Chapter 2 Review Questions

1. Why are viruses considered obligate intracellular pathogens? 

They are considered obligate intracellular pathogens because they depend on the cell’s internal environment to be able to create new virions. 

2. How does viral replication differ from cell reproduction? 

Viruses use the cell's mechanisms to replicate and create new virions from scratch, while cells will divide into two. 

3. What is the function of the capsid? Why must viruses repeat the same capsid protein subunits repeatedly, rather than having hundreds of different capsid proteins? 

The capsid is a protective shell that surrounds the nucleic acid of a virus. By repeating the same capsid protein, the capsid is stronger and flexible compared to having a bunch of different capsids. 

4. Explain what 2–3–5 symmetry is, pertaining to an icosahedron. 

2-3-5 symmetry refers to the ways that an icosahedron is able to rotate on its axis. 2-fold is when the axis is through the center of an edge. 3-fold is when the axis is through a face center. 5-fold is when the axis is through a vertex of an icosahedron. 

5. What is a structural unit? In a T = 3 virus that has three subunits per structural unit, how many total subunits form the capsid? 

A structural unit is a set of repeated viral proteins or a single viral protein that form the face of an icosahedron.  In a T = 3 virus, the total subunits that form the capsid is 3. 

6. List the seven groups of the Baltimore classification system.

Class 1: dsDNA

Class 2: ssDNA

Class 3: dsRNA

Class 4: +ssRNA

Class 5: -ssRNA 

Class 6: RNA viruses that can reverse transcribe

Class 7: DNA viruses that can reverse transcribe 

7. What taxa are used to classify viruses? How does this differ from the classification of a living organism? 

Viruses use: realm, kingdom, phylum, class, order, family, genus, and species. The realm in viruses is called the domain in living organisms. 

8. What viral properties are used to classify viruses?

There are several properties used to classify viruses. A virus's nucleic acid type and protein purpose is taken into account. The sequence of nucleic acid is also considered. The physical properties of viruses are also considered such as virion size, the capsid shape, whether it has an envelope or not.