AP Biology Reading Guide Name Chapter 19: Viruses Overview Experimental work with viruses has provided important evidence that genes are made of nucleic acids. Viruses were also important in working out the molecular mechanisms of DNA replication, transcription, and translation. Viruses have been important in the development of techniques of manipulating and transferring genes. As you learn about viruses in this chapter, you will build on the foundation necessary for an understanding of the molecular techniques of biotechnology. Concept 19.1 A virus consists of a nucleic acid surrounded by a protein coat 1. What are the four forms of viral genomes? (4) 2. What is a capsid? What are capsomeres? What different shapes may capsids have? 3. a. What is the significance of a membranous envelope (viral envelope)? What does it help the virus do? b. What makes up a viral envelope? 4. a. What type of virus is pictured to the right? b. What does its name mean? c. What is its host? d. Is the genome of this virus DNA, or RNA? 1
Concept 19.2 Viruses reproduce only in host cells 5. Viruses are considered non-living because they do not meet all the characteristics of a living organism. For example, viruses cannot reproduce on their own or make their own proteins, therefore they are non-living or acellular. In order to reproduce, a virus must hijack the replication machinery in a host cell. What property of a virus determines its attachment to a host cell membrane? 6. Compare the host range for West Nile virus to that of the human cold virus. 7. What components of the host cell does a virus use to reproduce itself? 8. Put the following events in order for an animal virus with a DNA genome. (Note there are two events below that will actually be occurring simultaneously within the host cell and the order of them won t matter). The host cell s DNA polymerase repeatedly replicates the viral DNA using host cell nucleotides. Hundreds to thousands of newly constructed viruses exit the host cell thus destroying the host cell. The virus enters the host cell releasing its viral genome (in this case DNA) and the capsid is disassembled. The host cell assembles new viruses. The new viruses can now infect additional cells. The host cell s RNA polymerase transcribes the viral DNA producing thousands of viral capsomere proteins for capsid construction. 9. Identify the two ways that some double-stranded DNA viruses can reproduce. 10. Identify 3 reasons why bacteria haven t been eliminated by virulent phages. 2
11. a. What are restriction enzymes and what is their role in bacteria? b. Explain why the bacteria s own DNA isn t effected by its own restriction enzymes. 12. Differentiate between a virulent phage and a temperate phage? 13. Differentiate between the lytic cycle and the lysogenic cycle. 14. What is a prophage? 15. Since cells that have incorporated phage DNA into their genome may continue to divide and propagate the viral genome, what might trigger the switchover from lysogenic to lytic mode? 16. What are two things that animal viruses tend to have that bacteriophages don t have? 17. Animal viruses are classified based on their genome. Identify the 5 classes of animal viruses. (4) (5) 18. Discus the interaction between the animal virus envelope and the host cell. 3
19. Put the following events in order for an animal virus with a single-stranded RNA genome. (Note there are two events below that will actually be occurring simultaneously within the host cell and the order of them won t matter) Viral enzymes use the viral RNA as a template to synthesize a complimentary RNA strand (RNA RNA; requires viral enzymes, no such enzymes exist in animal cells; these enzymes are located with the viral genome in the capsid). Viral enzymes use the complimentary RNA strand as a template to synthesize more viral RNA The capsid is digested releasing the viral RNA. Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, allowing entry into the host cell. Ribosomes in the cytosol can use the complimentary RNA as mrna, translating it into capsomere proteins for the capsid. Ribosomes on the rough ER can use the complimentary RNA as mrna, translating it into glycoproteins for the viral envelope. The glycoproteins are transported from the rough ER to the Golgi apparatus for processing, and then they are sent to the host cell s plasma membrane. The viral capsids leave the host cells by exocytosis, gaining new viral envelopes studded with viral glycoproteins derived from the host cell s plasma membrane. Viral RNA and viral enzymes are packaged in a capsid and sent to the host cell s plasma membrane. 20. What is a retrovirus? How do they get their name? 21. What is the function of reverse transcriptase? 22. It is very important that you know the HIV reproductive cycle as it may appear on the AP Exam. Therefore, put the following events in order for the HIV virus. (Note there are two events below that will actually be occurring simultaneously within the host cell and the order of them won t matter) Reverse transcriptase catalyzes the synthesis of a second DNA strand complimentary to the first. The host cell s RNA polymerase transcribes the proviral DNA into RNA molecules. The double-stranded DNA enters the cell s nucleus and integrates into the DNA of a chromosome, becoming a provirus. The host cell membrane and the viral envelope merge allowing the capsid to enter the cell. The capsid is then digested releasing the viral reverse transcriptase and two separate single-stranded RNA molecules. The newly transcribed RNA molecules can be used as mrna to synthesize caspeosomes for caspids and reverse transcriptase molecules; or used as mrna to synthesize glycoproteins that are transported to the host cell s plama membrane. The HIV virus glycoproteins located on the viral envelope are recognized by 2 different receptor proteins on a white blood cell (specifically a T cell). The newly made viruses are released from the host cell by exocytosis and are surrounded by viral envelope studded with viral glycoproteins derived from the host cell s plasma membrane. A DNA strand complimentary to the viral RNA is catalyzed by the viral reverse transcriptase enzymes. Two newly transcribed RNA molecules can be used as the viral genome and packaged into a capsid along with two reverse transcriptase enzymes. 4
23. Compare and contrast a prophage and a provirus. Which one are you likely to carry? 24. The final section in this concept is titled Evolution of Viruses. From this part, describe the two possible sources of viral genomes. You will see each of these important mobile genetic elements again. Plamids Description of the Mobile Genetic Element Transposons Concept 19.3 Viruses, viroids, and prions are formidable pathogens in animals and plants 25. What are three ways that viruses make us ill? 26. Why do we recover completely from a cold but not from polio? 27. What tools are in the medical arsenal against human viral diseases? 28. Emerging viruses such as HIV, Ebola, and SARS seem to burst upon the human scene. What are three processes that contribute to this sudden emergence? 5
29. Distinguish between horizontal transmission and vertical transmission in plants. 30. How do viruses spread throughout plant bodies? 31. a. What is a viroid? b. What important lesson do they teach? c. Name one viroid disease. 32. a. Prions strike fear into carnivores everywhere. What are they? b. How are they transmitted? c. What do they do? 33. Name four diseases caused by prions. 34. What are two alarming characteristics of prions? 6