生物專(zhuān)業(yè)英語(yǔ)第三版蔣悟生課文翻譯精華整理

1、 . 28/28Most of the properties we associate with life are properties of the cytoplasm. Much of the mass of a cell consists of this semifluid substance, which is bounded on the outside by the plasma membrane. Organelles are suspended within it, supported by t. of the cytoskeleton. Dissolved inluid ar

2、e nutrients, ions, soluble proteins, and other materials needed for cell 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(yíng)養(yǎng)物質(zhì)、離子、可溶蛋白以與維持細(xì)胞生理需求的其它物質(zhì)。The eukaryotic cell nucleus is the largest organelle and houses the genetic material (DNA) on chromosomes. (In pro

3、karyotes the hereditary material is found in the nucleoid.) The nucleus also contains one or two organelles-the nucleoli-that play a role in cell division. A pore-perforated sac called the nuclear envelope separates the nucleus and its contents from the cytoplasm. Small molecules can pass through th

4、e nuclear envelope, but larger molecules such as mRNA and ribosomes must enter and exit via the pores.真核細(xì)胞的細(xì)胞核是最大的細(xì)胞器，為染色體上遺傳物質(zhì)（DNA）提供空間（原核生物遺傳物質(zhì)存在于擬核中）。細(xì)胞核含有一或二個(gè)核仁，在細(xì)胞分裂中發(fā)揮作用。一個(gè)穿孔的囊叫核膜，將細(xì)胞核和其中含物與細(xì)胞質(zhì)分開(kāi)，小分子物質(zhì)可以自由通過(guò)核膜，但大分子物質(zhì)如mRNA 和核糖體必須通過(guò)核孔進(jìn)出核膜。All eukaryotic cells contain most of the various kinds of

5、 organelles, and each organelle performs a specialized function in the cell. Organelles described in this section include ribosomes, the endoplasmic reticulum, the Golgi complex, vacuoles, lysosomes, mitochondria, and the plastids of plant cells.所有的真核細(xì)胞都含有多種細(xì)胞器，每種細(xì)胞器在細(xì)胞中行使一項(xiàng)專(zhuān)門(mén)功能。本節(jié)主要介紹核糖體、質(zhì)網(wǎng)、高爾基體系、液

6、泡、溶酶體、線粒體和植物細(xì)胞中的質(zhì)體。The number of ribosomes within a cell may range from a few hundred to many thousands. This quantity reflects the fact that, ribosomes are the sites at which amino acids are assembled into proteins for export or for use in cell processes. A complete ribosome is composed of one larg

7、er and one smaller subunit. During protein synthesis the two subunits move along a strand of mRNA, reading the genetic sequence coded in it and translating that sequence into protein. Several ribosomes may become attached to a single mRNA strand; such a combination is called a polysome. Most cellula

8、r proteins are manufactured on ribosomes in the cytoplasm. Exportable proteins and membrane proteins are usually made in association with the endoplasmic reticulum.細(xì)胞中核糖體的數(shù)量變化從幾百到幾千，核糖體數(shù)量如此之多反映了核糖體是氨基酸組裝成蛋白質(zhì)并被運(yùn)出或在細(xì)胞過(guò)程中使用的位點(diǎn)這樣一個(gè)事實(shí)。完整的核糖體由大亞基和小亞基組成。在蛋白質(zhì)合成中兩個(gè)亞基沿mRNA 鏈移動(dòng)，閱讀其中編碼的遺傳序列，將序列翻譯成蛋白質(zhì)。一條mRNA上可能有

9、多個(gè)核糖體，這種組合稱(chēng)多聚核糖體。大多數(shù)細(xì)胞中的蛋白質(zhì)由細(xì)胞質(zhì)中的核糖體（游離核糖體）制造。運(yùn)輸?shù)鞍缀湍さ鞍淄ǔＳ筛街谫|(zhì)網(wǎng)上的核糖體制造。 The endoplasmic reticulum, a lacy array of membranous sacs, tubules, and vesicles, may be either rough (RER) or smooth (SER). Both types play roles in the synthesis and transport of proteins. The RER, which is studded with polyso

10、mes, also seems to be the source of the nuclear envelope after a cell divides.質(zhì)網(wǎng)是由膜狀囊、管和小泡等排列在一起，有粗面質(zhì)網(wǎng)和滑面質(zhì)網(wǎng)兩種類(lèi)型。兩種類(lèi)型都與蛋白質(zhì)的合成和運(yùn)輸有關(guān)。粗面質(zhì)網(wǎng)上散布著多聚核糖體，似乎也是細(xì)胞分裂后形成核膜的來(lái)源。SER lacks polysomes; it is active in the synthesis of fats and steroids and in the oxidation of toxic substances in the cell. Both types of

11、 endoplasmic reticulum serve as compartments within the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.光滑質(zhì)網(wǎng)上無(wú)核糖體/光滑質(zhì)網(wǎng)上缺乏多聚核糖體，主要作用是脂肪和類(lèi)固醇的合成以與細(xì)胞有毒物質(zhì)的氧化。兩種質(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/這兩種類(lèi)型的質(zhì)網(wǎng)都擔(dān)當(dāng)細(xì)胞中的隔斷，致使特殊產(chǎn)物能夠分離并最終逃避到細(xì)胞外的特定區(qū)域。Transport vesic

12、les may carry exportable molecules from the endoplasmic reticulum to another membranous organelle, the Golgi complex. Within the Golgi complex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.運(yùn)輸小泡能夠?qū)⒖蛇\(yùn)輸分子從質(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

13、質(zhì)中的其他場(chǎng)所。Vacuoles in cells appear to be hollow sacs but are actually filled with fluid and soluble molecules. The most prominent vacuoles appear in plant cells and serve as water reservoirs and storage sites for sugars and other molecules. Vacuoles in animal cells carry out phagocytosis (the intake o

14、f particulate matter) and pinocytosis (vacuolar drinking).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性分子。最典型的液泡存在于植物細(xì)胞中，儲(chǔ)備水，糖以與其它分子。動(dòng)物中的液泡起吞噬（顆粒物的吸收）和胞飲作用。 A subset of vacuoles are the organelles known as lysosomes, which contain digestive e(packaged in lysosomes in the Golgi complex) that can break down most biologi

15、cal macromolecules. They act to digest food particles and to degrade damaged cell parts.溶酶體是液泡亞單位，含有消化酶（在高爾基體中的溶酶體中包裝而成），能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/行使消化食物顆粒和分解損壞的細(xì)胞組件的作用。Mitochondria are the sites of energy-yielding chemical reactions in all cells. In addition, plant cells contain plastids that u

16、tilize light energy to manufacture carbohydrates in the process of photosynthesis. It is on the large surface area provided by the inner cristae of mitochondria that ATP-generating enzymes are located. Mitochondria are self-replicating, and probably they are the evolutionary descendants of what were

17、 once free-living prokaryotes.線粒體是所有細(xì)胞中發(fā)生產(chǎn)能化學(xué)反應(yīng)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。There are two types of plastids: leucoplasts, which lack pigments and serve as storage sites for starch, proteins, and oils; and chromoplasts, which contain pigment

18、s.The most important chromoplasts are chloroplasts-organelles that contain the chlorophyll used in photosynthesis. The internal structure of chloroplasts includes stacks of membranes called grana, which are embedded in a matrix called the stroma.質(zhì)體有兩種類(lèi)型：白色體，缺乏色素，是淀粉、蛋白質(zhì)和油類(lèi)的貯存場(chǎng)所；色質(zhì)體，含有色素。葉綠體是最重要的色質(zhì)體，

19、含有與光合作用有關(guān)的葉綠素。葉綠體的部結(jié)構(gòu)是由多層膜垛疊形成的葉綠體基粒，其中包埋在基質(zhì)中的基粒稱(chēng)子座。All eukaryotic cells have a cytoskeleton, which is a convoluted latticework of filaments and tubules that appears to fill all available space in the cell and provides support for various other organelles. A large portion of the cytoskeleton consist

20、s of threadlike microfilaments composed mainly of the contractile protein actin. They are involved in many types of intracellular movements in plant and animal cells. A second protein, myosin, is involve in the contraction of muscle cells. Another main structural component of the cytoskeleton consis

21、ts of microtubules, which are composed of the globular protein tubulin and together act as scaffolding that provides a stable cell shape. Cytoskeletal intermediate filaments appear tensile strength to the cell cytoplasm. Mechanoenzymes such as myosin, dynein, and kinesin interact with the cytoskelet

22、al filaments and tubules to generate forces that cause movements.所有的真核細(xì)胞都有細(xì)胞骨架，它是由細(xì)絲和管組成的回旋狀的網(wǎng)格，似乎充滿了細(xì)胞中的所有空間并為其它各種細(xì)胞器提供了支撐作用。細(xì)胞骨架大部分由線狀的微絲組成，微絲主要由可收縮的肌動(dòng)蛋白組成。動(dòng)物和植物細(xì)胞中多種類(lèi)型的細(xì)胞運(yùn)動(dòng)與肌動(dòng)蛋白有關(guān)。第二類(lèi)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)組分是微管，由球形蛋白質(zhì)微管蛋白組成，象腳手架那樣使細(xì)胞維持穩(wěn)定的形狀。細(xì)胞骨架的中間絲提供了細(xì)胞質(zhì)伸縮動(dòng)力。一些動(dòng)力酶如，肌球蛋白，動(dòng)力蛋白，驅(qū)動(dòng)蛋白與微絲、微管

23、相互作用產(chǎn)生動(dòng)力而引起細(xì)胞運(yùn)動(dòng)。Although the cytoskeleton provides some stability to cells, its microtubules and filaments and their associated proteins enable cells to move by creeping or gliding . Such movements require a solid substrate to which the cell can adhere and can be guided by the geometry of the surfac

24、e. Some cells also exhibit chemotaxis, the ability to move toward or away from the source of a diffusing chemical.盡管細(xì)胞骨架提供了細(xì)胞的某些穩(wěn)定性，微絲，微管與相關(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)胞具備趨化性，即趨向或逃離擴(kuò)散開(kāi)的化學(xué)源。Certain eukaryotic cells can swim freely in liquid environments, propelled by whiplike cilia or

25、flagella. Both cilia and flagella have the same internal structure: nine doublets (pairs of microtubules) are arranged in a ring and extend the length of the cilium or flagellum, and two more microtubules run down the center of the ring. Every cilium or flagellum grows only from the cell surface whe

26、re a basal body is located. Movement is based on the activities of tiny dynein side arms that extend from one of the microtubules of each doublet.某些真核細(xì)胞能在液體中自由運(yùn)動(dòng)，由纖毛或鞭毛推動(dòng)。纖毛和鞭毛具有同樣的部結(jié)構(gòu)：9個(gè)聯(lián)管（微管對(duì)）排列成環(huán)狀，沿纖毛或鞭毛縱向延伸，環(huán)中心是兩個(gè)或兩個(gè)以上微管組成。纖毛或鞭毛從細(xì)胞表面的基體長(zhǎng)出，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, and other m

27、aterials within most plant cells are moved about via cytoplasmic streaming. The process occursas myosin proteins attached to organelles push against microfilaments arrayed throughout the cell. Microfilaments and microtubules are responsible for almost all major cytoplasmic movements. During cell div

28、ision, microtubules of the spindle assembled fromtubutin subunits near organelles called centrioles move the chromosomes.大部分植物細(xì)胞的營(yíng)養(yǎng)物質(zhì)、蛋白質(zhì)和其它物質(zhì)通過(guò)胞質(zhì)流動(dòng)進(jìn)行運(yùn)動(dòng)。這個(gè)過(guò)程是由于依附在細(xì)胞器上的肌球蛋白推動(dòng)微絲在細(xì)胞中重排形成的。絕大部分細(xì)胞質(zhì)運(yùn)動(dòng)由微絲和微管完成。在細(xì)胞分裂過(guò)程中，中心粒周?chē)挠晌⒐艿鞍讈喕b配形成的紡錘體微管移向染色體。Photosynthesis occurs only in the chlorophyllchlorophyll-c

29、ontaining cells of green plants, algae, and certain protists and bacteria. Overall, it is a process that converts light energy into chemical energy that is stored in the molecular bonds. From the point of view of chemistry and energetics, it is the opposite of cellular respiration. Whereas cellular

30、respiration is highly exergonic and releases energy, photosynthesis requires energy and is highly endergonic.光合作用只發(fā)生在綠色植物，海藻，某些原生動(dòng)物和細(xì)菌等含有葉綠素的細(xì)胞中。總體來(lái)說(shuō)，這是一個(gè)將光能轉(zhuǎn)化成化學(xué)能，并將能量貯存在化學(xué)鍵中。從化學(xué)和能量學(xué)的角度來(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，而光合作用是需要能量并高吸能的過(guò)程。Photosynthesis starts with CO2 and H2O as raw materials and proceed

31、s through two sets of partial reactions. In the first set, called the light-dependent reactions, water molecules are split(oxidized), 02 is released, and ATP and NADPH are formed. These reactions must take place in the presence of light energy. In the second set, called light-independent reactions,

32、CO2 is reduced (via the addition of H atoms) to carbohydrate. These chemical events rely on the electron carrier NADPH and ATP generated by the first set of reactions.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)。第一步反應(yīng)稱(chēng)為光反應(yīng)，水分子分解（氧化），放出氧氣，形成ATP和NADPH。這些反應(yīng)必須有光能的存在才能進(jìn)行。第二步稱(chēng)為非光依賴型反應(yīng)（暗反應(yīng)），二氧化碳被還原（通過(guò)加氫）成碳水化合物，這步反應(yīng)依賴于第一步反應(yīng)產(chǎn)生的

33、NADPH和ATP 作為電子載體Both sets of reactions take place in chloroplasts. Most of the enzymes and pigments for the lightdependent reactions are embedded in the thylakoid membrane of chloroplasts. The dark reactions take place in the stroma. 以上兩步反應(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(lèi)囊體膜上。暗反應(yīng)發(fā)生在基質(zhì)中。The energy in l

34、ight photons in the visible part of the spectrum can be captured by biological molecules to do constructive work. The pigment chlorophyll in plant cells absorbs photons within a particular absorption spectrums statement of the amount of light absorbed by chlorophyll at different wavelengths. When li

35、ght is abt alters the arrangement of electrons in the absorbing molecule. The added energy of the photon boosts the energy condition of the molecule from a stable state to a less-stable excited state. During the light-dependent reactions of photosynthesis, as the absorbing molecule returns to the gr

36、ound state, the excess excitation energy is transmitted to other molecules and stored as chemical energy.生物分子能捕獲可見(jiàn)光譜中的光能用于建設(shè)性的工作。植物細(xì)胞中葉綠素在不同光波下吸收特定吸收光譜。當(dāng)光線被吸收時(shí)，光的作用使分子中的電子發(fā)生重排/它改變了吸收分子的電子排列。由光子增加的能量推進(jìn)分子的能量狀況從穩(wěn)定狀態(tài)（基態(tài)）轉(zhuǎn)變?yōu)椴环€(wěn)定的激發(fā)態(tài)。在光合作用的光反應(yīng)過(guò)程中，吸收分子又回到基態(tài)，多余的激發(fā)能傳遞到其它分子中，以化學(xué)能的形式儲(chǔ)存起來(lái)。 All photosynthetic org

37、anisms contain various classes of chlorophylls and one or more carotenoid (accessory) pigments that also contribute to photosynthesis. Groups of pigment molecules called antenna complexes are present on thylakoids. Light striking any one of the pigment molecules is funneled to a special chlorophyll

38、a molecule, termed a reaction-center chlorophyll, which directly participates in photosynthesis. Most photosynthetic organisms possess two types of reaction-center chlorophylls, P680 and P700, each associated with an electron acceptor molecule and an electron donor. These aggregations are known resp

39、ectively as photosystem (P700) and photosystem (P680).所有的光合作用生物含有不同等級(jí)的葉綠素和一種或多種類(lèi)胡蘿卜素（輔助色素），這些色素在光合作用中發(fā)揮作用。稱(chēng)作天線復(fù)合體的色素分子群存在于類(lèi)囊體中。到達(dá)任何一個(gè)色素分子的光能匯集到稱(chēng)為反應(yīng)中心葉綠素的特殊的葉綠素分子上，它直接參與光合作用。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)與電子受體分子和電子供體分子相關(guān)聯(lián)。這些集合體就是大家熟識(shí)的光合系統(tǒng) (P700)和光合系統(tǒng)(P680) 。The photosystems of the light-dependent

40、reactions are responsible for the packaging of light energy in the chemical compounds ATP and NADPH. This packaging takes place through a series of oxidation reduction reactions set in motion when light strikes the P680 reaction center in photosystem . In this initial event water molecules are cleav

41、ed, oxygen is released, and electrons are donated. These electrons are accepted first by plastoquinone and then by a series of carriers as they descend an electron transport chain. For each four electrons that pass down the chain, two ATPs are formed. The last acceptor in the chain is the P700 react

42、ion center of photosystem . At this point incoming photons boost the energy of the electrons, and they are accepted by ferredoxin. Ferredoxin is then reoxidized, and the coenzyme NADP+ is reduced to the NADPH. The ATP generated previously and the NADPH then take part in the light independent reactio

43、ns.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)的光反應(yīng)中心時(shí)，通過(guò)一系列的氧化還原反應(yīng)實(shí)現(xiàn)能量的傳遞。/這一包裝過(guò)程從光線到達(dá)光合系統(tǒng)II的活性中心P680這一步驟開(kāi)始，包括了一系列氧化還原反應(yīng)反應(yīng)。在第一個(gè)步驟中，水分子被分解，放出氧氣，提供電子。電子首先傳遞給質(zhì)體醌，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。鏈的最終受體是光合系統(tǒng)I的P700 活性中心。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。前面產(chǎn)生的ATP和NADPH進(jìn)入暗反應(yīng)。The production of ATP fro

44、m the transport of electrons excited by light energy down an electron transport chain is termed photophosphorylation. The one-way flow of electrons through photosystems II and I is called noncyclic photophosphorylation; plants also derive additional ATP through cyclic photophosphorylation, in which

45、some electrons are shunted back throughthe electron transport chain between photosystems and .由光能激發(fā)產(chǎn)生的電子沿電子傳遞鏈傳遞并產(chǎn)生ATP 的過(guò)程稱(chēng)為光合磷酸化。電子通過(guò)光合系統(tǒng)流經(jīng)光合系統(tǒng)的電子路徑稱(chēng)非循環(huán)式光合磷酸化；植物通過(guò)循環(huán)式光合磷酸化獲得額外的ATP，在這一過(guò)程中一些電子在光合系統(tǒng)和之間的電子傳遞鏈中回流。In the light-independent reactions of photosynthesis, which are driven by ATPand NADPH, C02

46、 is converted to carbohydrate. The reactions are also known as the Calvin-Benson cycle. Atmospheric CO2, is fixed as it reacts with ribulose biphosphate (RuBP), a reaction that is catalyzed by the enzyme ribulose biphosphate carboxylThe reduction Of C02 to carbohydrate (fructose diphosphate several

47、more steps of the cycle. Finally, RUBP is regenerated so that the cycle may continue.由ATP和NADPH驅(qū)動(dòng)的暗反應(yīng)中，CO2轉(zhuǎn)化成碳水化合物。即卡爾文-本森循環(huán)。在二磷酸核酮糖羧化酶的催化下，二磷酸核酮糖固定空氣中的CO2。CO2通過(guò)循環(huán)中的幾步反應(yīng)還原成為糖類(lèi)（二磷酸果糖），最終核酮糖二磷酸再生，這樣循環(huán)能夠持續(xù)進(jìn)行。High levels of oxygen in plant cells can disrupt photosynthesis and can also cause photorespira

48、tion-an inefficient fun of the dark reactions in which 02 is fixed rather than C02 and no carbohydrate is produced.植物細(xì)胞中高濃度的氧氣能夠破壞光合作用，同時(shí)也能夠引起光呼吸，這是一種效率差的暗反應(yīng)， 02而不是CO2被固定，不產(chǎn)生糖類(lèi)。 Most plants are C3 plants; they experience decreased carbohydrate production under hot, dry conditions as a result of the

49、effects of photorespiration. Among C4 plants, however, special leaf anatomy and a unique biochemical pathway enable the plant to thrive in and conditions. Thus C4 plants lessen photorespiration by carrying out photosynthesis only in cells that are insulated from high levels of CO2. They also possess

50、 a novel mechanism for carbon fixation.許多植物是C3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C4 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。這樣C4 植物并非在細(xì)胞中存在高濃度的CO2條件下才能進(jìn)行光合作用，這是碳固定的一個(gè)新機(jī)制。The cell nucleus is the main repository of genetic information. Within the nucleus are the chromosomes tightly coiled strands of DNA and

51、clusters of associated proteins. Long stretches of the continuous DNA molecule wind around these clusters of proteins, or histones, forming beadlike complexes known as nucleosomes. More coiling and supercoiling produces a dense chromosome structure. Each long strand of DNAcombines with histones and

52、nonhistone proteins to make up the substance chromatin.細(xì)胞核是貯藏遺傳信息的主要場(chǎng)所。DNA盤(pán)繞成螺旋線以與相關(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。這些螺旋或超螺旋形成致密的染色體組結(jié)構(gòu)。每個(gè)長(zhǎng)鏈DNA與組蛋白和非組蛋白一起構(gòu)成染色質(zhì)。A pictorial display of an organisms chromosomes in the coiled, condensed state is known as a karyotype. Karyotype reveal that in most cells a

53、ll but sex chromosomes are present as two copies, referred to as homologous pairs. Non-sex chromosomes are called autoso.Organisms whose cells contain two sets of parental chromosomes are called diploid; those with cells containing a single set of parental chromosomes are called haploid.以圖示方式展示出來(lái)的螺旋

54、的、濃縮狀態(tài)有機(jī)體染色體，稱(chēng)為染色體組型。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(chēng)同源染色體對(duì)。非性染色體稱(chēng)常染色體。生物細(xì)胞含有兩套親本染色體的有機(jī)體稱(chēng)為二倍體；含有單套染色體的有機(jī)體稱(chēng)單倍體。The cell cycle is a regular sequence in which the cell grows, prepares for division, and divides to form two daughter cells, each of which then repeats the cycle. Such cycling in effect makes single

55、-celled organisms immortal. Many cells in multicellular organisms, including animal muscle and nerve cells, either slow the cycle or break out of it altogether.細(xì)胞周期是一個(gè)有順序的過(guò)程：細(xì)胞生長(zhǎng)、準(zhǔn)備分裂、分裂形成兩個(gè)子細(xì)胞，子細(xì)胞再循環(huán)。此循環(huán)使得單細(xì)胞生物永生。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，能夠使細(xì)胞周期時(shí)間延長(zhǎng)或完全脫離細(xì)胞周期。 The normal cell cycle consists of four p

56、hases. The first three include G1, the periodof normal metabolism; S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized; and G2, a brief period of metabolism and additional growth. Toget G1, S, and G2 phases are called interphase.

57、The fourth phase of the cell cycle is M phase, the period of mitosis, during which the replicated chromosomes condense and move and the cell divides. It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as chalones.正常的細(xì)胞周期包括

58、四步。前3步包括G1(正常的代階段)；S期（持續(xù)進(jìn)行生物分子的正常合成，DNA復(fù)制，組蛋白合成）；G2期（代和再次生長(zhǎng)的短階段）。G1期、S期和G2期合起來(lái)稱(chēng)為間期。細(xì)胞周期的第四步為M 期（進(jìn)行有絲分裂的階段），在這一階段，復(fù)制的染色體濃縮、運(yùn)動(dòng)，細(xì)胞分裂。被認(rèn)為是細(xì)胞質(zhì)的特性和外部的刺激因子和抑制劑例如抑素控制了細(xì)胞周期。Biologists divide the mitotic cycle into four phases. At the beginning of prophase the chromosomes each consist of two highly condensed

59、chromatids attached to each other at a centromere. As prophase ends and metaphase begins, the condensed chromosomes become associated with the spindle. Eventually thein a plane (called the metaphase plate) at a rig spindle fibers. Next, during anaphase , the two sister chromatids of each chromosome

60、split, and one from each pair is drawn toward each pole of the cell. During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes place.生物學(xué)家將有絲分裂劃分為4個(gè)階段。分裂前期，每一條染色體包括兩條高度濃縮的染色單體，兩個(gè)染色單體通過(guò)著絲粒連接在一起。當(dāng)前期結(jié)束時(shí)，分裂中期開(kāi)始，濃縮的染色體與紡錘體相連，最后以與紡錘絲成垂直角度排列在赤道板上。在分