300-09%20Viruses1%20FINAL.ppt - BISC300L Mon Mar 23 2009...

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VIRUSES 1 3/23/09 1 1 BISC300L Mon., Mar. 23, 2009 Viruses 1 Chap. 14 FINAL VERSION. Print time 3/23/09 4:34 PM THIS SLIDE: Phage ! virions visualized by electron microscopy: from Ross Inman’s website. VIRUSES 1 3/23/09 2 2 General Introduction to Viruses VIRUSES 1 3/23/09 3 3 Bbbbbbbbbbbbbb Fig. 14.6 (left). Proliferation of a virus (in this case, a phage) Progeny phage can start another cycle by infecting another host cel . Parental phage A virus reproduces only inside a host cell. Though drawn here as a linear pathway, it is usually called a a cycle (since the progeny phage can start the process over again by infecting another cell). The virion is the virus particle, which is the extracellular state of a virus. The virion is dormant and metabolically inert.* A virion is not a living thing, but a set of instructions for taking over a cell and forcing it to produce more viruses of the same type. Viral proliferation occurs only in the intracellular state. * However, see the last slide. VIRUSES 1 3/23/09 4 4 Proliferation of a virus: The awesome power of an exponential function If we start with a single viral particle (virion) and an unlimited supply of host cells, we expect: Cycle Number of Virions at End of Cycle First 10 2 100 Second 10 4 10,000 Third 10 6 1,000,000 Fourth 10 8 100,000,000 Fifth 10 10 10,000,000,000 Sixth 10 12 1,000,000,000,000 This assumes that a single virion can start an infection (which is generally true for phages); that each infected cell produces 100 progeny virions; and that each progeny virus goes on to infect another cell. For some phages it is easy to obtain 10 12 (1 trillion) virions in a small liquid culture.
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VIRUSES 1 3/23/09 5 5 Classifcation oF viruses Viruses are classifed according to six characteristics oF the virion : 1. Viral genome: DNA vs. RNA. 2. Symmetry oF the capsid: Icosahedral (see ±ig. 14.2). Helical (see ±ig. 14.3). Complex (see ±ig. 14.5). 3. Naked vs. enveloped (see ±ig. 14.1). (to be continued) The virion is the virus particle, which is the extracellular state oF a virus. A virion consists oF a genome (DNA or RNA) encapsulated in a protein coat (capsid). The combination oF genome and capsid is called a nucleocapsid . Some viruses have an envelope surrounding the nucleocapsid. 1: Some virions are now known to contain both DNA and RNA. VIRUSES 1 3/23/09 6 6 ±ig. 14.2 Icosahedral symmetry An icosahedron is a regular polyhedron with 20 Faces, each oF which is an equilateral triangle. An icosahedron has ___ edges and ___ vertices. Part (B) illustrates a model For how to build an icosahedron by using 240 copies oF a single type oF capsid protein. The protein subunits ( monomers ; circles) are able to Form both pentamers (red) and hexamers (purple). The virion oF the Ross River virus appears to ft this model. VIRUSES 1
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300-09%20Viruses1%20FINAL.ppt - BISC300L Mon Mar 23 2009...

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