微生物专业英语chater4.docx

微生物专业英语chater4.docx

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微生物专业英语chater4

Chapter4VIRUSES第四章病毒Avirusisasubmicroscopicinfectiousparticlecomposedofaproteincoatandanucleicacidcore.Viruses，likecells，carrygeneticinformationencodedintheirnucleicacid，andcanundergomutationsandreproduce；however，theycannotcarryoutmetabolism，andthusarenotconsideredalive.Virusesareclassifiedbythetypeofnucleicacidtheycontain，andtheshapeoftheirproteincapsule.

　　GeneralCharacteristicsofViruses

　　1.     Dependingonone'sviewpoint，virusesmayberegardedas  exceptionally  complex  aggregationsofnonlivingchemicalsorasexceptionallysimplelivingmicrobes.

　　2.     Virusescontainasingletypeofnucleicacid（DNAorRNA）andaproteincoat，sometimesenclosedbyanenvelopecomposedoflipids，proteins，andcarbohydrates.

　　3.     Virusesareobligatoryintracellularparasites.Theymultiplybyusingthehostcell'ssynthesizingmachinerytocausethesynthesisofspecializedelementsthatcantransfertheviralnucleicacidtoothercells.

　　4.     Avirionisacomplete，fullydevelopedviralparticlecomposedofnucleicacidsurroundedbyacoat.

　　HOSTRANGE

　　1.      Hostrangereferstothespectrumofhostcellsinwhichaviruscanmultiply.

　　2.      Dependingonitshostrange，avirusisgenerallyclassifiedasananimalvirus，bacterialvirus（bacteriophage），orplantvirus.Aviruscaninfectonlycertainspecieswithineachclass.

　　3.      Hostrangeisdeterminedbythespecificattachmentsiteonthehostcell'ssurfaceandtheavailabilityofhostcellularfactors.

　　SIZE

　　1.      Viralsizeisdeterminedbyfiltrationthroughmembranefilters，ultracentrifugation，andelectronmicroscopy.

　　2.      Virusesrangefrom20to300nmindiameter.

　　ViralStructure

　　NUCLEICACID

　　1.     Theproportionofnucleicacidinrelationtoproteininvirusesrangesfromabout1%toabout50%.

　　2.     VirusescontaineitherDNAorRNA，neverboth，andthenucleicacidmaybesingle-ordouble-stranded，linearorcircular，ordividedintoseveralseparatemolecules.

　　CAPSIDANDENVELOPE

　　1.     Theproteincoatsurroundingthenucleicacidofavirusiscalledthecapsid（衣壳）。

　　2.     Thecapsidiscomposedofsubunits，thecapsomeres（衣壳粒），whichcanbeasingletypeofproteinorseveraltypes.

　　3.     Thecapsidofsomevirusesisenclosedbyanenvelopeconsistingoflipids，proteins，andcarbohydrates.

　　4.     Someenvelopesarecoveredwithcarbohydrate-proteincomplexescalledspikes（刺突）。

　　5.     Viruseswithoutenvelopesarecallednakedviruses.

　　GENERALMORPHOLOGY

　　1. Helical（螺旋对称）viruses（forexample，tobaccomosaicvirus）resemblelongrods，andtheircapsidsarehollowcylinderssurroundingthenucleicacid.

　　2.  Polyhedral（多面体）viruses（forexample，adenovirus，腺病毒）aremanysided.Usuallythecapsidisanicosahedron（二十面体）。

Envelopedvirusesarecoveredbyanenvelopeandareroughlysphericalbuthighlypleomorphic（多型性的）。

Therearealsoenvelopedhelicalviruses（forexample，influenzavirus）andenvelopedpolyhedralviruses（forexample，herpessimplexvirus）。

　　3. Complexviruseshavecomplexstructures.Forexample，manybacteriophageshaveapolyhedralcapsidwithahelicaltailattached.

　　TaxonomyofViruses

　　1.     Classificationofvirusesisbasedontypeofnucleicacid，morphologicalclass，sizeofcapsid，andnumberofcapsomeres.

　　2.     Otherclassificationschemestakeintoaccountthevirus'ssusceptibilitytomicrobialcontrolagents，immunologicalproperties，siteofmultiplication，andmethodoftransmission.

　　Isolation，Cultivation，andIdentificationofViruses

　　1.             Virusesmustbegrowninlivingcells.

　　2.              Theeasiestvirusestogrowarebacteriophages.

　　GROWTHOFBACTERIOPHAGESINTHELABORATORY

　　Theplaquemethod：

　　1.             Mixesbacteriophageswithhostbacteriaandnutrientagar.

　　2.             Afterseveralviralmultiplicationcycles，thebacteriaintheareasurroundingtheoriginalvirusaredestroyed；theareaoflysisiscalledaplaque.

　　3.             Eachplaquecanoriginatewithasingleviralparticleorwithmorethanone；theconcentrationofvirusesisgivenasplaque-formingunits.

　　GROWTHOFANIMALVIRUSESINTHELABORATORY

　　1.     Cultivationofsomeanimalvirusesrequireswholeanimals.

　　2.     Someanimalvirusescanbecultivatedinembryonatedeggs.

　　3.     Cellculturesarecellsgrowinginculturemediainthelaboratory.

　　4.     Viralgrowthcancausecytopathiceffectsinthecellculture.

　　VIRALIDENTIFICATION

　　1. Serologicaltestsareusedmostoftentoidentifyviruses.Virusesmaybeidentifiedbyrestrictionenzymefragmentsandnucleicacidbasesequencing.

　　2. ViralMultiplication

　　1.    Virusesdonotcontainenzymesforenergyproductionorproteinsynthesis.

　　2.    Foravirustomultiply，itmustinvadeahostcellanddirectthehost'smetabolicmachinerytoproduceviralenzymesandcomponents.

　　MULTIPLICATIONOFBACTERIOPHAGES

　　T-EvenBacteriophages

　　1.    TheT-evenbacteriophagesthatinfectEcolihavebeenstudiedextensively.

　　2.    Inattachment，sitesonthephage'stailfibersattachtocomplementaryreceptorsitesonthebacterialcell.

　　3.    Inpenetration，phagelysozymeopensaportionofthebacterialcellwall，thetailsheathcontractstoforcethetailcorethroughthecellwall，andphageDNAentersthebacterialcell.Thecapsidremainsoutside.

　　4.    Inbiosynthesis，transcriptionofphageDNAproducesmRNAcodingforproteinsnecessaryforphagemultiplication.PhageDNAisreplicated，andcapsidproteinsareproduced.Duringtheeclipseperiod，separatephageDNAandproteincanbefound.

　　5.    Duringmaturation，phageDNAandcapsidsareassembledintocompleteviruses.

　　6.    Duringrelease，phagelysozymebreaksdownthebacterialcellwall，andthemultipliedphagesarereleased.

　　7.    Thetimefromphageadsorptiontoreleaseiscalledbursttime（20to40minutes）。

Burstsize，thenumberofnewlysynthesizedphagesproducedfromasingleinfectedcell，rangesfrom50to200.

　　Lysogeny

　　1.   Duringalyticcycle，aphagecausesthelysisanddeathofahostcell.

　　2.   SomevirusescaneithercauselysisorhavetheirDNAincorporatedasaprophageintotheDNAofthehostcell.Thelattersituationiscalledlysogeny.

　　3.   Prophagegenesareregulatedbyarepressercodedforbytheprophage.Theprophageisreplicatedeachtimethecelldivides.

　　4.   Exposuretocertainmutagenscanleadtoexcisionoftheprophageandinitiationofthelyticcycle.

　　5.   Becauseoflysogeny，lysogeniccellsbecomeimmunetoreinfectionwiththesamephage，andthehostcellcanexhibitnewproperties.

　　6.   Alysogenicphagecantransferbacterialgenesfromonecelltoanotherthroughtransduction.Anygenescanbetransferredingeneralizedtransduction， andspecificgenescanbetransferredinspecializedtransduction.

　　MULTIPLICATIONOFANIMALVIRUSES

　　Animalvirusesattachtotheplasmamembraneofthehostcell.

　　1.   Penetrationofenvelopedvirusesoccursbyendocytosis.

　　2.   Animalvirusesareuncoatedbyviralorhostcellenzymes.TheDNAofmostDNAvirusesisreleasedintothenucleusofthehostcell.TranscriptionofviralDNAandtranslationproduceviralDNAand，later，capsidprotein.Capsidproteinissynthesizedinthecytoplasmofthehostcell.

　　3.   Retrovirusescarryreversetranscriptase（RNA-dependentDNApolymerase），whichtranscribesDNAfromRNA.

　　4.    Aftermaturation，virusesarereleased.Onemethodofrelease（andenvelopeformation）isbudding.Nakedvirusesarereleasedthroughrupturesinthehostcellmembrane.

　　EffectsofAnimalViralInfectiononHostCells

　　1.     Cytopathiceffects（CPE）areabnormalitiesthatleadtodamageordeathofahostcell.

　　2.     Cytopathiceffectsincludeinclusionbodies，polykaryocytes，andalteredfunction.

　　Interferon

　　Interferonisproducedbyvirus-infectedcellsandprotectsneighboringcellsfromviralinfection.

　　VirusesandDiseasesVirusescauseavarietyofdiseasesamongallgroupsoflivingorganisms.Viraldiseasesincludetheflu，commoncold，herpes，measles，chickenpox，smallpox，andencephalitis.Antibioticsarenoteffectiveagainstviruses.Vaccinationoffersprotectionforuninfectedindividuals.

　　EmergentViruses

　　Virusesareusuallyquitespecificastotheirhostsandeventothetypesofcellstheyinfectinamulticellularhost.Recentlysomevirusesappeartohaveshiftedtheirhost：

HIV，hantavirus，andebolaappeartobeeithervirusesshiftingtoanew（human）hostorelseviruseswhoseexistenceandeffectsarejustnowbeingrealizedbyscientistsandthegeneralpublic.

　　VirusesandCancer

　　1.     Anexcessoftissueduetounusuallyrapidcellmultiplicationiscalledatumor.Tumorsaremalignant（cancerous）orbenign（noncancerous）。

Metastasisreferstothespreadofcancertootherpartsofthebody.

　　2.     Tumorsareusuallynamedbyattachmentofthesuffixomatothenameofthetissuefromwhichthetumorarises.

　　3.     Theearliestrelationshipbetweencancerandviruseswasdemonstratedintheearly1900s，whenchickenleukemiaandchickensarcomaweretransferredtohealthyanimalsbycell-freefiltrates.

　　TRANSFORMATIONOFNORMALCELLSINTOTUMORCELLS

　　1.     Eucaryoticcellshaveproto-oncogenesthatcodeforproteinsnecessaryforthecells'normalgrowth.Whenactivatedtooncogenes，thesegenestransformnormalcellsintocancerouscells.

　　2.     Virusescapableofproducingtumorsarecalledoncogenicviruses.

　　3.     SeveralDNAvirusesandretrovirusesareoncogenic.

　　4. Thegeneticmaterialofoncogenicvirusesbecomesintegratedintothehostcell