Bài giảng Biology - Chapter 18: The Genetics of Viruses and Bacteria

Chapter 18The Genetics of Virusesand BacteriaOverview: Microbial Model SystemsViruses called bacteriophagesCan infect and set in motion a genetic takeover of bacteria, such as Escherichia coliFigure 18.10.5 mE. coli and its virusesAre called model systems because of their frequent use by researchers in studies that reveal broad biological principlesBeyond their value as model systemsViruses and bacteria have unique genetic mechanisms that are interesting in their own rightRecall that bacteria are prokaryotesWith cells much smaller and more simply organized than those of eukaryotesVirusesAre smaller and simpler stillFigure 18.20.25 m VirusAnimalcellBacteriumAnimal cell nucleusConcept 18.1: A virus has a genome but can reproduce only within a host cellScientists were able to detect viruses indirectlyLong before they were actually able to see themThe Discovery of Viruses: Scientific InquiryTobacco mosaic diseaseStunts the growth of tobacco plants and gives their leaves a mosaic colorationFigure 18.3In the late 1800sResearchers hypothesized that a particle smaller than bacteria caused tobacco mosaic diseaseIn 1935, Wendell StanleyConfirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)Structure of VirusesVirusesAre very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelopeViral GenomesViral genomes may consist ofDouble- or single-stranded DNADouble- or single-stranded RNAFigure 18.4a, b18  250 mm70–90 nm (diameter)20 nm50 nm(a) Tobacco mosaic virus(b) AdenovirusesRNADNACapsomereGlycoproteinCapsomereof capsidCapsids and EnvelopesA capsidIs the protein shell that encloses the viral genomeCan have various structuresSome viruses have envelopesWhich are membranous coverings derived from the membrane of the host cellFigure 18.4c80–200 nm (diameter)50 nm(c) Influenza virusesRNAGlycoproteinMembranousenvelopeCapsidBacteriophages, also called phagesHave the most complex capsids found among virusesFigure 18.4d80  225 nm50 nm(d) Bacteriophage T4DNAHeadTail fiberTail sheathGeneral Features of Viral Reproductive CyclesViruses are obligate intracellular parasitesThey can reproduce only within a host cellEach virus has a host rangeA limited number of host cells that it can infectViruses use enzymes, ribosomes, and small molecules of host cellsTo synthesize progeny virusesVIRUSCapsid proteinsmRNAViral DNAHOST CELLViral DNADNACapsidFigure 18.5Entry into cell anduncoating of DNAReplicationTranscriptionSelf-assembly of new virus particles and their exit from cellReproductive Cycles of PhagesPhagesAre the best understood of all virusesGo through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycleThe Lytic CycleThe lytic cycleIs a phage reproductive cycle that culminates in the death of the hostProduces new phages and digests the host’s cell wall, releasing the progeny virusesThe lytic cycle of phage T4, a virulent phagePhage assemblyHeadTailsTail fibersFigure 18.6Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.1Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.2Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.3Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.4Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.5The Lysogenic CycleThe lysogenic cycleReplicates the phage genome without destroying the hostTemperate phagesAre capable of using both the lytic and lysogenic cycles of reproductionThe lytic and lysogenic cycles of phage , a temperate phageMany cell divisions produce a large population of bacteria infected with the prophage.The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.Phage DNA integrates into the bacterial chromosome,becoming a prophage.New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.Certain factorsdetermine whetherThe phage attaches to ahost cell and injects its DNA.Phage DNAcircularizesThe cell lyses, releasing phages.Lytic cycleis inducedLysogenic cycleis enteredLysogenic cycleLytic cycleorProphageBacterialchromosomePhagePhageDNAFigure 18.7Reproductive Cycles of Animal VirusesThe nature of the genomeIs the basis for the common classification of animal virusesClasses of animal virusesTable 18.1Viral EnvelopesMany animal virusesHave a membranous envelopeViral glycoproteins on the envelopeBind to specific receptor molecules on the surface of a host cellRNACapsidEnvelope (withglycoproteins)HOST CELLViral genome (RNA)Template CapsidproteinsGlyco-proteinsmRNACopy ofgenome (RNA)ERFigure 18.8The reproductive cycle of an enveloped RNA virus Glycoproteins on the viral envelope bind to specific receptor molecules(not shown) on the host cell, promoting viral entry into the cell.1 Capsid and viral genomeenter cell2 The viral genome (red)functions as a template forsynthesis of complementary RNA strands (pink) by a viral enzyme.3 New copies of viralgenome RNA are madeusing complementary RNAstrands as templates.4Complementary RNAstrands also function as mRNA,which is translated into bothcapsid proteins (in the cytosol)and glycoproteins for the viralenvelope (in the ER).5Vesicles transportenvelope glycoproteins tothe plasma membrane.6 A capsid assemblesaround each viralgenome molecule.7New virus8RNA as Viral Genetic MaterialThe broadest variety of RNA genomesIs found among the viruses that infect animalsRetroviruses, such as HIV, use the enzyme reverse transcriptase To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirusFigure 18.9ReversetranscriptaseViral envelopeCapsidGlycoproteinRNA(two identicalstrands)The reproductive cycle of HIV, a retrovirusFigure 18.10mRNARNA genomefor the nextviral generationViral RNARNA-DNAhybridDNAChromosomalDNANUCLEUSProvirusHOST CELLReverse transcriptaseNew HIV leaving a cellHIV entering a cell0.25 µmHIVMembrane of white blood cell The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.1 Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.2 Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.3 The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.4 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.5 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).6 Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.7 Capsids areassembled aroundviral genomes and reverse transcriptase molecules.8 New viruses budoff from the host cell.9Evolution of VirusesViruses do not really fit our definition of living organismsSince viruses can reproduce only within cellsThey probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acidConcept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plantsDiseases caused by viral infectionsAffect humans, agricultural crops, and livestock worldwideViral Diseases in AnimalsViruses may damage or kill cellsBy causing the release of hydrolytic enzymes from lysosomesSome viruses cause infected cellsTo produce toxins that lead to disease symptomsVaccinesAre harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogenCan prevent certain viral illnessesEmerging VirusesEmerging virusesAre those that appear suddenly or suddenly come to the attention of medical scientistsSevere acute respiratory syndrome (SARS)Recently appeared in ChinaFigure 18.11 A, B(a) Young ballet students in Hong Kong  wear face masks to protect themselves  from the virus causing SARS.(b) The SARS-causing agent is a coronavirus  like this one (colorized TEM), so named for the  “corona” of glycoprotein spikes protruding from the envelope.Outbreaks of “new” viral diseases in humansAre usually caused by existing viruses that expand their host territoryViral Diseases in PlantsMore than 2,000 types of viral diseases of plants are knownCommon symptoms of viral infection includeSpots on leaves and fruits, stunted growth, and damaged flowers or rootsFigure 18.12Plant viruses spread disease in two major modesHorizontal transmission, entering through damaged cell walls Vertical transmission, inheriting the virus from a parentViroids and Prions: The Simplest Infectious AgentsViroidsAre circular RNA molecules that infect plants and disrupt their growthPrionsAre slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammalsPropagate by converting normal proteins into the prion versionFigure 18.13PrionNormalproteinOriginalprionNewprionMany prionsConcept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteriaBacteria allow researchersTo investigate molecular genetics in the simplest true organismsThe Bacterial Genome and Its ReplicationThe bacterial chromosomeIs usually a circular DNA molecule with few associated proteinsIn addition to the chromosomeMany bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosomeBacterial cells divide by binary fissionWhich is preceded by replication of the bacterial chromosomeReplicationforkOrigin of replicationTermination of replicationFigure 18.14Mutation and Genetic Recombination as Sources of Genetic VariationSince bacteria can reproduce rapidlyNew mutations can quickly increase a population’s genetic diversityFurther genetic diversityCan arise by recombination of the DNA from two different bacterial cellsMutantstrainarg trp+EXPERIMENTFigure 18.15 Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids.RESULTS Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar.Mutantstrainarg+ trp–MixtureColoniesgrewMutantstrainarg+ trp–Mutantstrainarg– trp+No colonies(control)No colonies(control)Mixture Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.CONCLUSIONMechanisms of Gene Transfer and Genetic Recombination in BacteriaThree processes bring bacterial DNA from different individuals togetherTransformationTransductionConjugationTransformationTransformationIs the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environmentTransductionIn the process known as transductionPhages carry bacterial genes from one host cell to another1Figure 18.16DonorcellRecipientcellA+B+A+A+B–A–B–A+Recombinant cellCrossingoverPhage infects bacterial cell that has alleles A+ and B+ Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell.A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid.Phage with the A+ allele from the donor cell infects a recipient A–B– cell, and crossing over (recombination)between donor DNA (brown) and recipient DNA(green) occurs at two places (dotted lines).The genotype of the resulting recombinant cell (A+B–) differs from the genotypes of both the donor (A+B+) and the recipient (A–B–).2345Phage DNAA+B+Conjugation and PlasmidsConjugationIs the direct transfer of genetic material between bacterial cells that are temporarily joinedFigure 18.17Sex pilus1 mThe F Plasmid and ConjugationCells containing the F plasmid, designated F+ cellsFunction as DNA donors during conjugationTransfer plasmid DNA to an F recipient cellConjugation and transfer of an F plasmid from an F+ donor to an F recipientFigure 18.18a A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid.A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow).2DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands.3The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+.4F PlasmidBacterial chromosomeBacterial chromosomeF+ cellF+ cellF+ cellMating bridge1Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient(a)F– cellChromosomal genes can be transferred during conjugationWhen the donor cell’s F factor is integrated into the chromosomeA cell with the F factor built into its chromosomeIs called an Hfr cellThe F factor of an Hfr cellBrings some chromosomal DNA along with it when it is transferred to an F– cellConjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombinationF+ cellHfr cellF factorThe circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line).1The resulting cell is called an Hfr cell (for High frequency of recombination). 2Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. 3A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA4The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D.5The mating bridgeusually breaks well before the entire chromosome andthe rest of the F factor are transferred.6Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green).7The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell.8TemporarypartialdiploidRecombinant F–bacteriumA+B+C+D+F– cellA–B–C–D–A–B–C–D–D–A–C–B–A+B+C+D+A+B+D+C+A+A+B+A–B–C–D–A–B+C–D–A+B+B–A+Hfr cellD–A–C–B–A+B+C+D+A+B+Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination(b)Figure 18.18bR plasmids and Antibiotic ResistanceR plasmidsConfer resistance to various antibioticsTransposition of Genetic ElementsTransposable elementsCan move around within a cell’s genomeAre often called “jumping genes”Contribute to genetic shuffling in bacteriaFigure 18.19a(a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that  encodes transposase, which catalyzes movement within the genome. The inverted repeats are  backward, upside-down versions of each other; only a portion is shown. The inverted repeat  sequence varies from one type of insertion sequence to another.Insertion sequenceTransposase geneInvertedrepeatInvertedrepeat3535A T C C G G TT A G G C C A A C C G G A TT G G C C T A Insertion SequencesAn insertion sequence contains a single gene for transposaseAn enzyme that catalyzes movement of the insertion sequence from one site to another within the genomeTransposonsBacterial transposonsAlso move about within the bacterial genomeHave additional genes, such as those for antibiotic resistanceFigure 18.19b(b) Transposons contain one or more genes in addition to the transposase gene. In the transposon  shown here, a gene for resistance to an antibiotic is located between twin insertion sequences.  The gene for antibiotic resistance is carried along as part of the transposon when the transposon  is inserted at a new site in the genome.Inverted repeatsTransposase geneInsertion sequenceInsertion sequenceAntibioticresistance geneTransposon5353Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expressionE. coli, a type of bacteria that lives in the human colonCan tune its metabolism to the changing environment and food sourcesThis metabolic control occurs on two levelsAdjusting the activity of metabolic enzymes already presentRegulating the genes encoding the metabolic enzymesFigure 18.20a, b(a) Regulation of enzyme  activityEnzyme 1Enzyme 2Enzyme 3Enzyme 4Enzyme 5Regulationof geneexpressionFeedbackinhibitionTryptophanPrecursor(b) Regulation of enzyme  productionGene 2Gene 1Gene 3Gene 4Gene 5––Operons: The Basic ConceptIn bacteria, genes are often clustered into operons, composed ofAn operator, an “on-off” switchA promoterGenes for metabolic enzymesAn operonIs usually turned “on”Can be switched off by a protein called a repressorThe trp operon: regulated synthesis of repressible enzymesFigure 18.21a(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the  promoter and transcribes the operon’s genes.Genes of operonInactiverepressorProteinOperatorPolypeptides that make upenzymes for tryptophan synthesisPromoterRegulatorygeneRNA polymeraseStart codon Stop codonPromotertrp operon53mRNA 5trpDtrpEtrpCtrpBtrpAtrpRDNAmRNAEDCBADNAmRNAProteinTryptophan(corepressor)Active repressorNo RNA madeTryptophan present, repressor active, operon off. As tryptophanaccumulates, it inhibits its own production by activating the repressor protein.(b)Figure 18.21bRepressible and Inducible Operons: Two Types of Negative Gene RegulationIn a repressible operonBinding of a specific repressor protein to the operator shuts off transcriptionIn an inducible operonBinding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcriptionThe lac operon: regulated synthesis of inducible enzymesFigure 18.22aDNAmRNAProteinActiverepressorRNApolymeraseNoRNAmadelacZlaclRegulatorygeneOperatorPromoterLactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.(a)53mRNA 5'DNAmRNAProteinAllolactose(inducer)InactiverepressorlacllaczlacYlacARNApolymerasePermeaseTransacetylase-Galactosidase53(b)Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.mRNA 5lac operonFigure 18.22bInducible enzymesUsually function in catabolic pathwaysRepressible enzymesUsually function in anabolic pathwaysRegulation of both the trp and lac operonsInvolves the negative control of genes, because the operons are switched off by the active form of the repressor proteinPositive Gene RegulationSome operons are also subject to positive controlVia a stimulatory activator protein, such as catabolite activator protein (CAP)PromoterLactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.(a)CAP-binding siteOperatorRNApolymerasecan bindand transcribeInactiveCAPActiveCAPcAMPDNAInactive lacrepressorlacllacZFigure 18.23aIn E. coli, when glucose, a preferred food source, is scarceThe lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP)When glucose levels in an E. coli cell increaseCAP detaches from the lac operon, turning it offFigure 18.23b(b)Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.Inactive lacrepressorInactiveCAPDNARNApolymerasecan’t bindOperatorlacllacZCAP-binding sitePromoter