BIOL125_Ch6 Text Notes

BIOL125_Ch6 Text Notes - BIOL 125(Microbiology Text Notes...

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BIOL 125 (Microbiology) Text Notes Chapter 6: Introduction to Viruses 6.1. THE SEARCH FOR THE ELUSIVE VIRUSES a. Ivanovski and Beijerinck showed that a tobacco disease was caused by a virus (TMV - Tobacco Mosaic Virus) in the 1890’s. b. Loeffler and Frosch discovered a virus that causes foot-and-mouth disease in cattle. 6.2. THE POSITION OF VIRUSES IN THE BIOLOGICAL SPECTRUM a. Viruses are considered the most abundant microbes on earth. b. Viruses can be described at infectious particles (rather than organisms). c. Viruses are a type of obligate intracellular parasite that cannot multiply unless it invades a specific host cell and instructs its genetic and metabolic machinery to make and release new viruses. 6.3. THE GENERAL STRUCTURE OF VIRUSES a. Size Range i. Viruses are ultramicroscopic , meaning they are so minute ( 0.2 μ m) that an electron microscope is necessary to detect them. ii. Negative staining uses very thin layers of an opaque salt to outline the shape of the virus against a dark background and to enhance textural features on the virus surface. iii. Positive staining reveals internal details, such as protein or nucleic acid. b. Viral Components: Capside, Nucleic Acids, and Envelopes i. Viral molecular structure is composed of regular, repeating subunits that give rise to their crystalline appearance. ii. Viruses contain only those parts needed to invade and control a host fell: an external coating and a core containing one or more nucleic acid strands of either DNA or RNA. 1) Capsid – a protein shell found in all viruses. 2) Nucleocapsid – term for capsid and nucleic acid together. 3) Naked (Nonenveloped) Viruses – those that contain only a nucleocapsid. 4) Enveloped Viruses – animal viruses which contain an additional covering external to the capsid, usually a modified piece of the host’s cell membrane. iii. The Viral Capsid: The Protective Outer Shell File: 51e5589e0f0dada2b8aa65c079e035aa4d02aaf4.doc Updated 8/14/09
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Microbiology Text Notes Chapter 6 Page 2 1) The capsid of any virus is constructed from a number of protein subunits called capsomers . a) Capsomers can spontaneously self-assemble into the finished capsid. There are two types of capsids: helical and icosahedral. i) Helical Capsids – have rod-shaped capsomers that bind together for form a series of hollow discs resembling a bracker. ii) Isocahedral Capsids – three-dimensional, 20-sided figures with 12 evenly spaced corners. 1 The nucleic acid is packed into the center, forming a nucleocapsid. iv. The Viral Envelope 1) When enveloped viruses (mostly animal) are released from the host cell, they take with them a bit of the membrane in the form of an envelope. 2)
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BIOL125_Ch6 Text Notes - BIOL 125(Microbiology Text Notes...

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