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

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Chapter 18 - The Genetics of Viruses and Bacteria Chapter 18 The Genetics of Viruses and Bacteria Lecture Outline 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. o Microbes such as E. coli and its viruses are called model systems because of their use in studies that reveal broad biological principles. o 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. o 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. o 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 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. o This disease stunts tobacco plant growth and mottles plant leaves. o Mayer concluded that the disease was infectious when he found that he could transmit the disease by rubbing sap from diseased leaves onto healthy plants. o He concluded that the disease must be caused by an extremely small bacterium. o Ten years later, 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 by demonstrating that the infectious agent could reproduce.
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o The sap from one generation of infected plants could be used to infect a second generation of plants that could infect subsequent generations. o Beijerinck 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). 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.
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