1935: Wendell M. Stanley, an American biologist, crystallized the infectious
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CHAPTER 18 MICROBIAL MODELS: THE GENETICS OF VIRUSESAND BACTERIA OUTLINE The Geneticsof Viruses A. Researchersdiscoveredvirusesby studying a plant disease:scienceas q process B. A virus is a genomeenclosedin a protectivecoat. C. Viruses can reproduceonly within a host cell'.an overview
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D. Phagesreproduceusing lytic or lysogeniccycles E,. Animal virusesare diversein their modesof infectionand of replication F. Plant viruses are seriousagriculturalpests G. Viroids and prions are infectious agentseven simpler than viruses IL
H. Viruses may have evolved from other mobile geneticelements The Geneticsof Bacteria A. The short generationspan of bacteriafacilitatestheir evolutionary adaptation to changing environments B. Geneticrecombinationproducesnew bacterialstrains C. The control of gene expression enables individual bacteria to adjust their metabolismto environmentalchange
OBJECTTVES After reading this chapter and attending lecture,the student should be able to: Recountthe history leadingup to the discoveryof virusesand discussthe contributions 1. of A. Mayer, D. Ivanowsky,Martinus Bei;erinck,and wendell Stanley. List and describestructuralcomponentsof viruses. 2. 3. 4. 5. 6. j. 8. g. 10. I 1. 12. 13.
Explain why virusesare obligateparasites. Describethree patternsof viral genomereplication' Explain the role of reversetranscriptasein retroviruses' Describehow virusesrecognizehost cells. Distinguish between lytic and lysogenicreproductivecycles using phage Ta and phage l" as examples. Outline the procedurefor measuringphageconcentrationin a liquid medium. Describeseveraldefensesbacteriahave againstphageinfection. Using viruses with envelopes and RNA viruses as examples, describevariations in replicationcyclesof animal viruses. Explain how virusesmay causediseasesymptoms, and describesome medical weapons used to fight viral infections. List some virusesthat have been implicated in human cancers,and explain how tumor virusestransformcells. Distinguishbetweenhorizontaland vertical routesof viral transmissionin plants.
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List some characteristicsthat viruses share with living organisms,and explain why viruses do not fit our usual definition of life. Provide evidencethat virusesprobablyevolvedfrom fragmentsof cellular nucleic acid. Describethe structureof a bacterialchromosome. Describethe processof binary fission in bacteria,and explain why replication of the bacterialchromosomeis consideredto be semiconservative. List and describethe three naturalprocessesof geneticrecombinationin bacteria. Distinguish between generaltransductionand specializedtransduction. Explain how the F plasmid controlsconjugationin bacteria. Explain how bacterial conjugation differs from sexual reproduction in eukaryotic organisms. For donor and recipient bacterial cells, predict the consequencesof conjugation between the following: 1) F- and F- cell, 2) Hfr and F cell. Define transposon, and describetwo essential types of nucleotide sequencesfound in transposonDNA. Distinguishbetweenan insertionsequenceand a complextransposon. and DNA polymerasein the processof transposition. Describethe role of transposase Explain how transposonscan generategeneticdiversity. Briefly describetwo main strategiescells use to control metabolism. Explain why grouping genesinto an operon can be advantageous. Using the trp operon as an example, explain the concept of an operon and the function of the operator, repressor,and corepressor. Distinguish between structuraland regulatorygenes. Describe how the lac operonfunctions and explain the role of the inducer allolactose. Explain how repressibleand inducible enzymes differ and how these differences reflect differences in the pathways they control. Distinguishbetweenpositiveand negativecontrol,and give examplesof each from the /ac operon. Explain how CAP is affectedby glucoseconcentration. Describe how E. coll uses the negative and positive controls of the lac operon to economizeon RNA and protein synthesis.
KEYTBRMS
R plasmid
operator operon repressor regulatory gene corepressor inducer cyclic amp (oAMP)
transposon insertionsequence
oAMP receptor protein (CRP)
provirus transformation capsid transduction retrovirus viral envelope bacteriophage(phage) reversetranscriptase conjugation F factor HIV host range episome AIDS lyic cycle F plasmid vaccine virulent virus lysogeniccycle temperatevirus prophage
virion prion nucleoid
LECTURENOTES Scientistsdiscoveredthe role of DNA in heredityby studyingthe simplestof biological systemsviruses and bacteria. Most of the molecular principles discoveredthrough microbe research appliesto higher organisms,but virusesand bacteriaalso have uniquegeneticfeatures.
ofVirusesandBaceria 255 Chapterl8 MicrobialModels:TheGenetics
Knowledge of these unique genetic features has helped scientists understand how viruses and bacteriacausedisease. Techniques for gene manipulation emerged from studying genetic peculiarities of microorganisms. The Genetics of Viruses A. Researchers discovered viruses by studying a plant disease: science as d process The discovery of viruses resultedfrom the searchfor the infectious agent causing tobacco mosiic disease.This diseasestuntsthe growth of tobacco plants and gives their leavesa mosaiccoloration(seeCampbell,Figure 18.8a)' 1883: A. Mayer, a German scientist demonstratedthat the diseasewas contagiousand proposedthai the infectious agent was an unusually small bacterium that could not be seenwith a microscoPe. . He successfullytransmitted the diseaseby spraying sap from infected plants onto the healthY ones. . Using a microscope,he examinedthe sap and was unable to identiff a microbe. 1890s:D. Ivanowsky, a Russianscientistproposedthat tobacco mosaic diseasewas causedby a bacterium that was either too small to be trapped by a filter or thal produced a filterable toxin. . To remove bacteria,he filtered sap from infected leaves' . Filtered sap still transmitted diseaseto healthy plants' 1897: Martinus Beijerinck, a Dutch microbiologist proposedthat the diseasewas caused by a reproducingpirticle much smaller and simpler than a bacterium. . He ruled out the theory that a filterable toxin caused the disease by demonstratingthat the infectious agent in filtered sap could reproduce. Plants were sprayedwith filtered sap from
Sprayedplants developedtobacco mosaic disease.
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Sap from newly infectedplants was used to infect others. for several generations. He concluded that the repeated This experiment was becauseits ability to infect was undiluted by reproducing pathogen must be to transfers from Plant Plant. He also noted that unlike bacteria,the pathogen:
Reproducedonly within the host it infected Could not be cultured on media . Could not be killed bY alcohol 1935: Wendell M. Stanley,an Americanbiologist,crystallizedthe infectiousparticle now known as tobacco mosaic virus(TMV). . .
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B. A virus is a genome enclosed in a protective coat In the 1950s,TMV and other viruses were finally observedwith electron mlcroscopes. Viral structureappearedto be unique from the simplest of cells. . The smallest viruses are only 20 nm in diameter. '
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The virus particle, consists of nucleic acid enclosed by a protein coat and sometimesa membranousenvelope. Viral genomes
Depending upon the virus, viral genomes: . May be double-strandedDNA, single-strandedDNA, double-strandedRNA, or single-strandedRNA ' Are organizedas single nucleic acid moleculesthat are linear or circular . May have as few as four genes or as many as several hundred 2. Capsids and envelopes Capsid: Protein coat that enclosesthe viral genome . Its structure may be rod-shaped,polyhedral, or complex . Composed of many capsomereg protein subunitsmade from only one or a few types of protein. Envelope: Membrane that cloaks some viral capsids . Helps viruses infect their host ' Derived from host cell membrane which is usually virus-modified and contains proteins and glycoproteins of viral origin The most complex capsidsare found among bacteriophagesor bacterial
