2018生物工程專業(yè)英語(yǔ)重點(diǎn)

第一章

Mostofthepropertiesweassociatewithlifearepropertiesofthecytoplasm.Muchofthemassofacell

consistsofthissemifluidsubstance,whichisboundedontheoutsidebytheplasmamembrane.

Organellesaresuspendedwithinit,supportedbythefilamentousnetworkofthecytoskeleton.

Dissolvedinthecytoplasmicfluidarenutrients,ions,solubleproteins,andothermaterialsneededforcell

functioning.

生命的大部分特征表現(xiàn)在細(xì)胞質(zhì)的特征上。細(xì)胞質(zhì)大部分由半流體物質(zhì)組成，并由細(xì)胞膜(原生質(zhì)膜)包

被。細(xì)胞器懸浮在其中，并由絲狀的細(xì)胞骨架支撐。細(xì)胞質(zhì)中溶解了大■的營(yíng)養(yǎng)物質(zhì)，離子，可溶蛋白以

及維持細(xì)胞生理需求的其它物質(zhì)。

Theeukaryoticcellnucleusisthelargestorganelleandhousesthegeneticmaterial(DNA)on

chromosomes.(Inprokaryotesthehereditarymaterialisfoundinthenucleoid.)Thenucleusalso

containsoneortwoorganelles-thenucleoli-thatplayaroleincelldivision.Apore-perforatedsaccalled

thenuclearenvelopeseparatesthenucleusanditscontentsfromthecytoplasm.Smallmoleculescan

passthroughthenuclearenvelope,butlargermoleculessuchasmRNAandribosomesmustenterand

exitviathepores.

真核細(xì)胞的細(xì)胞核是最大的細(xì)胞器，細(xì)胞核對(duì)染色體組有保護(hù)作用(原核細(xì)胞的遺傳物質(zhì)存在于擬核中卜

細(xì)胞核含有一或二個(gè)核仁，核仁促進(jìn)細(xì)胞分裂。核膜貫穿許多小孔，小分子可以自由通過(guò)核膜，而象mRNA

和核糖體等大分子必須通過(guò)核孔運(yùn)輸。

Alleukaryoticcellscontainmostofthevariouskindsoforganelles,andeachorganelleperformsa

specializedfunctioninthecell.Organellesdescribedinthissectionincluderibosomes,theendoplasmic

reticulum,theGolgicomplex,vacuoles,lysosomes,mitochondria,andtheplastidsofplantcells.

所有的真核細(xì)胞都含有多種細(xì)胞器，每個(gè)細(xì)胞器都有其特定功能。本節(jié)主要介紹核糖體，內(nèi)質(zhì)網(wǎng)，高爾基

體系，液泡，溶離體，線粒體和植物細(xì)胞中的質(zhì)體。

Thenumberofribosomeswithinacellmayrangefromafewhundredtomanythousands.Thisquantity

reflectsthefactthat,ribosomesarethesitesatwhichaminoacidsareassembledintoproteinsforexport

orforuseincellprocesses.Acompleteribosomeiscomposedofonelargerandonesmallersubunit.

DuringproteinsynthesisthetwosubunitsmovealongastrandofmRNA,“reading“thegeneticsequence

codedinitandtranslatingthatsequenceintoprotein.Severalribosomesmaybecomeattachedtoa

singlemRNAstrand;suchacombinationiscalledapolysome.Mostcellularproteinsaremanufactured

onribosomesinthecytoplasm.Exportableproteinsandmembraneproteinsareusuallymadein

associationwiththeendoplasmicreticulum.

核糖體的數(shù)=變化從幾百到幾千，核糖體是氨基酸組裝成蛋白質(zhì)的■要場(chǎng)所。完整的核糖體由大亞基和小

亞基組成。核糖體沿著mRNA移動(dòng)并閱讀遺傳密碼,18譯成蛋白質(zhì)。一條mRNA上可能有多個(gè)核糖體，

稱多聚核糖體。大多數(shù)細(xì)胞蛋白是由細(xì)胞質(zhì)中核糖體生產(chǎn)。輸出蛋白和膜蛋白通常與內(nèi)質(zhì)網(wǎng)有關(guān)。

Theendoplasmicreticulum,alacyarrayofmembranoussacs,tubules,andvesicles,maybeeitherrough

(RER)orsmooth(SER).Bothtypesplayrolesinthesynthesisandtransportofproteins.TheRER,

whichisstuddedwithpolysomes,alsoseemstobethesourceofthenuclearenvelopeafteracell

divides.

內(nèi)質(zhì)網(wǎng)，帶有花邊的生物家,有管狀，泡狀之分，以及光滑和粗糙面區(qū)別。兩種都與蛋白質(zhì)的合成和運(yùn)輸

有關(guān)。粗糙內(nèi)質(zhì)網(wǎng)上分布許多核糖體，也可能提供細(xì)胞分裂后所需的細(xì)胞膜。

SERlackspolysomes;itisactiveinthesynthesisoffatsandsteroidsandintheoxidationoftoxic

substancesinthecell.Bothtypesofendoplasmicreticulumserveascompartmentswithinthecell

wherespecificproductscanbeisolatedandsubsequentlyshuntedtoparticularareasinoroutsidethe

cell.

光滑內(nèi)質(zhì)網(wǎng)上無(wú)核糖體，主要作用是脂肪和類固醇的合成以及細(xì)胞內(nèi)有毒物質(zhì)的氧化。兩種內(nèi)質(zhì)網(wǎng)合成的

產(chǎn)物在其中進(jìn)行分流或運(yùn)輸?shù)郊?xì)胞外。

Transportvesiclesmaycarryexportablemoleculesfromtheendoplasmicreticulumtoanother

membranousorganelle,theGolgicomplex.WithintheGolgicomplexmoleculesaremodifiedand

packagedforexportoutofthecellorfordeliveryelsewhereinthecytoplasm.

運(yùn)輸小泡能夠?qū)⒖蛇\(yùn)輸分子從內(nèi)質(zhì)網(wǎng)運(yùn)輸?shù)礁郀柣鶑?fù)合體上。在高爾基復(fù)合體中修飾，包裝后輸出細(xì)胞或

傳遞到細(xì)胞質(zhì)中的其他場(chǎng)所。

Vacuolesincellsappeartobehollowsacsbutareactuallyfilledwithfluidandsolublemolecules.The

mostprominentvacuolesappearinplantcellsandserveaswaterreservoirsandstoragesitesforsugars

andothermolecules.Vacuolesinanimalcellscarryoutphagocytosis(theintakeofparticulatematter)

andpinocytosis(vacuolardrinking).

細(xì)胞中的液泡好象是中空的，但實(shí)際上充滿了液體和可溶分子。最典型的液泡存在于植物細(xì)胞中，儲(chǔ)備水，

糖以及其它分子。動(dòng)物中的液泡起吞噬和胞飲作用。

Asubsetofvacuolesaretheorganellesknownaslysosomes,whichcontaindigestiveenzymes

(packagedinlysosomesintheGolgicomplex)thatcanbreakdownmostbiologicalmacromolecules.

Theyacttodigestfoodparticlesandtodegradedamagedcellparts.

溶酶體是液泡亞單位，含有消化夠，降解大部分生物大分子。消化食物微粒和降解損傷的細(xì)胞殘片。

Mitochondriaarethesitesofenergy-yieldingchemicalreactionsinallcells.Inaddition,plantcells

containplastidsthatutilizelightenergytomanufacturecarbohydratesintheprocessofphotosynthesis.

ItisonthelargesurfaceareaprovidedbytheinnercristaeofmitochondriathatATP-generatingenzymes

arelocated.Mitochondriaareself-replicating,andprobablytheyaretheevolutionarydescendantsof

whatwereoncefree-livingprokaryotes.

線粒體是細(xì)胞中化學(xué)產(chǎn)能的場(chǎng)所。另外，植物細(xì)胞中的質(zhì)體在光合作用中利用光能產(chǎn)生碳水化合物，線粒

體內(nèi)埼上提供了很大的表面積并分布著產(chǎn)ATPI5。線粒體自我復(fù)制，并且可能是自由生活的原核生物在進(jìn)

化中形成的后代。

Therearetwotypesofplastids:leucoplasts,whichlackpigmentsandserveasstoragesitesforstarch,

proteins,andoils;andchromoplasts,whichcontainpigments.Themostimportantchromoplastsare

chloroplasts-organellesthatcontainthechlorophyllusedinphotosynthesis.Theinternalstructureof

chloroplastsincludesstacksofmembranescalledgrana,whichareembeddedinamatrixcalledthe

stroma.

質(zhì)體有兩種類型：白色體，缺乏色素，是淀粉，蛋白質(zhì)和油的儲(chǔ)備場(chǎng)所；色質(zhì)體，含有色素。葉綠體是最

直要的色質(zhì)體，含有與光合作用有關(guān)的葉綠素。葉綠體的內(nèi)部結(jié)構(gòu)是由多層膜形成的葉綠體基粒，其中包

埋在基質(zhì)中的基粒稱子座。

Alleukaryoticcellshaveacytoskeleton,whichisaconvolutedlatticeworkoffilamentsandtubulesthat

appearstofillallavailablespaceinthecellandprovidessupportforvariousotherorganelles.Alarge

portionofthecytoskeletonconsistsofthreadlikemicrofilamentscomposedmainlyofthecontractile

proteinactin.Theyareinvolvedinmanytypesofintracellularmovementsinplantandanimalcells.A

secondprotein,myosin,isinvolveinthecontractionofmusclecells.Anothermainstructural

componentofthecytoskeletonconsistsofmicrotubules,whicharecomposedoftheglobularprotein

tubulinandtogetheractasscaffoldingthatprovidesastablecellshape.Cytoskeletalintermediate

filamentsappeartoimparttensilestrengthtothecellcytoplasm.Mechanoenzymessuchasmyosin,

dynein,andkinesininteractwiththecytoskeletalfilamentsandtubulestogenerateforcesthatcause

movements.

所有的細(xì)胞都有細(xì)胞骨架，網(wǎng)絡(luò)結(jié)構(gòu)的奸絲充滿了它所能觸及的全部空間并且對(duì)細(xì)胞器提供支持作用。細(xì)

胞骨架大部分由微絲組成，微絲主要由可收縮的肌動(dòng)蛋白組成。動(dòng)植物細(xì)胞的許多種類型細(xì)胞內(nèi)運(yùn)動(dòng)與肌

動(dòng)蛋白有關(guān)。第二類蛋白是肌球蛋白，它與肌肉細(xì)胞的收縮有關(guān)。細(xì)胞骨架的另一個(gè)主要結(jié)構(gòu)成分是微管，

由球狀的微管蛋白組成，象腳手架一般維持細(xì)胞的穩(wěn)定形態(tài)。細(xì)胞骨架的中間絲提供了細(xì)胞質(zhì)伸縮動(dòng)力。

機(jī)械■,例如，肌球蛋白，動(dòng)力蛋白，驅(qū)動(dòng)蛋白與微絲，微管相互作用產(chǎn)生動(dòng)力而引起細(xì)胞運(yùn)動(dòng)。

Althoughthecytoskeletonprovidessomestabilitytocells,itsmicrotubulesandfilamentsandtheir

associatedproteinsenablecellstomovebycreepingorglidingSuchmovementsrequireasolid

substratetowhichthecellcanadhereandcanbeguidedbythegeometryofthesurface.Somecells

alsoexhibitchemotaxis,theabilitytomovetowardorawayfromthesourceofadiffusingchemical.

盡管細(xì)胞骨架提供了細(xì)胞的某些穩(wěn)定性，微絲，微管及相關(guān)蛋白能使細(xì)胞爬行或滑動(dòng)。這種運(yùn)動(dòng)需要固體

基質(zhì)依托并通過(guò)表面幾何形狀的改變而運(yùn)動(dòng)。某些細(xì)胞具備趨藥性，即趨向或逃離擴(kuò)散開的化學(xué)源。

Certaineukaryoticcellscanswimfreelyinliquidenvironments,propelledbywhiplikeciliaorflagella.Both

ciliaandflagellahavethesameinternalstructure:ninedoublets(pairsofmicrotubules)arearrangedina

ringandextendthelengthoftheciliumorflagellum,andtwomoremicrotubulesrundownthecenterof

thering.Everyciliumorflagellumgrowsonlyfromthecellsurfacewhereabasalbodyislocated.

Movementisbasedontheactivitiesoftinydyneinsidearmsthatextendfromoneofthemicrotubulesof

eachdoublet.

某些真核細(xì)胞能在液體液體中自由運(yùn)動(dòng)，由纖毛或鞭毛推動(dòng)。纖毛和鞭毛具有同樣的內(nèi)部結(jié)構(gòu)：九個(gè)雙微

管環(huán)形排列，縱向延伸，環(huán)中心是兩個(gè)或以上微管組成。纖毛或鞭毛從細(xì)胞表面的基體出生長(zhǎng)，雙微管的

動(dòng)力蛋白臂從一側(cè)延伸到另一側(cè)而引起運(yùn)動(dòng)。

Nutrients,proteins,andothermaterialswithinmostplantcellsaremovedaboutviacytoplasmic

streaming.Theprocessoccursasmyosinproteinsattachedtoorganellespushagainstmicrofilaments

arrayedthroughoutthecell.Microfilamentsandmicrotubulesareresponsibleforalmostallmajor

cytoplasmicmovements.Duringcelldivision,microtubulesofthespindleassembledfromtubutin

subunitsnearorganellescalledcentriolesmovethechromosomes.

大部分植物細(xì)胞的營(yíng)養(yǎng)，蛋白質(zhì)和其它物質(zhì)由細(xì)胞質(zhì)流運(yùn)輸。這個(gè)過(guò)程是由于依附在細(xì)胞器上的肌球蛋白

反推排列在細(xì)胞周圍的微絲形成的。絕大部分細(xì)胞質(zhì)運(yùn)動(dòng)由微絲和微管完成。在細(xì)胞分裂期間，中心粒周

圍的由微管蛋白亞基裝配形成的紡錘體微管移向染色體。

第二章

Photosynthesisoccursonlyinthechlorophyllchlorophyll-containingcellsofgreenplants,algae,and

certainprotistsandbacteria.Overall,itisaprocessthatconvertslightenergyintochemicalenergythat

isstoredinthemolecularbonds.Fromthepointofviewofchemistryandenergetics,itistheoppositeof

cellularrespiration.Whereascellularrespirationishighlyexergonicandreleasesenergy,

photosynthesisrequiresenergyandishighlyendergonic.

光合作用只發(fā)生在含有葉綠素的綠色植物細(xì)胞，海藻，某些原生動(dòng)物和細(xì)菌之中?？傮w來(lái)說(shuō)，這是一個(gè)將

光能轉(zhuǎn)化成化學(xué)能，并將能量貯存在分子鍵中，從化學(xué)和動(dòng)能學(xué)角度來(lái)看,它是細(xì)胞呼吸作用的對(duì)立面。

細(xì)胞呼吸作用是高度放能的，光合作用是需要能■并高吸能的過(guò)程。

PhotosynthesisstartswithCO2andH20asrawmaterialsandproceedsthroughtwosetsofpartial

reactions.Inthefirstset,calledthelight-dependentreactions,watermoleculesaresplit(oxidized),02is

released,andATPandNADPHareformed.Thesereactionsmusttakeplaceinthepresenceoflight

energy.Inthesecondset,calledlight-independentreactions,CO2isreduced(viatheadditionofH

atoms)tocarbohydrate.ThesechemicaleventsrelyontheelectroncarrierNADPHandATPgenerated

bythefirstsetofreactions.

光合作用以二場(chǎng)化碳和水為原材料并經(jīng)歷兩步化學(xué)反應(yīng)。第一步，稱光反應(yīng)，水分子分解，氧分子釋放，

ATP和NADPH形成。此反應(yīng)需要光能的存在。第二步，稱暗反應(yīng)，二氧化碳被還原成碳水化合物，這步

反應(yīng)依掇電子載體NADPH以及第一步反應(yīng)產(chǎn)生的ATPo

Bothsetsofreactionstakeplaceinchloroplasts.Mostoftheenzymesandpigmentsforthe

lightdependentreactionsareembeddedinthethylakoidmembraneofchloroplasts.Thedarkreactions

takeplaceinthestroma.

兩步反應(yīng)都發(fā)生在葉綠體中。光反應(yīng)需要的大部分璃和色素包埋在葉綠體的類索體膜上。暗反應(yīng)發(fā)生在

基質(zhì)中。

Theenergyinlightphotonsinthevisiblepartofthespectrumcanbecapturedbybiologicalmoleculesto

doconstructivework.Thepigmentchlorophyllinplantcellsabsorbsphotonswithinaparticular

absorptionspectrumsstatementoftheamountoflightabsorbedbychlorophyllatdifferentwavelengths.

Whenlightisabsorbeditaltersthearrangementofelectronsintheabsorbingmolecule.Theadded

energyofthephotonbooststheenergyconditionofthemoleculefromastablestatetoaless-stable

excitedstate.Duringthelight-dependentreactionsofphotosynthesis,astheabsorbingmolecule

returnstothegroundstate,the"excess"excitationenergyistransmittedtoothermoleculesandstored

aschemicalenergy.

生物分子能捕獲可見(jiàn)光譜中的光能。植物細(xì)胞中葉綠素在不同光波下吸收部分吸收光滑。在吸收分子中，

光的作用使分子中的電子發(fā)生重排。光子的能■激活了分子的能量狀態(tài)，使其從穩(wěn)定態(tài)進(jìn)入不穩(wěn)定的激活

態(tài)。

Allphotosyntheticorganismscontainvariousclassesofchlorophyllsandoneormorecarotenoid

(accessory)pigmentsthatalsocontributetophotosynthesis.Groupsofpigmentmoleculescalled

antennacomplexesarepresentonthylakoids.Lightstrikinganyoneofthepigmentmoleculesis

funneledtoaspecialchlorophyllamolecule,termedareaction-centerchlorophyll,whichdirectly

participatesinphotosynthesis.Mostphotosyntheticorganismspossesstwotypesofreaction-center

chlorophylls,P680andP700,eachassociatedwithanelectronacceptormoleculeandanelectrondonor.

TheseaggregationsareknownrespectivelyasphotosystemI(P700)andphotosystemII(P680).

所有的光合作用生物含有不同等級(jí)的葉綠素和一個(gè)或多個(gè)類胡蘿卜素。稱作天線復(fù)合體的色素分子群存在

于類囊體中。激活色素分子的光能進(jìn)入葉綠素反應(yīng)中心，其直接參與光合作用。大部分光反應(yīng)細(xì)胞器擁有

兩套反應(yīng)中心，P680和P700,每個(gè)光系統(tǒng)都含有一個(gè)電子受體和電子供體。這些集合體就是大家熟識(shí)的

光合系統(tǒng)I和光合系統(tǒng)II。

Thephotosystemsofthelight-dependentreactionsareresponsibleforthepackagingoflightenergyin

thechemicalcompoundsATPandNADPH.Thispackagingtakesplacethroughaseriesofoxidation

reductionreactionssetinmotionwhenlightstrikestheP680reactioncenterinphotosystemII.Inthis

initialeventwatermoleculesarecleaved,oxygenisreleased,andelectronsaredonated.These

electronsareacceptedfirstbyplastoquinoneandthenbyaseriesofcarriersastheydescendan

electrontransportchain.Foreachfourelectronsthatpassdownthechain,twoATPsareformed.The

lastacceptorinthechainistheP700reactioncenterofphotosystemI.Atthispointincomingphotons

boosttheenergyoftheelectrons,andtheyareacceptedbyferredoxin.Ferredoxinisthenreoxidized,

andthecoenzymeNADP+isreducedtotheNADPH.TheATPgeneratedpreviouslyandtheNADPH

thentakepartinthelightindependentreactions.

光反應(yīng)的光系統(tǒng)將光能轉(zhuǎn)化成化學(xué)復(fù)合物ATP和NADPHo當(dāng)光激活光系統(tǒng)II的光反應(yīng)中心時(shí)，通過(guò)一系

列的新化還原反應(yīng)實(shí)現(xiàn)能量的傳遞。反應(yīng)開始時(shí)，水被分解，場(chǎng)被釋放并提供電子。電子首先傳遞給質(zhì)體

K,然后通過(guò)一系列載體形成的電子傳遞鏈。每傳遞4個(gè)電子，形成2個(gè)ATP。最后一個(gè)受體存在于光反

應(yīng)系統(tǒng)I的反應(yīng)中心里。此處光子激活電子，電子傳遞給鐵叔還蛋白。鐵制還蛋白再II化，并且輔酶NADP+

還原成NADPHo早期產(chǎn)生的ATP和NADPH進(jìn)入暗反應(yīng)。

TheproductionofATPfromthetransportofelectronsexcitedbylightenergydownanelectrontransport

chainistermedphotophosphorylation.Theone-wayflowofelectronsthroughphotosystemsIIandIis

callednoncyclicphotophosphorylation;plantsalsoderiveadditionalATPthroughcyclic

photophosphorylation,inwhichsomeelectronsareshuntedbackthroughtheelectrontransportchain

betweenphotosystemsIIandI.

由電子傳遞鏈偶連產(chǎn)生ATP的過(guò)程稱為光合磷酸化。通過(guò)光合系統(tǒng)II流經(jīng)光合系統(tǒng)I的電子路徑稱非循環(huán)式

光合磷酸化；植物通過(guò)循環(huán)式光合磷酸化獲得顫外的ATP,一些電子在光合系統(tǒng)I和II之間的電子傳遞鏈中

回流。

Inthelight-independentreactionsofphotosynthesis,whicharedrivenbyATPandNADPH,C02is

convertedtocarbohydrate.ThereactionsarealsoknownastheCalvin-Bensoncycle.Atmospheric

CO2,isfixedasitreactswithribulosebiphosphate(RuBP),areactionthatiscatalyzedbytheenzyme

ribulosebiphosphatecarboxylase.ThereductionOfC02tocarbohydrate(fructosediphosphate)is

completedviaseveralmorestepsofthecycle.Finally,RUBPisregeneratedsothatthecyclemay

continue.

由ATP和NADPH驅(qū)動(dòng)的暗反應(yīng)中，二簟化碳轉(zhuǎn)化成碳水化合物。即卡爾文循環(huán)。二磷酸核明糖固定二制

化碳,由二磷酸核津糖竣化離催化。

MostplantsareC3plants;theyexperiencedecreasedcarbohydrateproductionunderhot,dryconditions

asaresultoftheeffectsofphotorespiration.AmongC4plants,however,specialleafanatomyanda

uniquebiochemicalpathwayenabletheplanttothriveinandconditions.ThusC4plantslessen

photorespirationbycarryingoutphotosynthesisonlyincellsthatareinsulatedfromhighlevelsofCO2.

Theyalsopossessanovelmechanismforcarbonfixation.

大部分植物是碳3植物，在高溫干旱條件下，由于光呼吸作用而使碳水化合物的合成降低。而在大多數(shù)的

碳4植物中，由于葉脈的特殊構(gòu)造和獨(dú)特的化學(xué)路徑使植物依然很茂盛。這是碳固定的一個(gè)新機(jī)制。

Thecellnucleusisthemainrepositoryofgeneticinformation.Withinthenucleusarethechromosomes

tightlycoiledstrandsofDNAandclustersofassociatedproteins.LongstretchesofthecontinuousDNA

moleculewindaroundtheseclustersofproteins,orhistones,formingbeadlikecomplexesknownas

nucleosomes.Morecoilingandsupercoilingproducesadensechromosomestructure.Eachlongstrand

ofDNAcombineswithhistonesandnonhistoneproteinstomakeupthesubstancechromatin.

細(xì)胞核是亡藏遺傳信息的主要場(chǎng)所。DNA盤繞成螺旋線以及相關(guān)的成簇蛋白質(zhì)。DNA螺旋線纏繞成簇的

組蛋白形成珠健狀的核小體。這些螺旋和超螺旋形成致癌的染色體組結(jié)構(gòu)。每個(gè)長(zhǎng)鏈DNA與組蛋白利非組

蛋白一起構(gòu)成染色質(zhì)物質(zhì)。

Apictorialdisplayofanorganism'schromosomesinthecoiled,condensedstateisknownasakaryotype.

Karyotyperevealthatinmostcellsallbutsexchromosomesarepresentastwocopies,referredtoas

homologouspairs.Non-sexchromosomesarecalledautosomes.Organismswhosecellscontaintwo

setsofparentalchromosomesarecalleddiploid;thosewithcellscontainingasinglesetofparental

chromosomesarecalledhaploid.

染色體致密的超螺旋狀態(tài)我們稱染色體組。除了性染色體外，大多數(shù)細(xì)胞的染色體組成對(duì)出現(xiàn)，稱同源染

色體對(duì)。非性染色體稱常染色體。生物細(xì)胞含有兩套父母本染色體的稱二倍體.含有單套染色體的稱單倍體。

Thecellcycleisaregularsequenceinwhichthecellgrows,preparesfordivision,anddividestoformtwo

daughtercells,eachofwhichthenrepeatsthecycle.Suchcyclingineffectmakessingle-celled

organismsimmortal.Manycellsinmulticellularorganisms,includinganimalmuscleandnervecells,

eitherslowthecycleorbreakoutofitaltogether.

在細(xì)胞生長(zhǎng)過(guò)程中，細(xì)胞循環(huán)遵循特定程序，分裂準(zhǔn)備，分裂成2個(gè)子細(xì)胞，子細(xì)胞再循環(huán)。此循環(huán)使得

單細(xì)胞永生。多細(xì)胞生物中的許多細(xì)胞，包括動(dòng)物肌肉和神經(jīng)細(xì)胞，要么降低循環(huán)速度，要么同時(shí)分裂。

Thenormalcellcycleconsistsoffourphases.ThefirstthreeincludeG1,theperiodofnormal

metabolism;Sphase,duringwhichnormalsynthesisofbiologicalmoleculescontinues,DNAisreplicated,

andhistonesaresynthesized;andG2,abriefperiodofmetabolismandadditionalgrowth.Togetherthe

G1,S,andG2phasesarecalledinterphase.ThefourthphaseofthecellcycleisMphase,theperiodof

mitosis,duringwhichthereplicatedchromosomescondenseandmoveandthecelldivides.Itis

believedthatpropertiesofthecellcytoplasmcontrolthecellcycle,alongwithexternalstimulatorsand

inhibitorssuchaschalones.

正常細(xì)胞循環(huán)由4個(gè)時(shí)期組成。頭三期包括G1,正常新陳代謝；S期，正常新陳代謝同時(shí)，DNA復(fù)制，組蛋

白合成；G2期，短期的新陳代謝和少許生長(zhǎng)。G1,S,和G2稱分裂間期。最后是M期，有絲分裂期，

復(fù)制的染色體組濃縮，移動(dòng)并細(xì)胞分裂。據(jù)稱是染色質(zhì)控制了細(xì)胞循環(huán).伴隨外部激活因子和抑制因子如抑

素。

Biologistsdividethemitoticcycleintofourphases.Atthebeginningofprophasethechromosomes

eachconsistoftwohighlycondensedchromatidsattachedtoeachotheratacentromere.Asprophase

endsandmetaphasebegins,thecondensedchromosomesbecomeassociatedwiththespindle.

Eventuallythechromosomesbecomearrangedinaplane(calledthemetaphaseplate)atarightangleto

thespindlefibers.Next,duringanaphase,thetwosisterchromatidsofeachchromosomesplit,and

onefromeachpairisdrawntowardeachpoleofthecell.Duringtelophasenuclearenvelopesbeginto

formaroundeachsetofchromosomes,anddivisionofthecytoplasmtakesplace.

生物學(xué)家將有絲分裂劃分為4個(gè)階段。分裂前期，高度濃縮的兩個(gè)染色單體通過(guò)著絲粒連接在一起。在分

裂前期后期和分裂中期前期，濃縮的染色體與紡錘體相連，最后以正確的角度排列在赤道板上。在分裂后

期，兩個(gè)姊妹單體分離，分別拽向細(xì)胞兩極。在分裂末期，在每套染色體周圍形成核膜，細(xì)胞質(zhì)發(fā)生分裂。

Asmitosisproceeds,thespindlemicrotubulesplayacrucialroleinensuringthatbothpairedand

separatedchromatidsmoveintherightdirectionsatthepropertimes.Eachhalfofthespindleforms

asmicrotubulesextendfromeachpoleofadividingcelltotheregionofthemetaphaseplate.During

prophase,othermicrotubules,thecentromericfibers,extendoutwardfromthespindlepolestostructures

onthechromosomescalledkinetochores.Duringanaphasethefibersbegintoshorten,andthe

chromatidsbegintomoveapart.

在有絲分裂過(guò)程中,是紡錘體微管確保了染色單體在通當(dāng)時(shí)間以正確方向進(jìn)行分離。紡錘體微管由兩極向

赤道板延伸。在分裂前期，其它微管，著絲粒纖維延伸到染色體的動(dòng)粒。在分裂后期，纖維開始變短，染

色單體分離。

Thespindleformsdifferentlyinplantandanimalcells.Inanimalsitisassociatedwithcentriole,whilein

plantandfungalcellsspindleformationisassociatedwithreionscalledmicrotubuleorganizingcenters.

植物和動(dòng)物細(xì)胞形成的紡錘體不同。動(dòng)物細(xì)胞與中心粒相連，而在植物和真菌細(xì)胞中，紡錘體與微管組織

中心的離子相連。

Thedivisionofthecellcytoplasmattheendofmitosisiscalledcytokinesis.Inanimalcellsittakes

placeasaringofactinfilamentscontractsaroundthecellequator,pinchingthecellintwo.Inplantcells,

whichareboundedbyacellwall,cytokinesisinvolvesthebuildingofanewcellplateacrossthedividing

cellatitsequator.Cellwallmaterialisthendepositedintheregionofthecellplate.

在動(dòng)物細(xì)胞中，環(huán)形肌動(dòng)蛋白絲延赤道板收縮而使細(xì)胞一分為二。在植物細(xì)胞中，在赤道板形成新的細(xì)胞

板。

Meiosisisaspecialformofcelldivisionthattakesplaceinthereproductiveorgansthatproducesexcells.

Likemitosis,ittakesplaceafterDNAreplicationhasoccurredandinvolvestwosequentialnuclear

divisions(meiosisIandmeiosisII).Thesedivisionsresultinfourdaughtercells,eachwithhalfthe

numberofchromosomesoftheparentcell.Thephenomenonofcrossingoverduringmeiosisresultsin

exchangesofgeneticinformationbetweenchromosomes.Hence,thehomologouschromosomes

distributedtodifferentprogenycellsarenotidentical.

減數(shù)分裂是性細(xì)胞分裂的特殊形式。如有絲分裂，它也是發(fā)生在DNA復(fù)制后并有連續(xù)的兩個(gè)核分裂。產(chǎn)生

4個(gè)子細(xì)胞，分別含有親本一半的染色體數(shù)。

Asinmitosistwochromatidsexistforeachchromosomeatthebeginningofprophase1.Duringthis

phasethehomologouschromosomesundergosy-napsis,orpairing,whichisbroughtaboutbya

bridgingstructureofproteinsandRNAcalledthesynaptonemalcomplex.Thehomologouspairsstay

togetherwhentheyalignonthemetaphaseplate.Unliketheanaphaseofmitosis,however,during

anaphaseIthetwochromatidsofeachchromosomestayjoinedatthecentromereandmovetogetherto

oneofthetwopolesofthecell.Itisthiseventthatresultsinthehalvingofthechromosomenumberin

thefourdaughtercellsthatresultfrommeiosis.

正如在有絲分裂中一樣，兩個(gè)同源染色單體通過(guò)蛋白質(zhì)和RNA橋配對(duì)形成聯(lián)會(huì)復(fù)合體。與有絲分裂不同的

是，每組染色體的兩個(gè)染色單體連接在著絲點(diǎn)上并一起移向細(xì)胞兩極的一級(jí)。由此而導(dǎo)致4個(gè)子細(xì)胞染色

體數(shù)減半。

DuringtelophaseInuclearenvelopesenclosethechromosomesinnuclei,andinmostspecies

cytokinesis(thefirstnucleardivision)follows.ThesecondnucleardivisionbeginswithmetaphaseII,in

whichthechromosomesineachdaughtercellagainalignonametaphaseplate.Thecentromeres

finallydivide,andeachsisterchromatidmovestooneofthepolesofthespindle.Thenextphaseis

telophase,followedagainbycytokinesis.Theresultoftheentireprocessisfourhaploidcellsinwhich

parentalchromosomesarerandomlydistributed.第二次核分裂開始于分裂中期，子細(xì)胞中染色體重新排列

在赤道板上。著絲粒最終分離，每個(gè)姊妹染色單體分向兩極。接著胞質(zhì)分裂。產(chǎn)生4個(gè)單倍體，父母染色

體隨機(jī)分配。

Mitosisandmeiosis,respectively,makesimplecelldivisionandsexualreproductionpossible.Each

meansofpassingonhereditaryinformationhasadvantages.Inasexualreproductiontheparent

organismgivesrisetooffspringthataregeneticclonesoftheparent.Theadvantagesofthistypeof

reproductionarethatitpreservestheparent'ssuccessfulgeneticcomplement,requireslittleorno

specializationofreproductiveorgans,andismorerapidthansexualreproduction.Amajor

disadvantageoftheasexualmodeisthatasinglecatastrophiceventordiseasemaydestroyanentire

populationofgeneticallyidenticalorganisms.Aprimebenefitofsexualreproductionisthatitprovides

geneticvariabilityandareadymechanismfortheeliminationofdeleteriousmutations.Italsoallows

"new"geneformstoariseandspreadthroughpopulations.

有絲分裂和減數(shù)分裂在傳遞遺傳信息過(guò)程中各有優(yōu)勢(shì)。體細(xì)胞的繁殖就是父母本的克隆，其優(yōu)勢(shì)是保留了

父母本的成功遺傳信息，不需要特殊器官，比性復(fù)制快的多。但一個(gè)簡(jiǎn)單災(zāi)難性事件或疾病都可能摧毀一

個(gè)細(xì)胞群體。性復(fù)制的優(yōu)勢(shì)是它提供了遺傳可變性和現(xiàn)存排除有害突變的機(jī)制。也可以產(chǎn)生新的基因并在

種群中蔓延。

ThefirstscientisttoinvestigatethequestionofhowgenesaffectphenotypewasSirArchibaldGarrod,

whosestudiesofalkaptonuriaimpliedarelationshipbetweengenesandenzymes.Thirtyyearslater

BeadleandEphrussishowedarelationshipbetweenparticulargenesandbiosyntheticreactions

responsibleforeyecolorinfruitflies.Next,inaseriesofclassicexperimentsontheeffectsofmutations

inthebreadmoldNeurosporacrassa,BeadleandTatumexploredtheone-gene-one-enzyme

hypothesis-theideathateachgenecodesforaparticularenzyme.Theirworkpavedthewayforother

researcherstoelucidatetheprecisewaysinwhichenzymesaffectcomplexmetabolicpathways.In1949,

inresearchontheroleofhemoglobininsicklecellanemia,LinusPaulinghelpedrefinethe

one-gene-one-enzymehypothesisintotheone-gene-one-polypeptidehypothesis.

ArchibaldGarrod是第一個(gè)研究基因是如何影響表型的科學(xué)家，他對(duì)尿黑酸癥的研究揭示了基因與峰之間

的關(guān)系。Beadle和Ephrussi在三十年后對(duì)果蠅眼睛顏色的研究發(fā)現(xiàn)特殊基因與相關(guān)反應(yīng)的生物合成有關(guān)。

接著對(duì)面包發(fā)騫粗糙脈抱菌的突變?cè)囼?yàn)得出一個(gè)基因一個(gè)酶的假說(shuō)。他們的工作為其他工作者鋪平了道路，

即精確地闡明了酶影響了復(fù)雜的新陳代謝途徑。在1949年，對(duì)鑲刀狀細(xì)胞貧血癥的研究對(duì)一個(gè)基因一個(gè)

例的假說(shuō)進(jìn)一步上升為一個(gè)基因一個(gè)多肽。

Nucleiacid,originallyisolatedbyJohannMiescherin1871,wasidentifiedasaprimeconstituentof

chromosomesthroughtheuseofthered-stainingmethoddevelopedbyFeulgenintheearly1900s.

FrederickGriffith'sexperimentswiththeRandSstainsofpneumococcishowedthatanasyetunknown

materialfromonesetofbacterialcouldalterthephysicaltraitsofasecondset.Inthe1940stheteamof

Avery,MacLeod,andMcCartyshowedthatthisunknownmaterialwasDNA.Ataboutthesametime

P.A.LevenediscoveredthatDNAcontainedfournitrogenousbases,eachofwhichwasattachedtoa

sugarmoleculeandaphosphategroup-acombinationLevenetermedanucleotide.

在1871年，核酸最初是由JohannMiescher分離成功，并由Feulgen在1900年證實(shí)核酸是染色體組最

基本的組成。FrederickGriffith對(duì)粗糙和光滑的肺炎球菌實(shí)驗(yàn)表明，不確定的某種物質(zhì)可以從一組細(xì)菌轉(zhuǎn)移

到另一種細(xì)菌中。在1940年，確認(rèn)該物質(zhì)為DNAo四個(gè)堿基和磷酸分子分別連接在糖分子上，稱核甘酸。

DisagreementoverwhetherDNAcouldcarrycomplexgeneticinformationwasendedintheearly1950s

byMarthaChaseandAlfredHershey,whoseworkwithE.colishowedclearlythatDNA,andnotprotein,

isthebearerofgeneticinformation.

直到1950年，通過(guò)對(duì)大腸桿菌實(shí)驗(yàn)發(fā)現(xiàn)，遺傳物質(zhì)是DNA,而不是蛋白質(zhì)。

EachDNAnucleotidecontainsafive-carbonsugar,deoxyribose,attachedtooneoffourbases:adenine,

guanine,cytosine,orthymine.Adenineandguaninemoleculesaredouble-ringstructurescalled

purines,whilecytosineandthyminearesingle-ringstructurescalledpyrimidines.Themoleculemade

upofabaseplusasugaristermedanucleoside.IneachmoleculeofDNAaphosphategrouplinksthe

five-carbonsugarofonenucleosidetothefive-carbonsugarofthenextnucleosideinthechain.This

phosphatebondingcreatesasugar-phosphatebackbone.

每個(gè)核昔酸都含有一個(gè)五碳脫氨核糖，分別連接4個(gè)堿基，即：腺曝吟,鳥曝吟，胞噫喔,胸腺噂院。堿

基連接糖稱核昔。磷酸鍵形成磷酸骨架。

Chargaffsrulesdescribethefactthat(1)theamountofadenineisequaltotheamountofthyminein

DNA,withamountofcytosineequaltothatofguanine,and(2)theratiosofAtoTandofCtoGvarywith

differentspecies.

(1)腺曝吟與胸腺需癥，胞蛻癥與鳥曝吟相等；(2)腺曝吟與胸腺噂癥，胞嚅嚏與鳥曝吟的比例隨物種不

同而不同。

Inthelate1940sandearly1950s,researcherslookingforthestructureofDNAdrewuponChargaffs

insight,Levene'sideasonDNAcomponents,andtwootherlinesofevidence.Onewasthesuggestion

ofLinusPaulingthatDNAmighthaveahelicalstructureheldinplacebyhydrogenbonds,andtheother

wasX-raydiffractionphotosofDNA,showingahelicalstructurewithdistancebetweenthecoils,takenby

FranklinandWilkins.

直到40