After studying this chapter you will be able to understand: A virus has a genome but can reproduce only within a host cell; viruses, viroids, and prions are formidable pathogens in animals and plants; rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria; individual bacteria respond to environmental change by regulating their gene expression;. | Chapter 18 The Genetics of Viruses and Bacteria Overview: Microbial Model Systems Viruses called bacteriophages Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure m 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 Figure m Virus Animal cell Bacterium Animal cell nucleus Concept : 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 Figure 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 Figure , b 18 250 mm 70–90 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses RNA DNA Capsomere Glycoprotein Capsomere of capsid 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 Figure 80–200 nm (diameter) 50 nm (c) Influenza viruses RNA . | Chapter 18 The Genetics of Viruses and Bacteria Overview: Microbial Model Systems Viruses called bacteriophages Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure m 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 Figure m Virus Animal cell Bacterium Animal cell nucleus Concept : 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 .
