Chapter 1 Review Questions

Chapter 1

1. What is a virion? What is the difference between a virus and a virion? 

A virion is an infectious virus particle. The difference between a virus and a virion is that a virion is an external form of a virus. It consists of a genome that is either DNA or RNA. It also has a capsid and may or may not have an envelope. The term virus is referring to the entity as whole, meaning it includes the viral cycle and characteristic differences. 

2. Describe three reasons why it is important to study viruses. 

The first and most important reason to study viruses is because they can cause diseases with some being fatal if not treated promptly. Viruses can cause epidemics or pandemics. Studying viruses has helped in understanding how systems worked in living organisms. This was exemplified by the Hershey-Chase experiment that revealed how DNA is a hereditary matter. Lastly, the studying of viruses revealed that they could be used for treatment of bacterial infections. 

3. How was it determined that DNA, and not protein, encodes genetic information? 

The Hershey-Chase experiment helped determine that DNA encodes genetic information. They used radioactive ³²P  and ³⁵S. Phosphorus is found in DNA and Sulfur is found in proteins. They observed that the P DNA went into the cell and that ³²P was passed down into a new set of bacteriophages. The same observation was not found in the ³⁵S. 

4. How are viruses used therapeutically? 

Viruses were used for phage therapy, which used viruses for the treatment of bacterial infections when antibiotics did not exist yet. Viruses were also used for gene therapy, which was replacing defective genes to treat disease. 

5. List the characteristics of living things. Describe why viruses do or do not satisfy each criterion. 

For an organism to be considered alive, it must be able to meet certain criteria. The first being that there must be some type of genomic material. Viruses do meet this criteria because depending on the virus it has either DNA or RNA genomic material. The next criteria needed is the organism must be able to take part in metabolic reactions. Viruses do not meet this requirement because they can’t independently engage in metabolic reactions, they must use the host cell’s mechanisms. An organism must be able to independently reproduce and grow. While a virus can technically reproduce and grow, it cannot do so independent of a host cell. An organism also needs to be able to quickly adjust to changes in the environment to maintain homeostasis. A virus is not able to maintain a level of homeostasis because they lack the mechanisms that living organisms have. Finally, for a living organism to be able to adapt to its environment, it must be able to evolve. Viruses do meet this criteria because they can evolve to adapt to different environments for survival. 

6. How is virus replication different from cell division? 

In cell division, they are able to reproduce independently. Cells replicate its DNA, grow and divide both the cell and DNA in two. Viruses can not reproduce independently, it must enter a cell for it to reproduce. Unlike in cell division, a virus will disassemble once it enters and will make new virions from scratch. 

7. What hypotheses exist for the origin of viruses? Which do you think is most likely, and why? 

There are three hypotheses that exist for the origin of viruses. The first is the Pre-Cellular hypothesis, which states that viruses were present before or alongside early cells. This hypothesis believes that RNA was the first genetic material to exist on Earth. The second hypothesis is called the Escape hypothesis. This hypothesis stated that viruses are pieces of cells that escaped and were able to travel from cell to cell. This hypothesis believes that cells came before viruses unlike in the Pre-cellular hypothesis. The last one is called the Regressive hypothesis. This hypothesis states that viruses were at one point independent and regressed back to a state where they can’t replicate independently. 

The one that is most likely is the Regressive hypothesis. This hypothesis explains that viruses, particularly large, evolved which means that viruses have cellular ancestors. Also, the pre-cellular and escape hypotheses are essentially the opposites of each other. Both contradict one another that either RNA is the starting genetic material or it’s not.  

8. What characteristics of viruses made it difficult for scientists to discover them? 

Koch’s Postulates were created as a way of making it easier to distinguish and identify viruses. In actuality, his Postulates made it quite difficult. Not every virus was able to match these characteristics and thus making it hard to discover them. 

9. How were Chamberland filters useful in the discovery of viruses? 

The Chamberland filters were useful because it helped identify if the infectious agent was bacterial or not. The Tobacco mosaic virus was much smaller than bacteria and was still infectious when passed through the filter. This discovery was useful because it provided a distinction between bacteria and viruses.  

10. How do light and electron microscopes work? What are the advantages and disadvantages of each?

Light microscopes work by having a light source shine through a specimen and lenses that focus the specimen to make it appear larger. Electron microscopes use electron beams that illuminate a specimen. The advantages of light microscopes is that they are less expensive, don’t need training, and can observe color. However, light microscopes have good resolution but it is not better than an electron microscope. The advantages of electron microscopes is that they have greater magnification and resolution. Electron microscopes allow for great visualization of the surface of a virus. The disadvantage is that electron microscopes are expensive and require training. They also don’t show color and specimens must be dead to be seen.