生物学英语教程10 18 19 20 23 24原文及翻译.docx

生物学英语教程10 18 19 20 23 24原文及翻译.docx

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生物学英语教程101819202324原文及翻译

10:

Thenatureofviruseshasbeenapparentonlywithinthelasthalfcentury,andthefirststeponthispathofdiscoverywastakenbytheRussianbotanistDmitriLvanovskyin1892whenhestudyingthetobaccomosaicdisease.

Virusesareverysmallentities,ranginginsizefrom0.02to0.3microns.Unliketheorganismsmakingupthefivetaxonomickingdomsofthelivingworld,virusesareacellular,theydon’tconsistofcellsandconductenergymetabolism-theydon’tproduceATPandincapableoffermentation,cellularrespirationorphotosynthesis.Asforthese,thequestionsofthevirusesoriginarise.Dotheyrepresentaprimitive‘nearlyliving’stageintheevolutionoflife?

Oraretheyorganismswhichhavelostallcellularcomponentsexceptthenucleus?

Couldvirusessimplybefragmentsofgeneticmaterialderivedfromcellularorganisms?

Noonereallyknowstheanswerstothesequestions,butwedoknowthatviruseshavebeenaroundforalongtime,andthatalmosteveryformoflifeissusceptibletoviralattack.

Thebasicunitsofavirusconsistofnucleicacidsurroundedbyacapsidorcoat,composedofoneoratmostafewkindsofproteins.Theseproteinsaresoassembledastogivethevirionacharacteristicshape.Astheybudthroughhostcellmembranes,manyanimalvirusesalsoacquireamembraneconsistingoflipidsandproteins,andmanybacterialviruseshavespecialized‘tails’madeofprotein.TheviralnucleicacidisusuallyasinglemoleculeandmaybecomposedofeitherDNAorRNA,butnotboth.DNAorRNAcanbedouble-strandedorsingle-stranded.

Virusesareobligateintracellularparasite,thatiswhytheymustdependuponspecifichostsfortheirreproductionanddevelopment.Thecellsofanimals,plantsandbacteriacanallserveashoststoviruses.Animalvirusesattachtospecialsitesontheplasmamembraneofthehostcellandarethentakenupbyendocytosis.Agivenviruscaninfectonlythosecellsthathaveareceptorsiteforthatvirus.Afterthemembranebreaksdown,theviralproteincapsidisbrokendownbycellularenzymesbeforetheviralnucleicacid,inaddition,theviralnucleicacidservestodirectthesynthesisofnewcapsidproteinbytheprotein-synthesizingsystemofthehost,andthecapsidcombinewithnewviralnucleicacidspontaneously;andinduecourse,thenewvirionsarereleasedbythehostcell.

Plantvirusesandbacteriophagesmustgetthroughacellwallaswellasthehostplasmamembrane.Infectionofaplantusuallyresultsfromattackbyavirion-ladeninsectvector.Theinsectusesitsproboscistopenetratethecellwall,andthevirionstheescapefromtheinsectintotheplant.Bacterialvirusesareoftenequippedwithtailassembliesthatinjectthenucleicacidintothehostbacteriumwhiletheproteincoatremainsoutside.Onceinsidethehostcell,thevirusgenestakesoverthemetabolicmachineryofthecellandgeneratetheirown.

Sometimes,viralDNAdoesnotimmediatelytakecontrolofthehostmetabolism,butinsertitselfintothehostchromosomeandpresentas‘silent’provirusuntilthehostcellisexposedtosomeenvironmentalinsult,suchasultravioletlightorradiation.

IftheviralnucleicacidisRNA,replicationneedsspecialenzymestomaketheprocessofRNA-to-RNAsynthesisoccurs.SomeRNAvirusescalledretrovirusesdonotcarryoutRNA-to-RNAtranscription.Instead,theirRNAistranscribedintoDNAisimmediately,thisreactioniscatalyzedbyreversetranscriptase,thennewlyformedDNAisinsertedintohostDNAandthentranscribedintoRNAandatlastnewvirusesareproduced.

Afterreplicationandcombination,mostvirusesarereleasedbylysisofthehostcell.Butinothercases,likethatoftheretroviruses,virusesarereleasedbyextrusion,aprocesssimilartobuddingtherebythevirusbecomesenvelopedinasmallpieceofcellmembraneasitmovesoutofthecell.Lysisresultinthedestructionofthecell,butextrusionallowsthecelltoremainaliveandcontinuetoproducenewvirusesforalongperiodoftime.

Acommonwaytoclassifyvirusesistoseparatesthemfirstonthenatureofthenucleicacidcomponent（DNAorRNA）andthenonwhetherthenucleicacidinthevirionissingle-ordouble-stranded.Furtherlevelsofclassificationdependonsuchfactorsastheoverallshapeofthevirusandthesymmetryofthecapsid.Mostcapsidmaybecategorizedashelical,icosahedralandsoon.Anotherlevelofcategorizationisbasedonthepresenceorabsenceofmembranousenvelopearoundthevirion;stillfurthersubdivisionreliesoncapsidsizeandothercriteria.

18:

Withinacell,energyisneededateverystagetodrivethereactionsthatkeeplifeinnormalstates.Ontheearth,almostalltheenergythatfuelslifetodaycomesfromthesunandiscapturedintheprocessofphotosynthesisbyplants.Mostnonphotosyntheticorganismsobtainenergybyingestingphotosyntheticorganismsorothersthathavethemselvesingestedphotosyntheticorganisms,andtheenergystoredbyphotosynthesisisusuallyreleasedthroughaprocessknownasrespiration.Inthischapter,weshalldiscussthesetwoprocesses.

Photosynthesisisalogicalstartingpointforourdiscussionofthebasicenergytransformationoflife.InsimpletermsphotosynthesisconsistsofthereductionofatmosphericCO2tocarbonhydratebyuseoflightenergy,withanassociatedreleaseofoxygenfromwater.Thisreactioncanbesummarizedbythefollowinggeneralizedequation.

Likemanyotherphysiologicalprocesses,photosynthesisconsistsofanumberofsequentialsteps:

①trappingoflightenergybychloroplasts;pigmentsotherthanchlorophyll（e.gcarotenoids）playanaccessoryroleinphotosynthesisbytransferringenergytochlorophylla.②splittingofwaterandreleaseofhigh-energyelectronsando2.③electrontransferleadingtogenerationofchemicalenergyintheformofATPandthereducingpowerasNADPH2.④terminalstepsinvolvingexpenditureofenergyofATPandthereducingpowerofNADPH2tofixCO2molecules,andfinallyconvertthiscompoundsintomorecomplexcarbohydrates,suchassucrose,starch,celluloseandsoon.

Carbondioxideisanexceedinglyenergy-poorcompound,whereascarbohydratesisenergy-rich.Photosynthesis,then,convertslightenergyintochemicalenergy.Inchemicalterms,theenergyissaidtobestoredbytheadditionofonemoreelectron-storesenergyinthesubstancebeingreduced.

Although,photosynthesiscanoccurinanychlorophyll-containingpartsoftheplant,leavesthatexposethegreatestareaofgreentissuetothelightaretheprincipalorgansofphotosynthesis.Throughamicroscopeitcanbeseenthattheoutersurfacesoftheleafhavealayerofepidermis,whichiscoveredbywaxylayerofcuticle.Theregionbetweentheupperandlowerepidermisconstitutesthemesophyllportionoftheleaf.Thecellsofmesophyllcontainmanychloroplasts,whichistheorganellesthatphotosynthesistakesplace.TheCO2requiredforphotosynthesiscanenterthroughsomeholescalledstomatabetweenthespacesofmesophyllcell.Chloroplastisboundedbytwoconcentricmembraneandathirdsetofinternalmembranesthatformaseriesofflattened,interconnectedsacsknownasthylakoids,wherechlorophyllmoleculesandmostoftheelectron-transport-chainmoleculesarelocated.Thelightreaction,inwhichlightenergyistrappedandconvertedintochemicalenergy,takesplaceonthethylakoidmembrane.Thedarkreaction,inwhichCO2isreducedtocarbohydrates,occursinthemorefluidstromathatsurroundsthethylakoidsacs.

19:

Itisevidentthatthephenomenonofinheritanceandvariationisofuniversalimportantinthelivingworld.Tounderstandit,wemustexplorehowhereditarymaterialexpressesitselfinnewcombinations,andwhetherprinciplescanbeformulatedaboutsocomplexevent.Thisexplorationisthestudyofthebranchofbiologyknownasgenetics.

SomeofthebasicconceptsofhereditygrewoutofexperimentsperformedbyGregorMendelinthemid-1800s.MendelspentmostofhislifetimeasamonkinanAustrianmonasteryandduringthistimehecultivatedgardenpeas,anddidaseriesofexperimentstostudyinheritanceinplants.Wenowknowthatowingtohiselaboratedesign,uniquetechnique,MendelformulatedtwoexcellentLawsofInheritancefromhisbreedingexperimentsonthegardenpeas:

oneistheLawofSegregation,theotheristheLawofIndependentAssortment.

TheLawofSegregationemphasizeonsingletraitsanditcanbestatedasfollows:

theinheritanceofeachindividualtraitisdeterminedbyhereditaryfactors（genes）,eachorganismpossessestwoinheritancefactorsforeachcharacter,whengametesareformed,thetwofactorsseparateintoseparatedgametes.Anoffspringformedbythefusionoftwogametesthereforereceivesonefactorforeachcharacterfromeachparent.TheLawofIndependentAssortmentinvolvingtwoormoretraitsandstatesthatduringgametesformation,whentwoormoregenesareinvolvedinacross,theallelesofonegeneareinheritedindependentlyoftheallelesofanothergene.Hisgreatachievementlaidasolidfoundationforthegenetics.Inordertofurtherunderstandingofthesetwolaws,somedetailscanbeknowneither:

1.Hereditarytraitsarecontrolledbydiscreteunitsthatpassunchangedfromgenerationtogeneration.Forexample,thetraitwhiteflowerseemstodisappearintheF1generation,butreappearsontheF2progeny,andthattherearenointermediatecolors,onlyredorwhite.

2.Eachtraitisproducedbytwohereditaryfactors.Thisisanecessaryassumptiontoaccountforthewayinwhichatraitsuchasflowercolorappearsinsuccessivegenerationsinapredictableratio.

3.Whentwocontrastinghereditaryfactorsarepresentin