CHAPTER 21 REVIEW

Which of the following statements is false?

An environmental stressor can cause the phage to initiate the lysogenic cycle.

The vaccine Gardasil that targets human papilloma virus (HPV), the etiological agent of genital warts, was developed after the anti-HPV medication podofilox. Why would doctors still want a vaccine created after anti-viral medications were available?

Anti-viral medications treat HPV after the skin of the genitals has been infected. Conversely, Gardasil stimulates the immune system to prevent infection of the tissue, even if a person is exposed to HPV. Since HPV is often asymptomatic, particularly in men, the vaccine also controls the spread of disease (patients will not seek treatment for a disease if they do not realize they are infected).

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

Antibiotics

Varicella-zoster virus is a double-stranded DNA virus that causes chickenpox. How does its genome structure provide an evolutionary advantage over a single-stranded DNA virus?

Both viruses are made of DNA, but single-stranded DNA viruses lack the ability to create the double helix. Thus, double-stranded DNA viruses have a more stable genome due to the complimentary base pairing, increasing the lifespan of the virus's genome.

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

Capsid and envelope

Which of the following is not associated with prions?

DNA

Which statement is true of viral replication?

During attachment, the virus attaches at specific sites on the cell surface.

Which of the following statements about virus structure is true?

Glycoproteins help the virus attach to the host cell.

A bacteriophage with a lytic life cycle develops a mutation that allows it to now also go through the lysogenic cycle. How would this provide an evolutionary advantage over the other bacteriophages that can only spread through lytic cycles?

In a lysogenic cycle, the bacteriophage integrates into the host bacterium's genome as a prophage, and is passed on to daughter cells every time a bacterium carrying the prophage replicates. This allows the prophage to be dispersed through a wide population without killing any of the host cells. Since the mutated bacteriophage also retains the ability to switch into the lytic cycle, it now has two methods to disseminate through the bacteria population.

Which statement is true of reverse transcriptase?

It transcribes RNA to make DNA.

Which statement is not true of viral replication?

Newly released virions can infect adjacent cells.

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 loss

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.

Classify the Rabies virus (a rhabdovirus family member) and HIV-1 with both the Baltimore and genomic structure systems. Compare your results. What conclusions can be made about these two different methods?

Rabies virus is a (-) strand RNA virus that transcribes mRNAs from its genome (Group V). HIV-1 is a single-stranded RNA retrovirus that uses reverse transcriptase to create a double-stranded DNA copy of its genome which is integrated into the host human's genome prior to making mRNAs (Group VI). The genome structure system classifies both viruses as single-stranded RNA viruses with linear genomes. Baltimore classification sorts Rabies virus and HIV-1 into two different groups, indicating that the two viruses have very different life cycles. However, genome structure classification does not distinguish between the two viruses. This leaves out important information regarding virus function and survival.

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.

A botanist notices that a tomato plant looks diseased. How could the botanist confirm that the agent causing disease is a viroid, and not a virus?

The botanist would need to isolate any foreign nucleic acids from infected plant cells, and confirm that an RNA molecule is the etiological agent of disease. The botanist would then need to demonstrate that the RNA can infect plant cells without a capsid, and that the RNA replicates, but is not translated to produce proteins.

Influenza virus is packaged in a viral envelope that fuses with the plasma membrane. This way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

The host cell can continue to make new virus particles.

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.

Which statement is true of viroids?

They are single-stranded RNA particles

How are viroids like viruses?

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

Which is true of DNA viruses?

They use the host cell's machinery to produce new copies of their genome.

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.

Viruses_______.

Vary in shape

Which statement is true?

Viruses are acellular.

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.

A bacteriophage can infect ________.

bacteria

Oncogenic virus cores can be_______.

either RNA or DNA

Vaccines_______.

stimulate an immune response