Ch18WordLectureOutline - CHAPTER 18 MICROBIAL MODELS THE...

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CHAPTER 18 MICROBIAL MODELS: THE GENETICS OF VIRUSES AND BACTERIA Introduction Viruses and bacteria are the simplest biological systems - microbial models where scientists find life’s fundamental molecular mechanisms in their most basic, accessible forms. 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. Viruses and bacteria also have interesting, unique genetic features with implications for understanding diseases that they cause. Bacteria are prokaryotic organisms. Their cells are much smaller and more simply organized that those of eukaryotes, such as plants and animals. Viruses are smaller and simpler still, lacking the structure and most metabolic machinery in cells. Most viruses are little more than aggregates of nucleic acids and protein - genes in a protein coat. A. The Genetics of Viruses 1. Researchers discovered viruses by studying a plant disease The story of how viruses were discovered begins in 1883 with research on the cause of tobacco mosaic disease by Adolf Mayer. This disease stunts the growth and mottles plant leaves.
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Mayer concluded that the disease was infectious when he found that he could transmit the disease by spraying sap from diseased leaves onto healthy plants. He concluded that the disease must be caused by an extremely small bacterium, but Dimitri Ivanovsky demonstrated that the sap was still infectious even after passing through a filter designed to remove bacteria. In 1897 Martinus Beijerinck ruled out the possibility that the disease was due to a filterable toxin produced by a bacterium and demonstrated that the infectious agent could reproduce. The sap from one generation of infected plants could be used to infect a second generation of plants that could infect subsequent generations. Bierjink also determined that the pathogen could reproduce only within the host, could not be cultivated on nutrient media, and was not inactivated by alcohol, generally lethal to bacteria. In 1935, Wendell Stanley crystallized the pathogen, the tobacco mosaic virus (TMV). 2. A virus is a genome enclosed in a protective coat Stanley’s discovery that some viruses could be crystallized was puzzling because not even the simplest cells can aggregate into regular crystals. However, viruses are not cells. They are infectious particles consisting of nucleic acid encased in a protein coat, and, in some cases, a membranous envelope. Viruses range in size from only 20nm in diameter to that barely resolvable with a light microscope. The genome of viruses includes other options than the double-stranded DNA that we have studied. Viral genomes may consist of double-stranded DNA, single-stranded
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Ch18WordLectureOutline - CHAPTER 18 MICROBIAL MODELS THE...

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