovirus was introduced to Australia to control the rabbit population. How does the virus keep the rabbit population relatively low without killing all of the rabbits?

 

ANS:

If there is a large population of rabbits, the rate of transmission of the virus increases. Since they are living closer together, it is more likely that they can pass the virus to other rabbits. Once the population decreases and they are more spread out, they are less likely to come in contact with rabbits carrying the virus. Some of the remaining rabbits are resistant to the virus and others don’t come in contact with it. As long as the population density is low, the virus is transmitted at a low rate. If the population density is high, the virus is more likely to be transmitted to others.

 

DIF:    Difficult         REF:   6.7                 TOP:   VII.B.i            MSC:  Understanding

 

  1. Discuss the role that marine viruses play in carbon balance.

 

ANS:

The carbon cycle in a marine environment is usually described as grazers consuming phytoplankton and then being consumed by carnivores. One must realize that viral infection of each of these leads to lysis and the formation of detritus. This organic material is consumed by bacteria that respire, returning carbon dioxide to the atmosphere.

 

DIF:    Medium         REF:   6.7                 TOP:   VII.B.ii           MSC:  Understanding