The Genetics of Viruses and Bacteria[病毒和细菌的遗传](-69).ppt

Chapter 18 The Genetics of Virusesand Bacteria Overview: Microbial Model Systems Viruses called bacteriophages Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli E. coli and its viruses Are called model systems because of their frequent use by researchers in studies that reveal broad biological principles Beyond their value as model systems Viruses and bacteria have unique genetic mechanisms that are interesting in their own right Recall that bacteria are prokaryotes With cells much smaller and more simply organized than those of eukaryotes Viruses Are smaller and simpler still Concept 18.1: A virus has a genome but can reproduce only within a host cell Scientists were able to detect viruses indirectly Long before they were actually able to see them The Discovery of Viruses: Scientific Inquiry Tobacco mosaic disease Stunts the growth of tobacco plants and gives their leaves a mosaic coloration In the late 1800s Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease In 1935, Wendell Stanley Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV) Structure of Viruses Viruses Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viral Genomes Viral genomes may consist of Double- or single-stranded DNA Double- or single-stranded RNA Capsids and Envelopes A capsid Is the protein shell that encloses the viral genome Can have various structures Some viruses have envelopes Which are membranous coverings derived from the membrane of the host cell Bacteriophages, also called phages Have the most complex capsids found among viruses General Features of Viral Reproductive Cycles Viruses are obligate intracellular parasites They can reproduce only within a host cell Each virus has a host range A limited number of host cells that it can infect Viruses use enzymes, ribos