1. How Were Viruses Discovered?

Biology 123 SI- Dr. Raut’s Class

Session 20- 4/13/2015

1. How were viruses discovered?

Farmers had been having problems with their tobacco plants being infected. A scientist wanted to see if it was an issue with bacterial infection. The blended up a plant and filtered it through a filter that captures bacteria and poured this liquid over unaffected tobacco plants. The tobacco plants were then diseased. This proved that they suffered from something other than bacteria.

2. What are the main structural components of a virus?

All viruses have a hereditary material (either DNA or RNA), and a protein coat called a capsid. Many viruses also have a viral envelope which is actually derived from the membrane of the host cell and will contain the phospholipid and some glycoproteins from the host cell.

3. Why are viruses not considered to be alive? This makes viruses ______parasites.

Viruses are not alive, because while they can reproduce, they cannot reproduce without the help of the host cell. The virus does not have any ribosomes and therefore cannot make its own proteins. Without proteins the virus cannot make its own enzymes or proteins which are necessary for reproduction. Obligate parasites

4. True or false. All viruses can affect all species of animals.

False. Viruses can only infect a specific number of hosts which are found within the viruses host range.

5. Draw and describe the lytic cycle.

In the lytic cycle, the DNA is immediately integrated into the host’s DNA, and production of new viruses begins immediately. These new viruses will cause the cell to lyse, and the viruses will be released. Viruses who only replicate through the lytic cycle are called virulent phages.

6. Draw and describe the lysogenic cycle.

In the lysogenic cycle, the DNA will be incorporated into the DNA of the host cell in the form of a prophage. The DNA will stay there without the host cell knowing and be replicated with the host cell’s DNA as cells are replicated. At some point environmental factors will be favorable, and the virus will enter into the lytic cycle. All of the cells that have been made with the prophage present will begin making new viruses and all of the cells will burst to release these viruses. A virus that can replicate by either the lytic or lysogenic cycle is called a temperate phage.

7. What are the main two categories of viruses?

The main two categories of viruses are RNA and DNA.

8. How do retroviruses work? What is an example of a retrovirus?

Retroviruses have RNA which cannot integrate into the host cell’s genome. Therefore, the retrovirus must go through the process of converting its RNA into DNA. They have a virus called reverse transcriptase which will convert this RNA to DNA so it can be put into the host cell’s genome. HIV is an important example of a retrovirus.

9. There are 306 human influenza viruses and more emerge fairly often. How does this happen?

Viruses have the ability to evolve fairly rapidly. Our cells have methods for fixing accidents that happen when replicating DNA, but they do not fix mistakes made when replicating viral DNA. Therefore, every time a mutation is made in the viral genome a new virus is made.

10. Briefly describe viroids and prions.

A viroid is a small, circular RNA molecule. They do not code for proteins, but do replicate within the host’s system, typically in plant cells.

A prion is an infectious protein. A prion is a misfolded protein, which then causes other proteins around it to fold incorrectly. An example of a disorder caused by prions is mad cow disease.

11. What are the main observations made by Darwin on adaptation?

Observation 1: Members of a population often vary in their inherited traits.

Observation 2: All species can produce more offspring than their environment can support, and many of these offspring fail to survive and reproduce.

Inference 1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals.

Inference 2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

12. Explain homology, vestigial structures, and convergent evolution.

Homology- similarities resulting from a common ancestor. Example: the similar structure in limbs across mammals.

Vestigial structures- remnants of features that served important functions in the organism’s ancestor. Example: Chill bumps are caused by the contraction of arrector pili muscles. In other mammals, this causes the hair to stand up which makes them both warm and look bigger as a defense mechanism. That is why we get chill bumps when we are cold or scared.

Convergent evolution-the independents evolution of similar features in different lineages; these are similar traits that are NOT due to a common ancestor, but rather, similar environments and needs. Example: flying squirrel and a sugar glider.

13. How does antibiotic resistance occur?

As bacteria evolve, they often become resistant to antibiotics, making treatment much more difficult.

14. _____Genetic Variation___ makes evolution possible.