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chap18studyguide - Chapter 18 The Genetics of Viruses and...

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Chapter 18 The Genetics of Viruses and Bacteria Study Guide Overview: Microbial Model Systems Viruses and bacteria are the simplest biological systems—microbial models in which scientists find life’s fundamental molecular mechanisms in their most basic, accessible forms. Molecular biology was born in the laboratories of microbiologists studying viruses and bacteria. ° Microbes such as E. coli and its viruses are called model systems because of their use in studies that reveal broad biological principles. ° Microbiologists provided most of the evidence that genes are made of DNA, and they worked out most of the major steps in DNA replication, transcription, and translation. ° Techniques enabling scientists to manipulate genes and transfer them from one organism to another were developed in microbes. In addition, viruses and bacteria have unique genetic features with implications for understanding the diseases that they cause. Bacteria are prokaryotic organisms, with cells that are much smaller and more simply organized than those of eukaryotes, such as plants and animals. Viruses are smaller and simpler still, lacking the structure and metabolic machinery of cells. ° Most viruses are little more than aggregates of nucleic acids and protein—genes in a protein coat. Concept 18.1 A virus has a genome but can reproduce only within a host cell A virus is a genome enclosed in a protective coat. Viruses are not cells. They are infectious particles consisting of nucleic acid encased in a protein coat and, in some cases, a membranous envelope. ° The tiniest viruses are only 20 nm in diameter—smaller than a ribosome. The genome of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus. ° A virus is called a DNA virus or an RNA virus, according to the kind of nucleic acid that makes up its genome. ° The viral genome is usually organized as a single linear or circular molecule of nucleic
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acid. ° The smallest viruses have only four genes, while the largest have several hundred. The 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. ° The capsid of the tobacco mosaic virus has more than 1,000 copies of the same protein. ° Adenoviruses have 252 identical proteins arranged into a polyhedral capsid—as an icosahedron. Some viruses have accessory structures to help them infect their hosts. A membranous envelope surrounds the capsids of flu viruses. ° These viral envelopes are derived from the membrane of the host cell. ° They also have some host cell viral proteins and glycoproteins, as well as molecules of
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chap18studyguide - Chapter 18 The Genetics of Viruses and...

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