Chapter 33

Summary		Questions		Media Resources
33.1 Viruses are strands of nucleic acid encased within a protein coat.
• Viruses are fragments of DNA or RNA surrounded by protein that are able to replicate within cells by using the genetic machinery of those cells. • The simplest viruses use the enzymes of the host cell for both protein synthesis and gene replication; the more complex ones are capable of synthesizing many structural proteins and enzymes. • Viruses are basically either helical or isometric. Most isometric viruses are icosahedral in shape.		1.  Why are viruses not considered to be living organisms? 2.  How did early scientists come to the conclusion that the infectious agents associated with hoof-and-mouth disease in cattle were not bacteria? 3.  What is the approximate size range of viruses and type of microscope is generally required to visualize viruses?		•Characteristics of Viruses
33.2 Bacterial viruses exhibit two sorts of reproductive cycles.
• Virulent bacteriophages infect bacterial cells by injecting their viral DNA or RNA into the cell, where it directs the production of new virus particles, ultimately lysing the cell. • Temperate bacteriophages, upon entering a bacterial cell, insert their DNA into the cell genome, where they may remain integrated into the bacterial genome as a prophage for many generations.		4.  What is a bacteriophage? How does a T4 phage infect a host cell?
33.3 HIV is a complex animal virus.
• AIDS, a viral infection that destroys the immune system, is caused by HIV (human immunodeficiency virus). After docking on a specific protein called CD4, HIV enters the cell and replicates, destroying the cell. • Considerable progress has been made in the treatment of AIDS, particularly with drugs such as protease inhibitors that block cleavage of HIV polyproteins into functional segments.		5.  What specific type of human cell does the AIDS virus infect? How does it recognize this specific kind of cell? 6.  How do many animal viruses penetrate the host cell? How does a plant virus infect its host? How does a bacterial virus infect its host?		•Life Cycles of Viruses • AIDS Vaccine •HIV's Waiting Game •Drug Therapy for Aids •Curing AIDS Just Got Harder • HIV Delivery Protein
33.4 Nonliving infectious agents are responsible for many human diseases.
• Viruses are responsible for many serious human diseases. Influenza, the most lethal virus in human history, evades the human immune system through a recombination of surface proteins. Some of the most serious viral diseases, like AIDS and Ebola, have only recently transferred to humans from some other animal host. • Proteins called prions may transmit serious brain diseases from one individual to another.		7.  Why is it so much more difficult to treat a viral infection than a bacterial one? Is this different from treating bacterial infections? 8.  What is a prion? How does it integrate into living systems?		• Prions •The Coming Plague by Garrett •Smallpox: Tomorrow's Nightmare? •Smallpox Questions •Hepatitis C •Mad Cows and Prions •Prions and Blood Supply •Increasing Mad Cow Disease •Mad Cow Epidemic in Europe • Maintaining Prions

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