Chapter 19 - Chapter 18 Viruses Updated Nov. 2008 Overview:...

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Unformatted text preview: Chapter 18 Viruses Updated Nov. 2008 Overview: A Borrowed Life Viruses are the simplest biological systems. Most viruses are little more than aggregates of nucleic acids and protein genes in a protein coat. Are viruses living or nonliving? Most virologists would probably agree that viruses are not alive but lead a kind of borrowed life. Molecular biology was born in the laboratories of microbiologists studying viruses & bacteria. They provided most of the evidence that genes are made of DNA They worked out most of the major steps in DNA replication, transcription, & translation They developed techniques to manipulate genes & transfer them from one organism to another The discover of viruses In 1883 Adolf Mayer guessed at the presence of viruses research tobacco mosaic disease This disease stunts tobacco plant growth & mottles plant leaves In 1935, Wendell Stanley crystallized the pathogen, the tobacco mosaic virus (TMV) Fig. 19-2 RESULTS 1 2 3 Extracted sap from tobacco plant with tobacco mosaic disease Passed sap through a porcelain filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected Bacteria & Viruses Bacteria are prokaryotic organisms with cells that are much smaller & more simply organized than those of eukaryotes, such as plants & animals Viruses are smaller & simpler still lacking the structure & metabolic machinery of cells Most viruses are little more than aggregates of nucleic acids & proteingenes in a protein coat Comparing the size of a virus, a bacterium, & an animal cell Structure of viruses A virus is a genome enclosed in a protective coat and, in some cases, a membranous envelope Viruses are not cells The tiniest viruses are only 20 nm in diameter smaller than a ribosome Virus genomes Depending on the kind of virus the genome may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA A virus is called a DNA virus or an RNA virus Usually organized as a single linear or circular molecule of nucleic acid Viruses range from four genes to several hundred Capsid is the protein shell enclosing the viral genome Capsids are built of a large number of protein subunits called capsomeres The number of different kinds of proteins making up the capsid is usually small Tobacco mosaic virus capsid has more than 1,000 copies of the same protein Adenoviruses have 252 identical proteins arranged into a polyhedral capsidas an icosahedron Viral structure Viral envelope Some viruses have accessory structures to help them infect their hosts A membranous envelope surrounds the capsids of flu viruses These envelopes are derived from the membrane of the host cell They also have some host cell viral proteins & glycoproteins as well as molecules of viral origin Some viruses carry a few viral enzyme molecules within their capsids Bacteriophages...
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Chapter 19 - Chapter 18 Viruses Updated Nov. 2008 Overview:...

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