Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 33 (p. 678)

1. Viruses are not considered to be living organisms because they cannot grow or replicate on their own.

2. Scientists in these fields discovered simultaneously that the agents of these diseases were not filterable out of solutions by the filters that typically caught bacteria.

3. Generally an electron microscope is required to see an actual viral particle, so the approximate size range is 17 to 1000 nanometers.

4. A bacteriophage is a virus that infects a bacterium. A T4 phage infects a bacterium by landing on it "whisker" side down, allowing the tail to make contact with the bacterial cell wall. The tail retracts, and the tube within it penetrates the bacterial cell wall, permitting the T4 nucleic acid to enter the bacterium.

5. The AIDS virus infects the CD4⁺T blood cell. Each HIV virus possesses a glycoprotein on its surface called gp120. This protein exactly fits the CD4 surface marker on the T cell.

6. Animal viruses penetrate the host cell via endocytosis. A plant virus infects its host by entering at a point of injury; bacterial viruses inject their nucleic acid into the bacterium through the cell wall.

7. It is difficult to treat a viral infection because the virus becomes so well integrated with the host genome that inhibiting the virus also inhibits the host cells. In bacterial infections, on the other hand, one can generally kill prokaryotic bacteria with little effect on the eukaryotic host cells.

8. A prion is a proteinaceous infective particle, which is the source of its name. The particle is protein. A prion can be injected or acquired through tissue transplants. However, it is most commonly acquired by ingestion of infected food, often the brain. Once ingested, it finds its way to the brain of the recipient. It apparently induces folding errors in other proteins in chair-reaction fashion.