19 - Viruses


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  • Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?
  • Figure 19.2 Inquiry: What causes tobacco mosaic disease?
  • Figure 19.3 Viral structure.
  • Figure 19.4 A simplified viral replicative cycle.
  • Figure 19.5 The lytic cycle of phage T4, a virulent phage.
  • Figure 19.6 The lytic and lysogenic cycles of phage  , a temperate phage.
  • Figure 19.7 The replicative cycle of an enveloped RNA virus.
  • Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
  • Figure 19.11 Model for how prions propagate.
  • 19 - Viruses

    1. 1. LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITIONJane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson© 2011 Pearson Education, Inc.Lectures byErin BarleyKathleen FitzpatrickVirusesChapter 19
    2. 2. Overview: A Borrowed Life• Viruses called bacteriophages can infect and setin motion a genetic takeover of bacteria, such asEscherichia coli• Viruses lead “a kind of borrowed life” between life-forms and chemicals• The origins of molecular biology lie in early studiesof viruses that infect bacteria© 2011 Pearson Education, Inc.
    3. 3. Figure 19.10.5 mm
    4. 4. The Discovery of Viruses: Scientific Inquiry• Tobacco mosaic disease stunts growth of tobaccoplants and gives their leaves a mosaic coloration• In the late 1800s, researchers hypothesized that aparticle smaller than bacteria caused the disease• In 1935, Wendell Stanley confirmed thishypothesis by crystallizing the infectious particle,now known as tobacco mosaic virus (TMV)© 2011 Pearson Education, Inc.
    5. 5. Figure 19.2Extracted sapfrom tobaccoplant withtobacco mosaicdiseaseRESULTSPassed sapthrough aporcelain filterknown to trapbacteriaHealthy plantsbecame infectedRubbed filteredsap on healthytobacco plants1 2 34
    6. 6. Structure of Viruses• Viruses are not cells• A virus is a very small infectious particleconsisting of nucleic acid enclosed in a proteincoat and, in some cases, a membranous envelope© 2011 Pearson Education, Inc.
    7. 7. Viral Genomes• Viral genomes may consist of either– Double- or single-stranded DNA, or– Double- or single-stranded RNA• Depending on its type of nucleic acid, a virus iscalled a DNA virus or an RNA virus© 2011 Pearson Education, Inc.
    8. 8. Capsids and Envelopes• A capsid is the protein shell that encloses the viralgenome• Capsids are built from protein subunits calledcapsomeres• A capsid can have various structures© 2011 Pearson Education, Inc.
    9. 9. Figure 19.3Capsomereof capsidRNA CapsomereDNAGlycoprotein GlycoproteinsMembranousenvelope RNACapsidHeadDNATailsheathTailfiber18 × 250 nm 80 × 225 nm70–90 nm (diameter) 80–200 nm (diameter)20 nm 50 nm 50 nm 50 nm(a)Tobaccomosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
    10. 10. • Some viruses have membranous envelopes thathelp them infect hosts• These viral envelopes surround the capsids ofinfluenza viruses and many other viruses found inanimals• Viral envelopes, which are derived from the hostcell’s membrane, contain a combination of viraland host cell molecules© 2011 Pearson Education, Inc.
    11. 11. • Bacteriophages, also called phages, are virusesthat infect bacteria• They have the most complex capsids foundamong viruses• Phages have an elongated capsid head thatencloses their DNA• A protein tail piece attaches the phage to the hostand injects the phage DNA inside© 2011 Pearson Education, Inc.
    12. 12. Viruses replicate only in host cells• Viruses are obligate intracellular parasites,which means they can replicate only within a hostcell• Each virus has a host range, a limited number ofhost cells that it can infect© 2011 Pearson Education, Inc.
    13. 13. General Features of Viral ReplicativeCycles• Once a viral genome has entered a cell, the cellbegins to manufacture viral proteins• The virus makes use of host enzymes, ribosomes,tRNAs, amino acids, ATP, and other molecules• Viral nucleic acid molecules and capsomeresspontaneously self-assemble into new viruses© 2011 Pearson Education, Inc.
    14. 14. VIRUS2134Entry anduncoatingReplicationTranscriptionand manufacture ofcapsid proteinsSelf-assembly ofnew virus particlesand their exit fromthe cellDNACapsidHOSTCELLViral DNAViralDNAmRNACapsidproteinsFigure 19.4
    15. 15. Replicative Cycles of Phages• Phages are the best understood of all viruses• Phages have two reproductive mechanisms: thelytic cycle and the lysogenic cycle© 2011 Pearson Education, Inc.
    16. 16. The Lytic Cycle• The lytic cycle is a phage replicative cycle thatculminates in the death of the host cell• The lytic cycle produces new phages and lyses(breaks open) the host’s cell wall, releasing theprogeny viruses• A phage that reproduces only by the lytic cycle iscalled a virulent phage• Bacteria have defenses against phages, includingrestriction enzymes that recognize and cut upcertain phage DNA© 2011 Pearson Education, Inc.
    17. 17. Figure 19.5-5Attachment21543Entry of phageDNA anddegradationof host DNAReleaseSynthesis ofviral genomesand proteinsAssemblyPhage assemblyHead Tail Tailfibers
    18. 18. The Lysogenic Cycle• The lysogenic cycle replicates the phagegenome without destroying the host• The viral DNA molecule is incorporated into thehost cell’s chromosome• This integrated viral DNA is known as a prophage• Every time the host divides, it copies the phageDNA and passes the copies to daughter cells© 2011 Pearson Education, Inc.
    19. 19. • An environmental signal can trigger the virusgenome to exit the bacterial chromosome andswitch to the lytic mode• Phages that use both the lytic and lysogeniccycles are called temperate phages© 2011 Pearson Education, Inc.
    20. 20. Figure 19.6New phage DNA and proteinsare synthesized and assembledinto phages.The cell lyses, releasing phages.PhagePhageDNAThe phageinjects its DNA.BacterialchromosomeLytic cyclelytic cycleis inducedorPhage DNAcircularizes.Certain factorsdetermine whetherlysogenic cycleis enteredLysogenic cycleProphageDaughter cellwith prophageOccasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.Cell divisionsproduce apopulation ofbacteria infectedwith the prophage.The bacterium reproduces,copying the prophage andtransmitting it to daughtercells.Phage DNA integrates intothe bacterial chromosome,becoming a prophage.
    21. 21. Viral Envelopes• Many viruses that infect animals have amembranous envelope• Viral glycoproteins on the envelope bind to specificreceptor molecules on the surface of a host cell• Some viral envelopes are formed from the hostcell’s plasma membrane as the viral capsids exit• Other viral membranes form from the host’snuclear envelope and are then replaced by anenvelope made from Golgi apparatus membrane© 2011 Pearson Education, Inc.
    22. 22. Figure 19.7CapsidRNAEnvelope (withglycoproteins)Capsid and viral genomeenter the cellHOST CELLViral genome(RNA)TemplatemRNAERCapsidproteinsCopy ofgenome(RNA)New virusGlyco-proteins
    23. 23. RNA as Viral Genetic Material• The broadest variety of RNA genomes is found inviruses that infect animals• Retroviruses use reverse transcriptase to copytheir RNA genome into DNA• HIV (human immunodeficiency virus) is theretrovirus that causes AIDS (acquiredimmunodeficiency syndrome)© 2011 Pearson Education, Inc.
    24. 24. GlycoproteinReversetranscriptase HIVViral envelopeCapsidRNA (twoidenticalstrands)HOSTCELLViral RNAReversetranscriptaseRNA-DNAhybridDNANUCLEUSProvirusChromosomalDNARNA genomefor thenext viralgenerationmRNANew virusHIVMembraneof whiteblood cell0.25 µmHIV entering a cellNew HIV leaving a cellFigure 19.8
    25. 25. • The viral DNA that is integrated into the hostgenome is called a provirus• Unlike a prophage, a provirus remains apermanent resident of the host cell• The host’s RNA polymerase transcribes theproviral DNA into RNA molecules• The RNA molecules function both as mRNA forsynthesis of viral proteins and as genomes for newvirus particles released from the cell© 2011 Pearson Education, Inc.
    26. 26. Evolution of Viruses• Viruses do not fit our definition of living organisms• Since viruses can replicate only within cells, theyprobably evolved as bits of cellular nucleic acid• Candidates for the source of viral genomes areplasmids, circular DNA in bacteria and yeasts, andtransposons, small mobile DNA segments• Plasmids, transposons, and viruses are all mobilegenetic elements© 2011 Pearson Education, Inc.
    27. 27. Viruses, viroids, and prions areformidable pathogens in animals andplants• Diseases caused by viral infections affect humans,agricultural crops, and livestock worldwide• Smaller, less complex entities called viroids andprions also cause disease in plants and animals,respectively© 2011 Pearson Education, Inc.
    28. 28. • Vaccines are harmless derivatives of pathogenicmicrobes that stimulate the immune system tomount defenses against the harmful pathogen• Vaccines can prevent certain viral illnesses• Viral infections cannot be treated by antibiotics• Antiviral drugs can help to treat, though not cure,viral infections© 2011 Pearson Education, Inc.
    29. 29. Viroids and Prions: The Simplest InfectiousAgents• Viroids are small circular RNA molecules thatinfect plants and disrupt their growth• Prions are slow-acting, virtually indestructibleinfectious proteins that cause brain diseases inmammals• Prions propagate by converting normal proteinsinto the prion version• Scrapie in sheep, mad cow disease, andCreutzfeldt-Jakob disease in humans are allcaused by prions© 2011 Pearson Education, Inc.
    30. 30. Figure 19.11PrionNormalproteinOriginalprionNewprionAggregatesof prions