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6.E: Acellular Pathogens (Exercises)

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    6.1: Viruses

    Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found. Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid. Viruses are obligate intracellular parasites.

    Multiple Choice

    The component(s) of a virus that is/are extended from the envelope for attachment is/are the:

    1. capsomeres
    2. spikes
    3. nucleic acid
    4. viral whiskers


    Which of the following does a virus lack? Select all that apply.

    1. ribosomes
    2. metabolic processes
    3. nucleic acid
    4. glycoprotein

    A and B

    The envelope of a virus is derived from the host’s

    1. nucleic acids
    2. membrane structures
    3. cytoplasm
    4. genome


    In naming viruses, the family name ends with ________ and genus name ends with _________.

    1. −virus; −viridae
    2. −viridae; −virus
    3. −virion; virus
    4. −virus; virion


    What is another name for a nonenveloped virus?

    1. enveloped virus
    2. provirus
    3. naked virus
    4. latent virus



    True or False: Scientists have identified viruses that are able to infect fungal cells.



    Fill in the Blank

    A virus that infects a bacterium is called a/an ___________________.



    A/an __________ virus possesses characteristics of both a polyhedral and helical virus.



    A virus containing only nucleic acid and a capsid is called a/an ___________________ virus or __________________ virus.


    naked or nonenveloped

    The ____________ _____________ on the bacteriophage allow for binding to the bacterial cell.


    tail fibers

    Short Answer

    Discuss the geometric differences among helical, polyhedral, and complex viruses.

    What was the meaning of the word “virus” in the 1880s and why was it used to describe the cause of tobacco mosaic disease?

    Critical Thinking

    Name each labeled part of the illustrated bacteriophage.

    An illustrated bacteriophage is shown. There are arrows pointing to its different parts labeled A, B, C, and D.

    In terms of evolution, which do you think arises first? The virus or the host? Explain your answer.

    Do you think it is possible to create a virus in the lab? Imagine that you are a mad scientist. Describe how you would go about creating a new virus.

    6.2: The Viral Life Cycle

    Many viruses target specific hosts or tissues. Some may have more than one host. Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release. Bacteriophages have a lytic or lysogenic cycle. The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host genome.

    Multiple Choice

    Which of the following leads to the destruction of the host cells?

    1. lysogenic cycle
    2. lytic cycle
    3. prophage
    4. temperate phage


    A virus obtains its envelope during which of the following phases?

    1. attachment
    2. penetration
    3. assembly
    4. release


    Which of the following components is brought into a cell by HIV?

    1. a DNA-dependent DNA polymerase
    2. RNA polymerase
    3. ribosome
    4. reverse transcriptase


    A positive-strand RNA virus:

    1. must first be converted to a mRNA before it can be translated.
    2. can be used directly to translate viral proteins.
    3. will be degraded by host enzymes.
    4. is not recognized by host ribosomes.


    What is the name for the transfer of genetic information from one bacterium to another bacterium by a phage?

    1. transduction
    2. penetration
    3. excision
    4. translation


    Fill in the Blank

    An enzyme from HIV that can make a copy of DNA from RNA is called _______________________.


    reverse transcriptase

    For lytic viruses, _________________ is a phase during a viral growth curve when the virus is not detected.



    Short Answer

    Briefly explain the difference between the mechanism of entry of a T-even bacteriophage and an animal virus.

    Discuss the difference between generalized and specialized transduction.

    Differentiate between lytic and lysogenic cycles.

    Critical Thinking

    Label the five stages of a bacteriophage infection in the figure:

    A shows the bacteriophage sitting on the outside of the cell; the phage DNA is inside the phage. The host cell shows host DNA. B shows phage DNA entering host. C shows phage DNA replicates as well as protein particles. D shows new bacteriophages forming. E shows new phage particles exiting host bacterial cell.

    Bacteriophages have lytic and lysogenic cycles. Discuss the advantages and disadvantages for the phage.

    How does reverse transcriptase aid a retrovirus in establishing a chronic infection?

    Discuss some methods by which plant viruses are transmitted from a diseased plant to a healthy one.

    6.3: Isolation, Culture, and Identification of Viruses

    Viral cultivation requires the presence of some form of host cell (whole organism, embryo, or cell culture). Viruses can be isolated from samples by filtration. Viral filtrate is a rich source of released virions. Bacteriophages are detected by presence of clear plaques on bacterial lawn. Animal and plant viruses are detected by cytopathic effects, molecular techniques (PCR, RT-PCR), enzyme immunoassays, and serological assays (hemagglutination assay, hemagglutination inhibition assay).

    Multiple Choice

    Which of the followings cannot be used to culture viruses?

    1. tissue culture
    2. liquid medium only
    3. embryo
    4. animal host


    Which of the following tests can be used to detect the presence of a specific virus?

    1. EIA
    2. RT-PCR
    3. PCR
    4. all of the above


    Which of the following is NOT a cytopathic effect?

    1. transformation
    2. cell fusion
    3. mononucleated cell
    4. inclusion bodies


    Fill in the Blank

    Viruses can be diagnosed and observed using a(n) _____________ microscope.



    Cell abnormalities resulting from a viral infection are called ____________ _____________.


    cytopathic effects

    Short Answer

    Briefly explain the various methods of culturing viruses.

    Critical Thinking

    Label the components indicated by arrows.

    A plate with a smooth beige background labeled (A). There are holes in this background; these holes are labeled B.

    (credit: modification of work by American Society for Microbiology)

    What are some characteristics of the viruses that are similar to a computer virus?

    6.4: Viroids, Virusoids, and Prions

    Other acellular agents such as viroids, virusoids, and prions also cause diseases. Viroids consist of small, naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to establish an infection. Prions are proteinaceous infectious particles that cause transmissible spongiform encephalopathies. Prions are extremely resistant to chemicals, heat, and radiation.

    Multiple Choice

    Which of these infectious agents do not have nucleic acid?

    1. viroids
    2. viruses
    3. bacteria
    4. prions


    Which of the following is true of prions?

    1. They can be inactivated by boiling at 100 °C.
    2. They contain a capsid.
    3. They are a rogue form of protein, PrP.
    4. They can be reliably inactivated by an autoclave.


    Fill in the Blank

    Both viroids and virusoids have a(n) _________ genome, but virusoids require a(n) _________ to reproduce.


    RNA, helper virus

    Short Answer

    Describe the disease symptoms observed in animals infected with prions.

    Critical Thinking

    Does a prion replicate? Explain.

    6.E: Acellular Pathogens (Exercises) is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.

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