Mastering Concepts

1. What features do all viruses share?

All viruses contain genetic information (either DNA or RNA) and are surrounded by a protein coat.

2. What determines a virus’s host range?

A virus’s host range is determined by the presence of a specific receptor on the host cell.

3. How do viruses evolve?

Viruses evolve by natural selection. Mutations in viral DNA or RNA create genetic variation; some variants are more successful than others at infecting cells and leaving descendants.

1. Describe the five steps in viral replication.

(1) Attachment: virus adheres to host cell receptor. (2) Penetration: virus enters the cell. (3) Synthesis: multiple copies of the viral genome and proteins are produced by the host cell. (4) Assembly: the viral genetic information is packaged in a protein coat. (5) Release: new viruses leave host cell.

2. What is the source of energy and raw materials for the synthesis of viruses in a host cell?

The source of energy and raw materials is the host cell’s ATP and its stores of nucleotides and amino acids.

1. What is a lytic viral infection?

In a lytic viral infection, the virus immediately replicates after infecting a host cell and then bursts out of the host cell, killing it.

2. How is a lysogenic viral infection similar to and different from a lytic cycle?

A lysogenic viral infection is similar to a lytic viral cycle in that both allow begin with the attachment and penetration steps. In a lytic infection, the remaining steps (synthesis, assembly, and release) occur immediately, destroying the host cell. In contrast, a lysogenic virus inserts its DNA into the host chromosome, where it is carried on into daughter cells. Eventually an environmental change triggers a switch to the lytic pathway, destroying the infected host cells.

1. How can a person acquire and transmit a viral infection?

Viral infections can be acquired by inhaling respiratory droplets or ingesting contaminated food and water. Some viruses are also acquired through the bloodstream and transmitted by blood transfusion, sexual contact, or contaminated needles.

2. How do symptoms of a viral infection develop?

A viral infection can cause symptoms by killing host cells in the respiratory tract, skin, immune system, or other body parts. In addition, the viral infection triggers an immune response that causes whole-body symptoms like fever and inflammation.

3. What is a latent animal virus?

In a latent infection, a virus has infected a host cell, but the viral genetic information is not being expressed. The host cell is therefore not producing new viruses.

4. How are some latent viral infections linked to cancer?

Some latent viruses signal host cells to divide continuously, a strategy that increases the number of infected cells but can also cause cancer.

5. Describe how HIV replicates in host cells.

After attachment and penetration, the HIV enzyme reverse transcriptase copies the viral RNA to DNA. The viral DNA then inserts itself into the host cell's DNA. New HIV particles are synthesized and assembled inside the host cell and are released by budding. A new virus acquires its envelope from the host cell’s membrane as it emerges.

6. How are viral infections treated and prevented?

Viral infections are difficult to treat, in part because viruses infect living host cells; it is difficult to destroy the virus without also destroying the host cell. A few antiviral drugs do prevent viral replication, but viruses are genetically variable, and new treatments quickly select for resistant varieties. The best weapon for prevention is the vaccine, which builds immunity to a virus before a person is exposed.

1. How do viruses enter plant cells and spread within a plant? One of the most common ways for viruses to enter plant cells is by hitching a ride on the contaminated mouthparts of plant-feeding insects. The viruses spread within the plant via plasmodesmata or in vascular tissue.

2. What are some symptoms of a viral infection in plants?

Some symptoms include small dead spots, mottled leaves, abnormal growth, or even streaking in some flowers.

1. How are viroids and prions different from viruses?

All viruses have genetic information and a protein coat. Viroids consist only of RNA and therefore lack a protein coat. Prions are abnormally shaped proteins with no genetic information.

2. How do viroids and prions cause disease?

Viroids interfere with the production of important proteins. An abnormal prion protein “recruits” normal proteins to refold into the abnormal configuration, triggering a chain reaction that results in cell death.

3. What is the best way to avoid prion diseases?

The best way to avoid prion disease is to keep animal brains and spinal cords out of the human food chain and out of medical products.

1. What hypothesis was Hahn’s team investigating, and how did they test their hypothesis?

Hahn’s team hypothesized that the source of the HIV-1 was a form of SIV found in central chimpanzees. They tested the hypothesis by investigating the amino acid sequences of key proteins in HIV-1 and in SIV from four chimpanzees. The researchers also documented an overlap between the habitat of central chimpanzees and regions where HIV-1 occurs. A followup study measured the infection rate of SIV in wild chimpanzees at up to 18%.

2. Explain how the relationships in figure 16.10 support the researchers’ hypotheses.

The amino acid sequences of SIV from chimpanzees were nested within the sequences of HIV-1. If HIV-1 did not originate from SIVcpz, the sequences would not have remained separate and not clustered together so tightly. Write It Out

1. Describe the basic parts of a virus and how each contributes to viral replication.

The two basic parts of a virus are the protein coat and genetic information. The protein coat protects the virus and (in non-enveloped viruses) attaches to host cell receptors. The genetic information encodes the viral molecules that are synthesized and assembled in the host cell.

2. Your biology lab instructor gives you a petri dish of agar covered with visible colonies. Your lab partner says the colonies are viruses, but you disagree. How do you know the colonies are bacteria?

Viruses do not produce colonies on agar; they require living host cells to reproduce. Many bacteria, on the other hand, can live and reproduce using the nutrients present in agar.

3. Why is it inaccurate to refer to the “growth” of viruses?

Because viruses are not cells, they cannot grow. Instead, new viruses are assembled inside cells, somewhat like new cars being assembled in a factory.

4. Rhinoviruses replicate in the mucus-producing cells in a person’s nose, throat, and lungs, causing the common cold. Papillomaviruses, which infect skin cells, cause growths called warts. HIV infects T cells and causes AIDS. How do these three types of viruses “know” which human cells to infect?

The virus “knows” which cells are suitable for infection by attaching to unique receptors on the surface of a host cell.

5. As described in this chapter’s Burning Question box, human papillomavirus (HPV) infects cells of the skin and genitals, but it has never been shown to infect T cells. If a researcher put HPV and T cells in the same petri dish, which step of HPV replication would fail? Explain your answer.

The attachment step would fail because T cells lack the receptors that HPV requires to infect a cell.

6. This chapter’s Apply It Now box describes many treatments for HIV. Considering the high mutation rate of the HIV genome, why might HIV-positive patients take several anti-HIV drugs in combination?

Patients typically take multiple anti-HIV drugs to reduce the chance that HIV acquires resistance to the drugs. It is relatively unlikely that one virus would simultaneously undergo all of the mutations necessary to acquire resistance to all of the drugs.

7. Refer to figure 16.B, then explain why a mutation in a gene encoding a T cell coreceptor might be beneficial. HIV cannot infect a T cell unless both the receptor and a coreceptor are present. A mutation in the gene encoding a coreceptor might cause the coreceptor to be misshapen or absent. The cell with the mutated gene would therefore be immune to HIV infection.

8. Chapter 7 compared chromosomes to cookbooks and genes to recipes. How could you incorporate viruses into this analogy?

Viruses would be analogous to a small number of new recipes that are slipped into a preexisting cookbook.

9. Search the Internet for information about the injectable flu vaccine (a “flu shot”). Why is the flu shot administered annually when many other vaccines last for years? Is it possible for a flu shot to cause influenza?

The influenza virus mutates so rapidly that this year’s vaccine will be ineffective against next year's virus. The injectable flu vaccine consists of killed viruses, which cannot cause influenza.

10. Why do antibiotics kill bacteria but leave viruses unharmed?

Viruses lack the cell walls, ribosomes, and enzymes that antibiotic drugs target.

11. Use the Internet to find three examples of viruses that infect humans but are not mentioned in the chapter. Describe the symptoms associated with each infection.

[Answers will vary]

12. Gene therapy aims to replace faulty or disease-causing genes with healthy DNA sequences. How might you use a virus to deliver these new genes into a cell?

You would begin by selecting a virus that infects the target cell type and that typically inserts its genetic information into the host cell’s DNA. The virus would be modified to include the DNA sequence desired and to remove the genes required for viral replication. The altered viruses would be allowed to infect and deliver their payload into host cells.

13. How is a biological virus similar to and different from a computer virus?

Both types of viruses can severely disrupt the functions of the host; both also have the potential to pass rapidly from host to host. Moreover, a biological virus needs a living host to replicate, just as a computer virus needs a computer. However, biological viruses evolve rapidly as their genetic material mutates. In addition, a biological virus triggers the production of a physical product (DNA/RNA and proteins), whereas a computer virus exists only in the digital realm.

Pull It Together

1. What are the four types and configurations of nucleic acids in viruses? A viral genome can be made of double-stranded DNA, single-stranded DNA, double-stranded RNA or single-stranded RNA.

2. How is a virus similar to and different from a bacterium, a viroid, and a prion?

Like a viroid and a prion, a virus does not consist of cells, whereas bacteria are cells. As a result, only bacteria have a functional cell membrane and can replicate in the absence of a host cell. Viruses, bacteria, and prions all consist partly or entirely of proteins; viroids lack protein. Viruses, bacteria, and viroids all consist partly or entirely of genetic material; prions lack genetic material.

3. Add reservoir, latent, and vaccines to the concept map.

“Viruses” could connect with the phrase “that infect humans may persist in a nonhuman” to “Reservoir.” “Latent” could connect with the phrase “infections do not proceed to” to “Synthesis.” “Vaccines” could connect with the phrase “prevent infections by” to “Viruses.”