Bio171-F10-Lec 28 - Biology 171 Friday, November 19, 2010...

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Lecture 28: Viruses Announcements No discussion next week Text Reading Lecture 28 : 4 th : Chapter 35 (675-692) 3 rd : Chapter 35 (769-789) Lecture 29 : 4 th : Chapter 29 (519-537) 3 rd : Chapter 29 (593-623) Characteristics/Discovery of Viruses History of Smallpox Diversity of Viruses Lytic & Lysogenic Infections Influenza Retroviruses & HIV Genomic Impact of Retroviruses & Mobile Elements Biology 171 Friday, November 19, 2010 1
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Viruses Viruses are tiny, non-cellular parasites that infect virtually every type of cell known. They cannot perform metabolism on their own —meaning outside a parasitized cell— and are not considered to be alive. Different types of viruses are specialized for infecting particular species and types of cells. 2
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Viruses are very small relative to eukaryotic or even bacterial cells ( Figure 35.5 ). 3
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Viruses are Ubiquitous in Natural Environments, Greatly Exceeding Cellular Life Forms in Abundance 4
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In 1892 a Russian scientist named Dmitri Ivanovsky was studying tobacco mosaic disease, which destroys the leaves of tobacco plants. The disease was clearly infectious; plants that came into contact with the sap from diseased plants were damaged as well. This ability to reproduce itself eliminated the possibility that the damaging agent might be a simple toxin. Ivanovsky ran an extract of diseased leaves through a very fine filter, with pores small enough to trap any known type of bacteria. But he found that whatever caused the disease went right through his filter. By the 1930s filters could finally be manufactured with pores tiny enough to prove that viruses are particulate after all, rather than being fluid in nature. The earliest electron microscopes also appeared in the 1930s, and viruses could at last be seen. Virus = “poisonous fluid” Discovery of viruses 6
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Of all human infectious diseases, smallpox, caused by the Variola virus (below right), is believed to have resulted in more human deaths throughout history than from any other single pathogen. 7
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of smallpox. Case-fatalities ranged from 20% to 60% (adults) and up to 80-90% for infants. Unknown in the New World, smallpox devastated native American populations and was instrumental in the fall of the Aztec and Inca empires and in the European settlement of North America. During the French-Indian War (1754–1767), Sir Jeffrey Amherst, the commander of the British forces, suggested the deliberate use of smallpox to diminish the Native American population hostile to the British. 8
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Bio171-F10-Lec 28 - Biology 171 Friday, November 19, 2010...

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