Chapter 18Overview: Microbial Model SystemsSlide 3Slide 4LE 18-2Concept 18.1: A virus has a genome but can reproduce only within a host cellThe Discovery of Viruses: Scientific InquirySlide 8Structure of VirusesViral GenomesCapsids and EnvelopesLE 18-4aLE 18-4bSlide 14LE 18-4cSlide 16LE 18-4dGeneral Features of Viral Reproductive CyclesLE 18-5Reproductive Cycles of PhagesThe Lytic CycleLE 18-6The Lysogenic CycleLE 18-7Reproductive Cycles of Animal VirusesSlide 26Slide 27Slide 28Slide 29Viral EnvelopesLE 18-8RNA as Viral Genetic MaterialLE 18-9Slide 34LE 18-10Evolution of VirusesConcept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plantsViral Diseases in AnimalsSlide 39Emerging VirusesLE 18-11Viral Diseases in PlantsSlide 43Slide 44Viroids and Prions: The Simplest Infectious AgentsLE 18-13Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteriaThe Bacterial Genome and Its ReplicationLE 18-14Mutation and Genetic Recombination as Sources of Genetic VariationLE 18-15Mechanisms of Gene Transfer and Genetic Recombination in BacteriaTransformationTransductionLE 18-16Conjugation and PlasmidsSlide 57LE 18-17The F Plasmid and ConjugationLE 18-18_1LE 18-18_2LE 18-18_3LE 18-18_4R plasmids and Antibiotic ResistanceTransposition of Genetic ElementsInsertion SequencesLE 18-19aTransposonsLE 18-19bConcept 18.4: Individual bacteria respond to environmental change by regulating their gene expressionLE 18-20Operons: The Basic ConceptLE 18-21aLE 18-21b_1LE 18-21b_2Repressible and Inducible Operons: Two Types of Negative Gene RegulationLE 18-22aLE 18-22bSlide 79Positive Gene RegulationLE 18-23aLE 18-23bCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsPowerPoint Lectures for Biology, Seventh EditionNeil Campbell and Jane ReeceLectures by Chris RomeroChapter 18Chapter 18The Genetics of Virusesand BacteriaCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsOverview: 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 rightCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings•Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes•Viruses are smaller and simpler than bacteriaLE 18-2LE 18-2VirusBacteriumAnimalcellAnimal cell nucleus0.25 µmCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 18.1: A virus has a genome but can reproduce only within a host cell•Scientists detected viruses indirectly long before they could see them•The story of how viruses were discovered begins in the late 1800sCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Discovery of Viruses: Scientific Inquiry•Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration•In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease•In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsStructure of Viruses•Viruses are not cells•Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelopeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsViral Genomes•Viral genomes may consist of–Double- or single-stranded DNA–Double- or single-stranded RNA•Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virusCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCapsids and Envelopes•A capsid is the protein shell that encloses the viral genome•A capsid can have various structuresLE 18-4aLE 18-4aCapsomereof capsidRNA18  250 mmTobacco mosaic virus20 nmLE 18-4bLE 18-4bCapsomereGlycoprotein70–90 nm (diameter)DNAAdenoviruses50 nmCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings•Some viruses have structures have membranous envelopes that help them infect hosts •These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals•Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell moleculesLE 18-4cLE 18-4cGlycoprotein80–200 nm (diameter)RNACapsidInfluenza viruses50 nmMembranousenvelopeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings•Bacteriophages, also called phages, are viruses that infect bacteria•They have the most complex capsids found among viruses•Phages have an elongated capsid head that encloses their DNA•A protein tailpiece attaches the phage to the host and injects the phage DNA insideLE 18-4dLE 18-4d80  225 nmDNAHeadTailsheathTailfiberBacteriophage T450 nmCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsGeneral Features of Viral Reproductive Cycles•Viruses are obligate intracellular parasites, which means 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, ribosomes, and small host molecules to synthesize progeny virusesLE 18-5LE 18-5DNAVIRUSCapsidHOST CELLViral DNAReplicationEntry into cell anduncoating of DNATranscriptionViral DNAmRNACapsidproteinsSelf-assembly ofnew virus particlesand their exit from cellCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsReproductive Cycles of Phages•Phages are the best understood of all viruses•Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycleCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Lytic Cycle•The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell•The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny