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21.E: Viruses (Exercises)

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  • 21.1: Viral Evolution, Morphology, and Classification

    Viruses are diverse entities. They vary in their structure, their replication methods, and in their target hosts. Nearly all forms of life—from bacteria and archaea to eukaryotes such as plants, animals, and fungi—have viruses that infect them. While most biological diversity can be understood through evolutionary history, such as how species have adapted to conditions and environments, much about virus origins and evolution remains unknown.

    Review Questions

    Which statement is true?

    1. A virion contains DNA and RNA.
    2. Viruses are acellular.
    3. Viruses replicate outside of the cell.
    4. Most viruses are easily visualized with a light microscope.


    The viral ________ plays a role in attaching a virion to the host cell.

    1. core
    2. capsid
    3. envelope
    4. both b and c



    1. all have a round shape
    2. cannot have a long shape
    3. do not maintain any shape
    4. vary in shape


    Free Response

    The first electron micrograph of a virus (tobacco mosaic virus) was produced in 1939. Before that time, how did scientists know that viruses existed if they could not see them? (Hint: Early scientists called viruses “filterable agents.”)

    Viruses pass through filters that eliminated all bacteria that were visible in the light microscopes at the time. As the bacteria-free filtrate could still cause infections when given to a healthy organism, this observation demonstrated the existence of very small infectious agents. These agents were later shown to be unrelated to bacteria and were classified as viruses.

    21.2: Virus Infections and Hosts

    Viruses can be seen as obligate, intracellular parasites. A virus must attach to a living cell, be taken inside, manufacture its proteins and copy its genome, and find a way to escape the cell so that the virus can infect other cells. Viruses can infect only certain species of hosts and only certain cells within that host. Cells that a virus may use to replicate are called permissive.

    Review Questions

    Which statement is not true of viral replication?

    1. A lysogenic cycle kills the host cell.
    2. There are six basic steps in the viral replication cycle.
    3. Viral replication does not affect host cell function.
    4. Newly released virions can infect adjacent cells.


    Which statement is true of viral replication?

    1. In the process of apoptosis, the cell survives.
    2. During attachment, the virus attaches at specific sites on the cell surface.
    3. The viral capsid helps the host cell produce more copies of the viral genome.
    4. mRNA works outside of the host cell to produce enzymes and proteins.


    Which statement is true of reverse transcriptase?

    1. It is a nucleic acid.
    2. It infects cells.
    3. It transcribes RNA to make DNA.
    4. It is a lipid.


    Oncogenic virus cores can be_______.

    1. RNA
    2. DNA
    3. neither RNA nor DNA
    4. either RNA or DNA


    Which is true of DNA viruses?

    1. They use the host cell’s machinery to produce new copies of their genome.
    2. They all have envelopes.
    3. They are the only kind of viruses that can cause cancer.
    4. They are not important plant pathogens.


    A bacteriophage can infect ________.

    1. the lungs
    2. viruses
    3. prions
    4. bacteria


    Free Response

    Why can’t dogs catch the measles?

    The virus can’t attach to dog cells, because dog cells do not express the receptors for the virus and/or there is no cell within the dog that is permissive for viral replication.

    One of the first and most important targets for drugs to fight infection with HIV (a retrovirus) is the reverse transcriptase enzyme. Why?

    Reverse transcriptase is needed to make more HIV-1 viruses, so targeting the reverse transcriptase enzyme may be a way to inhibit the replication of the virus. Importantly, by targeting reverse transcriptase, we do little harm to the host cell, since host cells do not make reverse transcriptase. Thus, we can specifically attack the virus and not the host cell when we use reverse transcriptase inhibitors.

    In this section, you were introduced to different types of viruses and viral diseases. Briefly discuss the most interesting or surprising thing you learned about viruses.

    Answer is open and will vary.

    Although plant viruses cannot infect humans, what are some of the ways in which they affect humans?

    Plant viruses infect crops, causing crop damage and failure, and considerable economic losses.

    21.3: Prevention and Treatment of Viral Infections

    Viruses cause a variety of diseases in animals, including humans, ranging from the common cold to potentially fatal illnesses like meningitis . These diseases can be treated by antiviral drugs or by vaccines, but some viruses, such as HIV, are capable of both avoiding the immune response and mutating to become resistant to antiviral drugs.

    Review Questions

    Which of the following is NOT used to treat active viral disease?

    1. vaccines
    2. antiviral drugs
    3. antibiotics
    4. phage therapy



    1. are similar to viroids
    2. are only needed once
    3. kill viruses
    4. stimulate an immune response


    Free Response

    Why is immunization after being bitten by a rabid animal so effective and why aren’t people vaccinated for rabies like dogs and cats are?

    Rabies vaccine works after a bite because it takes week for the virus to travel from the site of the bite to the central nervous system, where the most severe symptoms of the disease occur. Adults are not routinely vaccinated for rabies for two reasons: first, because the routine vaccination of domestic animals makes it unlikely that humans will contract rabies from an animal bite; second, if one is bitten by a wild animal or a domestic animal that one cannot confirm has been immunized, there is still time to give the vaccine and avoid the often fatal consequences of the disease.

    21.4: Other Acellular Entities - Prions and Viroids

    Prions and viroids are pathogens (agents with the ability to cause disease) that have simpler structures than viruses but, in the case of prions, still can produce deadly diseases.

    Review Questions

    Which of the following is not associated with prions?

    1. replicating shapes
    2. mad cow disease
    3. DNA
    4. toxic proteins


    Which statement is true of viroids?

    1. They are single-stranded RNA particles.
    2. They reproduce only outside of the cell.
    3. They produce proteins.
    4. They affect both plants and animals.


    Free Response

    Prions are responsible for variant Creutzfeldt-Jakob Disease, which has resulted in over 100 human deaths in Great Britain during the last 10 years. How do humans obtain this disease?

    This prion-based disease is transmitted through human consumption of infected meat.

    How are viroids like viruses?

    They both replicate in a cell, and they both contain nucleic acid.