Notes to Instructors
Chapter 19 Viruses
What is the focus of these activities?
Although almost all students recognize that viruses exist and are pathogenic organisms,
What are the particular activities designed to do?
Activity 19.1 How do viruses, viroids, and prions affect host cells?
Activity 19.1 provides students with a mechanism to sort out what viruses are and how
What misconceptions or difficulties can these activities reveal?
Activity 19.1
Like most people, many students do not understand that antibiotics are designed to treat
Answers
Activity 19.1 How do viruses, viroids, and prions affect host cells?
1. By definition, viruses are obligate intracellular parasites. What does this mean?
An obligate intracellular parasite is one that can neither survive long term nor
2. In general, how are viruses classified?
Viruses are often classified based on the type of nucleic acid they contain—e.g., ss
Activity 19.1 133
3. What is reverse transcriptase?
a. Where was reverse transcriptase first found?
b. Of what use is reverse transcriptase to viruses? Of what use is it to scientists?
It is useful in genetic recombination to make cDNA from mRNA and avoid the
4. Why must viruses invade other cells to reproduce?
a. Describe the general process. Include a discussion of lytic and lysogenic viral
cycles.
Some viruses are only lytic. They enter the cell and take over the cell’s
b. Which types of viruses are more likely to have a lysogenic phase?
A number of double-stranded DNA viruses and retroviruses that can produce
5. More than 100 different viruses can cause the “common cold” in humans. Many of
these are rhinoviruses. Other viruses—influenza viruses—cause the flu. While there
are many different antibiotics for treating bacterial infections, there are relatively
few drugs available to treat viral infections. Explain.
Antibiotics used to counteract bacterial infections target bacteria-specific structures or
134 Activity 19.1
6. a. How do viruses, viroids and prions differ in terms of both composition and
function?
Composition: Viruses ⫽nucleic acid core with protein coat or capsid.
b. Compared to viral infections, do these differences make it easier or harder to
treat viroid and prion infections? Explain your reasoning.
Because viroids are basically bare RNA and prions are altered proteins, there are
Activity 19.1 135