Chapter 17 - Study Questions

Use the following information to answer the question below. Some viruses can be crystallized and their structures analyzed. One such virus is yellow mottle virus, which infects beans. This virus has a single-stranded RNA genome containing about 6,300 nucleotides. Its capsid is 25-30 nm in diameter and contains 180 identical capsomeres. If the yellow mottle virus capsid has 20 facets, how many proteins form each facet?

9

What is the function of reverse transcriptase in retroviruses?

It uses viral RNA as a template for DNA synthesis.

Which of the following is the most probable fate of a newly emerging virus that causes high mortality in its host?

The newly emerging virus will die out rather quickly or will mutate to be far less lethal.

A researcher lyses a cell that contains nucleic acid molecules and capsomeres of tobacco mosaic virus (TMV). The cell contents are left in a covered test tube overnight. The next day this mixture is sprayed on tobacco plants. Which of the following would be expected to occur?

The plants would develop the typical symptoms of TMV infection.

Which of the following describes plant virus infections?

They are spread via the plasmodesmata.

In many ways, the regulation of the genes of a particular group of viruses will be similar to the regulation of the host genes. Therefore, which of the following would you expect of the genes of the bacteriophage?

control of more than one gene in an operon

Which of the following accounts for someone who has had a herpesvirus-mediated cold sore or genital sore getting flare-ups for the rest of his or her life?

copies of the herpesvirus genome permanently maintained in host nuclei

Emerging viruses arise by

all of the above

Figure 17.1 Which of the three types of viruses shown in Figure 17.1 would you expect to include a capsid(s)?

all three

Viral envelopes can best be analyzed with which of the following techniques?

antibodies against specific proteins not found in the host membranes

RNA viruses require their own supply of certain enzymes because

host cells lack enzymes that can replicate the viral genome.

Which of the following characteristics, structures, or processes is common to both bacteria and viruses?

genetic material composed of nucleic acid

Which of the following can be effective in preventing the onset of viral infection in humans?

getting vaccinated

Antiviral drugs that have become useful are usually associated with which of the following properties?

interference with viral replication

Which viruses have single-stranded RNA that acts as a template for DNA synthesis?

retroviruses

The host range of a virus is determined by

the proteins on its surface and that of the host.

Use the following information to answer the following questions. In 1971, David Baltimore described a scheme for classifying viruses based on how the virus produces mRNA. Table 17.1 shows the results of testing five viruses (A—E) for nuclease specificity, the ability of the viral genome to act as an mRNA, and the presence (+) or absence (—) of each virus's own polymerase. Table 17.1 Based on Table 17.1, which virus meets the requirements for a bacteriophage?

A

Which of the following is characteristic of the lytic cycle?

A large number of phages are released at a time.

Which of the following series best reflects what we know about how the flu virus moves between species?

An animal such as a pig is infected with more than one virus, genetic recombination occurs, the new virus mutates and is passed to a new species such as a bird, and the virus mutates and can be transmitted to humans.

Use the following information to answer the following questions. In 1971, David Baltimore described a scheme for classifying viruses based on how the virus produces mRNA. Table 17.1 shows the results of testing five viruses (A—E) for nuclease specificity, the ability of the viral genome to act as an mRNA, and the presence (+) or absence (—) of each virus's own polymerase. Table 17.1 Based on Table 17.1, which virus meets the Baltimore requirements for a retrovirus?

D

Figure 17.1 Which of the three types of viruses shown in Figure 17.1 would you expect to include glycoproteins?

I and II only

Refer to the treatments listed below to answer the following questions. You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious If you already knew that the infectious agent was either bacterial or viral, which treatment would allow you to distinguish between these two possibilities?

III

Why do RNA viruses appear to have higher rates of mutation?

Replication of their genomes does not involve proofreading

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced will have

T4 protein and T4 DNA.

Which of the following statements describes the lysogenic cycle of lambda (λ) phage?

The phage genome replicates along with the host genome.

Use the following information to answer the few questions. The herpesviruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex virus (HSV) types I and II, causing facial and genital lesions, and the varicella zoster virus (VSV), causing chicken pox and shingles. Each of these three actively infects nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. In electron micrographs of HSV infection, it can be seen that the intact virus initially reacts with cell-surface proteoglycans, then with specific receptors. This is later followed by viral capsids docking with nuclear pores. Afterward, the capsids go from being full to being "empty." Which of the following best fits these observations?

The viral envelope mediates entry into the cell, the capsid enters into the nuclear membrane, and the genome is all that enters the nucleus.

Most molecular biologists think that viruses originated from fragments of cellular nucleic acid. Which of the following observations supports this theory?

Viral genomes are usually similar to the genome of the host cell.

Which of the following is the best predictor of how much damage a virus causes?

ability of the infected cell to undergo normal cell division

A human pandemic is

an epidemic that extends around the world.

Use the following information to answer the few questions. The herpesviruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex virus (HSV) types I and II, causing facial and genital lesions, and the varicella zoster virus (VSV), causing chicken pox and shingles. Each of these three actively infects nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. In order to be able to remain latent in an infected live cell, HSV must be able to shut down what process?

apoptosis of a virally infected cell

Refer to the treatments listed below to answer the following questions. You isolate an infectious substance that is capable of causing disease in plants, but you do not know whether the infectious agent is a bacterium, virus, viroid, or prion. You have four methods at your disposal that you can use to analyze the substance in order to determine the nature of the infectious agent. I. treating the substance with nucleases that destroy all nucleic acids and then determining whether it is still infectious II. filtering the substance to remove all elements smaller than what can be easily seen under a light microscope III. culturing the substance by itself on nutritive medium, away from any plant cells IV. treating the sample with proteases that digest all proteins and then determining whether it is still infectious If you already knew that the infectious agent was either a viroid or a prion, which treatment would allow you to distinguish between these two possibilities?

either I or IV

Figure 17.2 In Figure 17.2, at the arrow marked II, what enzyme(s) are being utilized?

host cell DNA polymerase

Use the following information to answer the few questions. The herpesviruses are very important enveloped DNA viruses that cause disease in all vertebrate species and in some invertebrates such as oysters. Some of the human ones are herpes simplex virus (HSV) types I and II, causing facial and genital lesions, and the varicella zoster virus (VSV), causing chicken pox and shingles. Each of these three actively infects nervous tissue. Primary infections are fairly mild, but the virus is not then cleared from the host; rather, viral genomes are maintained in cells in a latent phase. The virus can then reactivate, replicate again, and be infectious to others. If scientists are trying to use what they know about HSV to devise a means of protecting other people from being infected, which of the following would have the best chance of lowering the number of new cases of infection?

interference with new viral replication in preexisting cases

Figure 17.2 In Figure 17.2, when new viruses are being assembled (IV), what mediates the assembly?

nothing; they self-assemble

The difference between vertical and horizontal transmission of plant viruses is that

vertical transmission is transmission of a virus from a parent plant to its progeny, and horizontal transmission is one plant spreading the virus to another plant.