1、 /27LessonOne（4學(xué)時(shí)）InsidetheLivingCell:StructureandFunctionofInternalCellParts1.Cytoplasm:TheDynamic,MobileFactory細(xì)胞質(zhì):動力工廠Mostofthepropertiesweassociatewithlifearepropertiesofthecytoplasm.Muchofthemassofacellconsistsofthissemifluidsubstance,whichisboundedontheoutsidebytheplasmamembrane.Organellesares

2、uspendedwithinit,supportedbythefilamentousnetworkofthecytoskeleton.Dissolvedinthecytoplasmicfluidarenutrients,ions,solubleproteins,andothermaterialsneededforcellfunctioning.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)中溶解了大量的營養(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)。2.TheNucleus:Informa

3、tionCentral（細(xì)胞核:信息中心）Theeukaryoticcellnucleusisthelargestorganelleandhousesthegeneticmaterial（DNA）onchromosomes.（Inprokaryotesthehereditarymaterialisfoundinthenucleoid.）Thenucleusalsocontainsoneortwoorganelles-thenucleoli-thatplayaroleincelldivision.Apore-perforatedsaccalledthenuclearenvelopeseparat

4、esthenucleusanditscontentsfromthecytoplasm.Smallmoleculescanpassthroughthenuclearenvelope，butlargermoleculessuchasmRNAandribosomesmustenterandexitviathepores.真核細(xì)胞的細(xì)胞核是最大的細(xì)胞器，細(xì)胞核對染色體組有保護(hù)作用（原核細(xì)胞的遺傳物質(zhì)存在于擬核中）。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核膜，而象mRNA和核糖體等大分子必須通過核孔運(yùn)輸。3.0rganelles:SpecializedWorkUn

5、its（細(xì)胞器:特殊的功能單位）Alleukaryoticcellscontainmostofthevariouskindsoforganelles，andeachorganelleperformsaspecializedfunctioninthecell.Organellesdescribedinthissectionincluderibosomes，theendoplasmicreticulum，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

6、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，溶酶體，線粒體和植物細(xì)胞中的質(zhì)體。Thenumberofribosomeswithinacellmayrangefromafewhundredtomanythousands.Thisquantityreflectsthefactthat，ribosomesarethesitesatwhichaminoacidsareassembledintoproteinsforexportorforuseincellprocesses.Acompleteribosomeiscomposedofonelargerandonesmallersubunit

7、.DuringproteinsynthesisthetwosubunitsmovealongastrandofmRNA，readingthegeneticsequencecodedinitandtranslatingthatsequenceintoprotein.SeveralribosomesmaybecomeattachedtoasinglemRNAstrand;suchacombinationiscalledapolysome.Mostcellularproteinsaremanufacturedonribosomesinthecytoplasm.Exportableproteinsan

8、dmembraneproteinsareusuallymadeinassociationwiththeendoplasmicreticulum.核糖體的數(shù)量變化從幾百到幾千,核糖體是氨基酸組裝成蛋白質(zhì)的重要場所。完整的核糖體由大亞基和小亞基組成。核糖體沿著mRNA移動并閱讀遺傳密碼，翻譯成蛋白質(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)

9、orsmooth(SER).Bothtypesplayrolesinthesynthesisandtransportofproteins.TheRER,whichisstuddedwithpolysomes,alsoseemstobethesourceofthenuclearenvelopeafteracelldivides.內(nèi)質(zhì)網(wǎng)，帶有花邊的生物囊，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;itisactiveinthesynthesisoffatsandstero

10、idsandintheoxidationoftoxicsubstancesinthecell.Bothtypesofendoplasmicreticulumserveascompartmentswithinthecellwherespecificproductscanbeisolatedandsubsequentlyshuntedtoparticularareasinoroutsidethecell.光滑內(nèi)質(zhì)網(wǎng)上無核糖體，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。Transportvesiclesmaycarryexportable

11、moleculesfromtheendoplasmicreticulumtoanothermembranousorganelle，theGolgicomplex.WithintheGolgicomplexmoleculesaremodifiedandpackagedforexportoutofthecellorfordeliveryelsewhereinthecytoplasm.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)中的其他場所。Vacuolesincellsappeartobehollowsacsbutareactuallyf

12、illedwithfluidandsolublemolecules.Themostprominentvacuolesappearinplantcellsandserveaswaterreservoirsandstoragesitesforsugarsandothermolecules.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，儲備水，糖以及其它分

13、子。動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，含有消化酶，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。Mitochondriaarethesit

14、esofenergy-yieldingchemicalreactionsinallcells.Inaddition，plantcellscontainplastidsthatutilizelightenergytomanufacturecarbohydratesintheprocessofphotosynthesis.ItisonthelargesurfaceareaprovidedbytheinnercristaeofmitochondriathatATP-generatingenzymesarelocated.Mitochondriaareself-replicating,andproba

15、blytheyaretheevolutionarydescendantsofwhatwereoncefree-livingprokaryotes.線粒體是細(xì)胞中化學(xué)產(chǎn)能的場所。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)嵴上提供了很大的表面積并分布著產(chǎn)ATP酶。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，whichc

16、ontainpigments.Themostimportantchromoplastsarechloroplasts-organellesthatcontainthechlorophyllusedinphotosynthesis.Theinternalstructureofchloroplastsincludesstacksofmembranescalledgrana,whichareembeddedinamatrixcalledthestroma.質(zhì)體有兩種類型:白色體，缺乏色素,是淀粉，蛋白質(zhì)和油的儲備場所;色質(zhì)體，含有色素。葉綠體是最重要的色質(zhì)體，含有與光合作用有關(guān)的葉綠素。葉綠體的內(nèi)部

17、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。TheCytoskeleton（細(xì)胞骨架）Alleukaryoticcellshaveacytoskeleton，whichisaconvolutedlatticeworkoffilamentsandtubulesthatappearstofillallavailablespaceinthecellandprovidessupportforvariousotherorganelles.Alargeportionofthecytoskeletonconsistsofthreadlikemicrofilamentscomposedmain

18、lyofthecontractileproteinactin.Theyareinvolvedinmanytypesofintracellularmovementsinplantandanimalcells.Asecondprotein，myosin,isinvolveinthecontractionofmusclecells.Anothermainstructuralcomponentofthecytoskeletonconsistsofmicrotubules,whicharecomposedoftheglobularproteintubulinandtogetheractasscaffol

19、dingthatprovidesastablecellshape.Cytoskeletalintermediatefilamentsappeartoimparttensilestrengthtothecellcytoplasm.Mechanoenzymessuchasmyosin，dynein,andkinesininteractwiththecytoskeletalfilamentsandtubulestogenerateforcesthatcausemovements.所有的細(xì)胞都有細(xì)胞骨架，網(wǎng)絡(luò)結(jié)構(gòu)的纖絲充滿了它所能觸及的全部空間并且對細(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

20、要由可收縮的肌動蛋白組成。動植物細(xì)胞的許多種類型細(xì)胞內(nèi)運(yùn)動與肌動蛋白有關(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)伸縮動力。機(jī)械酶，例如，肌球蛋白，動力蛋白，驅(qū)動蛋白與微絲，微管相互作用產(chǎn)生動力而引起細(xì)胞運(yùn)動。CellularMovements（細(xì)胞運(yùn)動Althoughthecytoskeletonprovidessomestability穩(wěn)固tocells，itsmicrotubules英imaikr9UltjU：bjU：landfilaments絲狀物andtheir

21、associatedproteinsenablecellstomovebycreeping爬行orgliding滑動.Suchmovementsrequireasolid固體的substratetowhichthecellcanadhere附著andcanbeguidedbythegeometrydibmitri幾何形狀ofthesurface.Somecellsalsoexhibitigibit展覽chemotaxis,kemJt比ksis趨藥性theabilitytomovetowardorawayfromthesourceofadiffusing擴(kuò)散chemical.盡管細(xì)胞骨架提供了細(xì)

22、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。這種運(yùn)動需要固體基質(zhì)依托并通過表面幾何形狀的改變而運(yùn)動。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,propelledbywhiplikeciliasilim纖毛orflagella英flmdselm鞭毛.Bothciliaandflagellahavethesameinternalstructure:ninedoublets(pairsofmicrotubules)arearrangedinaringandextendth

23、elengthoftheciliumorflagellum,andtwomoremicrotubulesrundownthecenterofthering.Everyciliumorflagellumgrowsonlyfromthecellsurfacewhereabasalbody基體islocated.Movementisbasedontheactivitiesoftinydynein動力蛋白sidearmsthatextendfromoneofthemicrotubulesofeachdouble成對的東西.某些真核細(xì)胞能在液體液體中自由運(yùn)動，由纖毛或鞭毛推動。纖毛和鞭毛具有同樣的內(nèi)部結(jié)

24、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，雙微管的動力蛋白臂從一側(cè)延伸到另一側(cè)而引起運(yùn)動。Nutrients,proteins,andothermaterialswithinmostplantcellsaremovedaboutviacytoplasmicstreaming.Theprocessoccursasmyosinmaiausin肌凝蛋白proteinsattachedtoorganelles纟田胞器,小器官pushagainstmicrofilamentsarrayedarei展示throughoutthecell.Microfi

25、lamentsandmicrotubulesareresponsibleforalmostallmajorcytoplasmicmovements.Duringcelldivision，microtubulesofthespindleassembled集合,收集fromtubutinsubunitsnearorganellescalledcentriolesmovethechromosomes.大部分植物細(xì)胞的營養(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

26、裝配形成的紡錘體微管移向染色體。課后作業(yè):第一第二篇閱讀材料答案:lB,2C,3B,4B,5D,6c;1A,2B,3D,4A（2學(xué)時(shí)）LessonTwoPhotosynthesisPhotosynthesisoccursonlyinthechlorophyllchlorophyll葉綠素-containingcellsofgreenplants,algae藻，andcertainprotists原生生物andbacteria.Overall,itisaprocessthatconvertslightenergyintochemicalenergythatisstoredinthemolecul

27、arbonds.Fromthepointofviewofchemistryandenergetics,itistheoppositeofcellularrespiration.Whereas然而cellular纟田胞的respiration呼吸ishighlyexergonic吸收能量的andreleasesenergy,photosynthesis光合作用requiresenergyandishighlyendergonic.光合作用只發(fā)生在含有葉綠素的綠色植物細(xì)胞，海藻，某些原生動物和細(xì)菌之中。總體來說,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)和動能學(xué)角度來看，它是細(xì)胞呼

28、吸作用的對立面。細(xì)胞呼吸作用是高度放能的，光合作用是需要能量并高吸能的過程。PhotosynthesisstartswithCO2andH2Oasrawmaterialsandproceedsthroughtwosetsofpartialreactions.Inthefirstset,calledthelight-dependentreactions,watermoleculesaresplit裂開(oxidized),02isreleased,andATPandNADPHareformed.Thesereactionsmusttakeplaceinthepresenceof在面前l(fā)ighte

29、nergy.Inthesecondset，calledlight-independentreactions,CO2isreduced(viatheadditionofHatoms)tocarbohydrate.ThesechemicaleventsrelyontheelectroncarrierNADPHandATPgeneratedbythefirstsetofreactions.光合作用以二氧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)依賴電子載體NADPH

30、以及第一步反應(yīng)產(chǎn)生的ATP。Bothsetsofreactionstakeplaceinchloroplasts.Mostoftheenzymesandpigments色素forthelightdependentreactionsareembedded深入的內(nèi)含的inthethylakoid類囊體membrane膜隔膜ofchloroplasts葉綠體.Thedarkreactionstakeplaceinthestroma.基質(zhì)兩步反應(yīng)都發(fā)生在葉綠體中。光反應(yīng)需要的大部分酶和色素包埋在葉綠體的類囊體膜也暗反應(yīng)發(fā)生在基質(zhì)中。HowLightEnergyReachesPhotosynthetic

31、Cells(光合細(xì)胞如何吸收光能的)Theenergyinlightphotonsinthevisiblepartofthespectrumcanbecapturedbybiologicalmoleculestodoconstructivework.Thepigmentchlorophyllinplantcellsabsorbsphotonswithinaparticularabsorptionspectrumsstatementoftheamountoflightabsorbedbychlorophyllatdifferentwavelengths.Whenlightisabsorbedit

32、altersthearrangementofelectronsintheabsorbingmolecule.Theaddedenergyofthephotonbooststheenergyconditionofthemoleculefromastablestatetoaless-stableexcitedstate.Duringthelight-dependentreactionsofphotosynthesis，astheabsorbingmoleculereturnstothegroundstate，theexcessexcitationenergyistransmittedtoother

33、moleculesandstoredaschemicalenergy.生物分子能捕獲可見光譜中的光能。植物細(xì)胞中葉綠素在不同光波下吸收部分吸收光譜。在吸收分子中，光的作用使分子中的電子發(fā)生重排。光子的能量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.Groupsofpigmentmoleculescalledante

34、nnacomplexesarepresentonthylakoids.Lightstrikinganyoneofthepigmentmoleculesisfunneledtoaspecialchlorophyllamolecule,termedareaction-centerchlorophyll,whichdirectlyparticipatesinphotosynthesis.Mostphotosyntheticorganismspossesstwotypesofreaction-centerchlorophylls,P680andP700,eachassociatedwithanelec

35、tronacceptormoleculeandanelectrondonor.TheseaggregationsareknownrespectivelyasphotosystemI(P700)andphotosystemII(P680)所有的光合作用生物含有不同等級的葉綠素和一個(gè)或多個(gè)類胡蘿卜素(光合作用的輔助色素)。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的光合系統(tǒng)I和光合系統(tǒng)II。TheLight-Dependen

36、tReaction:ConvertingSolarEnergyintoChemical-BondEnergy光反應(yīng):光能轉(zhuǎn)化成化學(xué)鍵能Thephotosystemsofthelight-dependentreactionsareresponsibleforthepackagingoflightenergyinthechemicalcompoundsATPandNADPH.ThispackagingtakesplacethroughaseriesofoxidationreductionreactionssetinmotionwhenlightstrikestheP680reactioncente

37、rinphotosystemI.Inthisinitialeventwatermoleculesarecleaved,oxygenisreleased,andelectronsaredonated.Theseelectronsareacceptedfirstbyplastoquinoneandthenbyaseriesofcarriersastheydescendanelectrontransportchain.Foreachfourelectronsthatpassdownthechain,twoATPsareformed.ThelastacceptorinthechainistheP700

38、reactioncenterofphotosystemI.Atthispointincomingphotonsboosttheenergyoftheelectrons,andtheyareacceptedbyferredoxin.Ferredoxinisthenreoxidized,andthecoenzymeNADP+isreducedtotheNADPH.TheATPgeneratedpreviouslyandtheNADPHthentakepartinthelightindependentreactions.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NADPH。當(dāng)光激活光系統(tǒng)II的光反

39、應(yīng)中心時(shí)，通過一系列的氧化還原反應(yīng)實(shí)現(xiàn)能量的傳遞。反應(yīng)開始時(shí),水被分解,氧被釋放并提供電子。電子首先傳遞給質(zhì)體醌,然后通過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，電子傳遞給鐵氧還蛋白。鐵氧還蛋白再氧化，并且輔酶NADP+還原成NADPH。早期產(chǎn)生的ATP和NADPH進(jìn)入暗反應(yīng)。TheproductionofATPfromthetransportofelectronsexcitedbylightenergydownanelectrontransportchainistermedphotophosphorylatio

40、nTheone-wayflowofelectronsthroughphotosystemsIIandIiscallednoncyclicphotophosphorylation;plantsalsoderiveadditionalATPthroughcyclicphotophosphorylation,inwhichsomeelectronsareshuntedbackthroughtheelectrontransportchainbetweenphotosystemsIandI.由電子傳遞鏈偶連產(chǎn)生ATP的過程稱為光合磷酸化。通過光合系統(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

41、通過循環(huán)式光合磷酸化獲得額外的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。TheLight-IndependentReactions:BuildingCarbohydrates暗反應(yīng):碳水化合物的形成Inthelight-independentreactionsofphotosynthesis,whicharedrivenbyATPandNADPH,C02isconvertedtocarbohydrate.ThereactionsarealsoknownastheCalvin-Bensoncycle.AtmosphericCO2,isfixedasitreactswithribu

42、losebiphosphate(RuBP),areactionthatiscatalyzedbytheenzymeribulosebiphosphatecarboxylase.ThereductionOfC02tocarbohydrate(fructosediphosphate)iscompletedviaseveralmorestepsofthecycle.Finally,RUBPisregeneratedsothatthecyclemaycontinue.由ATP和NADPH驅(qū)動的暗反應(yīng)中,二氧化碳轉(zhuǎn)化成碳水化合物。即卡爾文循環(huán)。二磷酸核酮糖固定二氧化碳，由二磷酸核酮糖羧化酶催化。Oxyg

43、en:AnInhibitorofphotosynthesis氧:光合作用的抑制因子)Highlevelsofoxygeninplantcellscandisruptphotosynthesisandcanalsocausephotorespiration-aninefficientfunofthedarkreactionsinwhich02isfixedratherthanC02andnocarbohydrateisproduced.ReprievefromPhotorespiration:TheC4PathwayMostplantsareC3plants;theyexperiencedecr

44、easedcarbohydrateproductionunderhot,dryconditionsasaresultoftheeffectsofphotorespiration.AmongC4plants,however,specialleafanatomyandauniquebiochemicalpathwayenabletheplanttothriveinandconditions.ThusC4plantslessenphotorespirationbycarryingoutphotosynthesisonlyincellsthatareinsulatedfromhighlevelsofC

45、O2.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。課后作業(yè):第一篇閱讀材料答案:lB,2A,3A,4D,5C,6cLessonThree(2學(xué)時(shí))CellularReproduction:MitosisandMeiosisTheNucleusandChromosomesThecellnucleusisthemainrepositoryofgeneticinforma

46、tion.WithinthenucleusarethechromosomestightlycoiledstrandsofDNAandclustersofassociatedproteins.LongstretchesofthecontinuousDNAmoleculewindaroundtheseclustersofproteins,orhistones,formingbeadlikecomplexesknownasnucleosomes.Morecoilingandsupercoilingproducesadensechromosomestructure.EachlongstrandofDN

47、Acombineswithhistonesandnonhistoneproteinstomakeupthesubstancechromatin.細(xì)胞核是貯藏遺傳信息的主要場所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鏈狀的核小體。這些螺旋和超螺旋形成致密的染色體組結(jié)構(gòu)。每個(gè)長鏈DNA與組蛋白和非組蛋白一起構(gòu)成染色質(zhì)物質(zhì)。Apictorialdisplayofanorganismschromosomesinthecoiled,condensedstateisknownasakaryotype.Karyotyperevealthatinmostcellsallbuts

48、exchromosomesarepresentastwocopies,referredtoashomologouspairs.Non-sexchromosomesarecalledautosomes.Organismswhosecellscontaintwosetsofparentalchromosomesarecalleddiploid;thosewithcellscontainingasinglesetofparentalchromosomesarecalledhaploid.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)胞的染色體組成對出現(xiàn)，稱同源染色體對。非性染色體稱常

49、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。TheCellCycleThecellcycleisaregularsequenceinwhichthecellgrows，preparesfordivision，anddividestoformtwodaughtercells，eachofwhichthenrepeatsthecycle.Suchcyclingineffectmakessingle-celledorganismsimmortal.Manycellsinmulticellularorganisms，includinganimalmuscleandnerv

50、ecells，eitherslowthecycleorbreakoutofitaltogether.在細(xì)胞生長過程中，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，theperiodofnormalmetabolism;Sphase，duringwhichnormalsynthesisofbiologicalmoleculescontinues，DNAi

51、sreplicated，andhistonesaresynthesized;andG2，abriefperiodofmetabolismandadditionalgrowth.TogethertheG1，S，andG2phasesarecalledinterphase.ThefourthphaseofthecellcycleisMphase，theperiodofmitosis，duringwhichthereplicatedchromosomescondenseandmoveandthecelldivides.Itisbelievedthatpropertiesofthecellcytopl

52、asmcontrolthecellcycle，alongwithexternalstimulatorsandinhibitorssuchaschalones.正常細(xì)胞循環(huán)由4個(gè)時(shí)期組成。頭三期包括G，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G,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。Mitosis:PartitioningtheHereditaryMaterialBiologistsdividethemitoticcycleintofou

53、rphases.Atthebeginningofprophase前期thechromosomeseachconsistoftwohighlycondensedchromatidsattachedtoeachotheratacentromere.Asprophaseendsandmetaphase中期begins，thecondensed濃縮的chromosomesbecomeassociatedwiththespindle紡錘體.Eventuallythechromosomesbecomearrangedinaplane(calledthemetaphaseplate)atarightangl

54、etothespindlefibers.Next，duringanaphase后期,thetwosisterchromatidsofeachchromosomesplit，andonefromeachpairisdrawntowardeachpoleofthecell.Duringtelophase末期nuclearenvelopes包膜begintoformaroundeachsetofchromosomes,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)染色單體通過著絲粒連接在一起。在分裂前期后期和分裂中期

55、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,thespindlemicrotubulesplayacrucialroleinensuringthatbothpairedandseparatedchromatids染色單體moveintherightdirectionsatthepropertimes.Eachhalfofthespindleformsasmicrotubulesextendfromeachpoleofadividingce

56、lltotheregionofthemetaphaseplate.Duringprophase，othermicrotubules，thecentromeric著絲粒fibers，extendoutwardfromthespindlepolestostructuresonthechromosomescalledkinetochores著絲粒.Duringanaphasethefibersbegintoshorten，andthechromatidsbegintomoveapart.在有絲分裂過程中,是紡錘體微管確保了染色單體在適當(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

57、，其它微管，著絲粒纖維延伸到染色體的動粒。在分裂后期，纖維開始變短，染色單體分離。Thespindleformsdifferentlyinplantandanimalcells.Inanimalsitisassociatedwithcentriole中心粒，whileinplantandfungal真菌cellsspindleformationisassociatedwithreionscalledmicrotubuleorganizingcenters.植物和動物細(xì)胞形成的紡錘體不同。動物細(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。Cytokinesis:Part

58、itioningtheCytoplasm胞質(zhì)分裂:細(xì)胞質(zhì)分離Thedivisionofthecellcytoplasmattheendofmitosisiscalledcytokinesis細(xì)胞漿移動.Inanimalcellsittakesplaceasaringofactinfilamentscontracts使縮短合同契約aroundthecellequator赤道,pinching收聚thecellintwo.Inplantcells，whicharebounded有限制的byacellwall，cytokinesisinvolvesthebuildingofanewcellplate

59、acrossthedividingcellatitsequator.Cellwallmaterialisthendeposited存放堆積intheregionofthecellplate.在動物細(xì)胞中，環(huán)形肌動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。Meiosis:TheBasisofSexualReproductionMeiosis減數(shù)isaspecialformofcelldivisionthattakesplaceinthereproductive生殖organsthatproducesexcells.Likemitosis，ittakesplacea

60、fterDNAreplicationhasoccurredandinvolvestwosequential連續(xù)的nucleardivisions(meiosisIandmeiosisII).Thesedivisionsresultinfourdaughtercells,eachwithhalfthenumberofchromosomesoftheparentcell.Thephenomenonofcrossingoverduringmeiosisresultsinexchangesofgeneticinformationbetweenchromosomes.Hence因此,thehomolog