chapt06_lecture - Microbiology A Systems Approach 2nd ed...

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Microbiology: A Systems Approach, 2 nd ed. Chapter 6: An Introduction to the Viruses
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6.1 The Search for the Elusive Virus Viruses were too small to be seen with the first microscopes The cause of viral infections was unknown for years Louis Pasteur first proposed the term virus 1890s Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus Loeffler and Frosch discovered an animal virus that causes foot –and-mouth disease in cattle Many years of experimentation showed what we know today and by the 1950s virology had grown
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6.2 The Position of Viruses in the Biological Spectrum Can infect every type of cell Cannot exist independently from the host cell, so aren’t considered living things However, since they can direct life processes they are often considered more than lifeless molecules- hence living!! Referred to as infectious particles, either active or inactive
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Obligate intracellular parasites Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses
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Checkpoint What organisms do viruses infect? Are viruses true organisms? Why? What is an obligate intracellular parasite?
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6.3 The General Structure of Viruses Figure 6.1
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Size Range Most are so small, they can only be seen with an electron microscope Animal viruses Paroviruses- around 20 nm in diameter Mimiviruses- up to 450 nm in length
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Viral Components: Capsids, Nucleic Acids, and Envelopes No resemblance to a cell Molecular structure- composed of regular, repeating subunits that give rise to their crystalline appearance Fig 6.3
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Nucleocapsid Contain only those parts needed to invade and control a host cell External coating Protein Capsid Envelope - modified host cell membrane If no envelope, called naked virus Core DNA RNA The capsid and the nucleic acid together are called the nucleocapsid Fully formed virus that is able to establish an infection in a host cell- virion
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Figure 6.4
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Checkpoint Which major components of virus? What is Virion?
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The Viral Capsid: The Protective Outer Shell Constructed from identical subunits called capsomers Made up of protein molecules They self assemble to form capsid
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Common shapes Depending on shape there are two different types Helical eg. TMV Rod-shaped capsomers Assemble in to helical nucleocapsid I cosahderal Three-dimensional, 20-sided figure with 12 evenly spaced corners One or more types of capsomers
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Figure 6.5
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Figure 6.6
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Figure 6.7
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Figure 6.9 Complex viruses : which have different types of proteins and asymmetrical shapes
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Figure 6.10
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