生物專業(yè)英語教學(xué)-細(xì)胞的繁殖課件

1、Unit3 Cellular Reproduction: Mitosis and MeiosisWarmingupWords and phrasesText AnalysisSimple exercises ContentsWarmingup有絲分裂:無絲分裂:減數(shù)分裂:一種特殊方式的有絲分裂細(xì)胞增殖方式與體細(xì)胞的形成有關(guān)不出現(xiàn)紡綞體，沒有染色體等變化。如蛙的紅細(xì)胞。有絲分裂和減數(shù)分裂的比較比較有絲分裂減數(shù)分裂不同點相同點分裂后形成( )分裂后形成( )細(xì)胞分裂 （ ）次，產(chǎn)生（ ）個子細(xì)胞細(xì)胞分裂（ ）次，產(chǎn)生（ ）個子細(xì)胞子細(xì)胞和母細(xì)胞的染色體數(shù)（ ）子細(xì)胞染色體數(shù)是母細(xì)胞的（ ）不出現(xiàn)聯(lián)

2、會、四分體現(xiàn)象減數(shù)I發(fā)生同源染色體的聯(lián)會、四分體現(xiàn)象細(xì)胞分裂過程中均出現(xiàn)（ ）和（ ），染色體復(fù)制（ ）次體細(xì)胞生殖細(xì)胞1224相同一半染色體紡錘體11. interphase n. 分裂間期 int(:)feiz Interphase- The stage in the CELL CYCLE when the nucleus is not in a state of division. Interphase is divisible into various stages each characterized by a differing physiological activity.間期：

3、是細(xì)胞周期的一個時期，在這個時期里細(xì)胞核不分裂。根據(jù)細(xì)胞生理活性差異可以把間期分成不同的階段。Words and phrasesCell cycle Interphase 間期: G1 + S + G2 M phase (mitosis 有絲分裂):分裂間期1、DNA復(fù)制2、中心粒復(fù)制其中G1和G2期主要是合成有關(guān)蛋白質(zhì)和RNA，S期則完成DNA 的復(fù)制。2. prophase prufeiz n. 分裂前期 3. metaphase n. 分裂中期 metfeiz 4. anaphase nfeiz n. 分裂后期5. telophase telfeiz n. 分裂末期 有絲分裂前期1）染色

4、體出現(xiàn)2）每條染色體包含 兩條染色單體3）復(fù)制的中心粒 向兩極移動4）紡錘體出現(xiàn)5）染色體中部和 紡錘絲相連6）核膜、核仁消失prophase prufeiz n. 分裂前期 The first stage of mitosis, during which individual chromosomes become visible.有絲分裂中期1）中心粒到達細(xì)胞兩極2）染色體排列在細(xì)胞中央的赤道板上赤道板（equatorial plane）3. metaphase n. 分裂中期 metfeiz The second stage in mitosis, during which the chr

5、omosomes align at the equatorial plane.1）著絲粒分裂，染色單體分離2）兩條染色體受紡錘絲 牽引，向細(xì)胞兩極移動有絲分裂后期4. anaphase nfeiz n. 分裂后期The third stage of mitosis, during which the centromeres split and the chromosomes move to the poles.1）細(xì)胞質(zhì)分裂2）赤道板附近的細(xì)胞膜凹陷3）細(xì)胞一分為二4）植物則是在細(xì)胞中 央形成細(xì)胞板，再 形成細(xì)胞壁中央凹陷5. telophase telfeiz n. 分裂末期 有絲分裂末期T

6、he last stage in mitosis, during which daughter nuclei are formed.有絲分裂全過程interphaseprophasemetaphaseanaphase telophaseprometaphaseDuring mitosis, sister chromatids remain joined by their centromere until anaphase.在有絲分裂期間，姐妹染色單體在分裂后期之前都是通過著絲點連接的。For example：A plant cell structure that begins to form

7、in the center of the cell and proceeds to the cell membrane,resulting in cytokinesis.6. Cell platen. 細(xì)胞板The cell-plate membranes eventually form the two new plasma membranes of the daughter cells.For example：細(xì)胞板的膜最終形成兩個新的子細(xì)胞的質(zhì)膜。 7. centromere n. 著絲粒;著絲點 sentr,mi The small area of a chromosome that d

8、oes not stain with basic dyes(堿性燃料) during mitosis and meiosis; at interphase it is single while the rest of the chromosome is made up of two chromatids.8. chalonen. 抑素 klun 9. chromatidn. 染色單體 krumtid A replicated chromosome physically attached to an identical chromatid at centromere.一對同源染色體（含4條染色單

9、體）一條染色單體10. chromatinn. 染色質(zhì) krumtin Chromatin is the material that makes up chromosomes; it consists of DNA complexed with proteins.由DNA和蛋白質(zhì)組成的染色質(zhì)，是構(gòu)成染色體的原料。 11. cytokinesis n. 胞質(zhì)分裂 ,saitukaini:sis Division（mitosis or meiosis)of the cytoplasm of one cell into two new cells. 12. diploidn. 二倍體 diplid

10、Any nucleus,cell,or organism that posesses twice the haploid number of chromosomes.During the process of fertilization a zygote with the diploid set is produced. 通過受精作用使受精卵形成一個二倍體。13. haploidn. 單倍體 hplid Genome- A complete haploid chromosome set.基因組：即一套完整的單倍體染色體。 Any nucleus,cell,or organism that po

11、sesses twice a single set of unpaired chromosome.14. histonen. 組蛋白 histun The small proteins have five families or classes :H1,H2A,H2B,H3,H4;and have a large positive charge (rich in Lys、Arg ) which can bind very strongly to the negtively charged DNA in forming chromatin.DNA和組蛋白組裝成的結(jié)構(gòu)叫做染色體。15. homol

12、ogous pair humlgs pn. 同源染色體對一對同源染色體（含4條染色單體）一條染色單體16. karyotypen. 核型，染色體組型 kritaip The appearance of the chromosome complement of an organism or cell.核型：一個給定物種的染色體所表現(xiàn)出的物理學(xué)特征。 17. meiosisn. 減數(shù)分裂 maiusis The process by which a nucleus divides into four daughter nuclei,each containing half the number o

13、f chromosomes of the parent nucleus.18.mitosis mitusis n. 有絲分裂,間接核分裂 通過有絲分裂，每條染色體精確復(fù)制成的兩條染色單體并均等地分到兩個子細(xì)胞，使子細(xì)胞含有同母細(xì)胞相同的遺傳信息。細(xì)胞有絲分裂過程，可以區(qū)分為：前期，中期，后期和末期。 有絲分裂和減數(shù)分裂的比較比較有絲分裂減數(shù)分裂不同點相同點分裂后形成( )分裂后形成( )細(xì)胞分裂 （ ）次，產(chǎn)生（ ）個子細(xì)胞細(xì)胞分裂（ ）次，產(chǎn)生（ ）個子細(xì)胞子細(xì)胞和母細(xì)胞的染色體數(shù)（ ）子細(xì)胞染色體數(shù)是母細(xì)胞的（ ）不出現(xiàn)聯(lián)會、四分體現(xiàn)象減數(shù)I發(fā)生同源染色體的聯(lián)會、四分體現(xiàn)象細(xì)胞分裂過程中均

14、出現(xiàn)（ ）和（ ），染色體復(fù)制（ ）次體細(xì)胞生殖細(xì)胞1224相同一半染色體紡錘體1 19. metaphase platen. 中期板，赤道板赤道板（equatorial plane）20.nucleosome nju:klisum n. 生化核小體 The basic unit of chromatin structurein eukaryotic cells. A nucleosome consists of 8 histone molecules,together with about 140 base pairs of DNA coiled around it.Nucleosome i

15、s the basic unit of chromatin structure 21.spindle spindl n. 紡錘體An array of microtubule extending from pole tp pole and used in the movement of chromosomes. Reviewing1. anaphase n. 分裂后期2. Cell plate n. 細(xì)胞板3. centromere n. 著絲粒4. chalone n. 抑素5. chromatid n. 染色單體6. chromatin n. 染色質(zhì)7. cytokinesis n. 胞質(zhì)

16、分裂8. diploid n. 二倍體9. haploid n. 單倍體10. histone n. 組蛋白11. Homologous pair n. 同源染色體對nfezsentrmikln; ke-krmtdkrmtn,satokniss12. interphase n. 分裂間期 13. karyotype n. 核型，染色體組型14. meiosis n. 減數(shù)分裂15. Metaphase n. 中期16.metaphase plate n. 中期板，赤道板17.mitosis n. 有絲分裂18.nucleosome n. 核小體 19.prophase n. 前期 20.spi

17、ndle n. 紡錘體21.telophase n. 末期 kr,tapPre-reading Activities:Listen to a video and think about the concept and mechanism of cell reproduction.Text analysisText AnalysisText Analysis1. The nucleus and chromosomesThe cell nucleus is the main repository of genetic information. Within the nucleus are the

18、chromosomes-tightly coiled strands of DNA and clusters of associated proteins. 細(xì)胞核是遺傳信息的主要貯存室。細(xì)胞核內(nèi)部是染色體，包括緊密纏繞的線狀DNA和與之相連的蛋白質(zhì)串。Long stretches of the continuous DNA molecule wind around these clusters of proteins, or histones, forming beadlike complexes known as nucleosomes. 長的伸展的持續(xù)的DNA分子圍繞蛋白質(zhì)串（組蛋白）纏

19、繞，形成串珠狀的復(fù)合體，被稱為核小體。More coiling and supercoiling produces a dense chromosome structure. Each long strand of DNA combines with histones and nonhistone proteins to make up the substance chromatin.更多的螺旋或超螺旋形成一個密集的染色體結(jié)構(gòu)。每一個DNA長鏈與組蛋白或非組蛋白結(jié)合構(gòu)成了染色質(zhì)。A pictorial display of an organisms chromosomes in the coi

20、led, condensed state is known as a karyotype. 有機體的染色體以螺旋的、濃縮的狀態(tài)用圖示的方式展示出來，稱為染色體組型.Karyotypes reveal that in most cells all but sex chromosomes are present as two copies, referred to as homologous pairs. 染色體組型顯示出大多數(shù)細(xì)胞除了性染色體以外的其他染色體以2個拷貝的形式存在，被稱為同源染色體對。Non-sex chromosomes are called autosomes. Organis

21、ms whose cells contain two sets of parental chromosomes are called diploid; those with cells containing a single set of parental chromosomes are called haploid. 非性染色體稱為常染色體。細(xì)胞中含有兩套親本染色體的有機體稱為二倍體；細(xì)胞中含有一套親本染色體的有機體稱為單倍體。2 The cell cycleThe cell cycle is a regular sequence in which the cell grows, prepa

22、res for division, and divides to form two daughter cells, each of which then repeats the cycle. 細(xì)胞周期是一個有順序的過程：細(xì)胞生長、準(zhǔn)備分裂、分裂形成兩個子細(xì)胞，每個子細(xì)胞再重新開始細(xì)胞周期。Such cycling in effect makes single-celled organisms immortal. Many cells in multicellular organisms, including animal muscle and nerve cells, either slow

23、the cycle or break out of it altogether.這種有效的循環(huán)使得單細(xì)胞生物永生。多細(xì)胞生物的許多細(xì)胞包括動物肌肉和神經(jīng)細(xì)胞能夠使細(xì)胞周期時間延長或完全脫離細(xì)胞周期。The normal cell cycle consists of four phases. The first three include G1, the period of normal metabolism; S phase, during which normal synthesis of biological molecules continues, DNA is replicated,

24、and histones are synthesized; 正常的細(xì)胞周期由四個階段構(gòu)成。前三個階段包括G1，即正常的代謝的階段；S期，期間持續(xù)進行生物分子的正常合成，即DNA復(fù)制和組蛋白的合成；and G2, a brief period of metabolism and additional growth. Together the G1, S, and G2 phases are called interphase. G2期（代謝和再次生長的短階段）。G1、S和G2期合起來稱為間期。The fourth phase of the cell cycle is M phase, the p

25、eriod of mitosis, during which the replicated chromosomes condense and move and the cell divides. 細(xì)胞周期的第四步為M 期（進行有絲分裂的階段），在這一階段，復(fù)制的染色體濃縮、運動，細(xì)胞分裂。It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as chalones.被認(rèn)為是細(xì)胞質(zhì)的特性和外部的

26、刺激因子和抑制劑例如抑素控制了細(xì)胞周期。3 Mitosis: Partitioning the hereditary materialBiologists divide the mitotic cycle into four phases. At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. 生物學(xué)者把有絲分裂周期分為四步，分裂前期開始時，每一條染色體包括兩條高度濃縮的染色單體，兩者

27、在著絲粒位置連在一起。As prophase ends and metaphase begins, the condensed chromosomes become associated with the spindle. Eventually the chromosomes become arranged in a plane (called the metaphase plate) at a right angle to the spindle fibers. 當(dāng)前期結(jié)束時，分裂中期開始。高度濃縮的染色體與紡錘體相連，最終染色體排在一個叫做中期板的平面上，與紡錘絲成垂直角度。 Next,

28、during anaphase, the two sister chromatids of each chromosomes split, and one from each pair drawn toward each pole of the cell. 接著，在分裂后期，每個染色體上的兩個姊妹染色單體分開，每一對的一個分別移向細(xì)胞的一極。During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes place. 在分裂末

29、期，圍繞每一套染色體開始形成核膜，然后開始細(xì)胞質(zhì)的分裂。As mitosis proceeds, the spindle microtubules play a crucial role in ensuring that both paired and separated chromatids move in the right directions at the proper times. 當(dāng)有絲分裂進行時，紡錘體的微管在保證成對的和分離的染色單體適時向正確的方向運動起到關(guān)鍵作用。Each half of the spindle forms as microtubules extend fr

30、om each pole of a dividing cell to the region of the metaphase plate. 當(dāng)微管從每個分裂細(xì)胞的兩極延伸至赤道板時，形成了紡錘體的每一半。During prophase, other microtubules, the centromeric fibers, extend outward from the spindle poles to structures on the chromosomes called kinetochores. During anaphase the fibers begin to shorten,

31、and the chromatids begin to move apart.在有絲分裂前期，另外一種微管，著絲粒纖維從紡錘體的兩極向外延伸到染色體上叫做動粒的結(jié)構(gòu)上。在分裂后期，纖維開始縮短，染色單體開始分開。knitk; ka-kinetochore動粒The spindle forms differently in plant and animal cells. In animals it is associated with centriole, while in plant and fungal cells spindle formation is associated with r

32、egions called microtubule organizing centers.在植物和動物細(xì)胞中紡錘體的形成不同，動物細(xì)胞中，與中心體有關(guān)。在植物和真菌細(xì)胞中紡錘體的形成與叫做微管組織中心的區(qū)域有關(guān)。4 Cytokinesis: Partitioning the cytoplasmThe division of the cell cytoplasm at the end of mitosis is called cytokinesis. In animal cells it takes place as a ring of actin filaments contracts aro

33、und the cell equator, pinching the cell in two. 有絲分裂結(jié)束時細(xì)胞質(zhì)的分裂稱為胞質(zhì)分裂。動物細(xì)胞中胞質(zhì)分裂過程中，首先肌動蛋白形成的環(huán)在細(xì)胞赤道周圍進行收縮，將細(xì)胞收縮為兩部分。In plant cells, which are bounded by a cell wall, cytokinesis involves the building of a new cell plate across the dividing cell at its equator. Cell wall material is then deposited in th

34、e region of the cell plate.植物細(xì)胞有細(xì)胞壁包被，胞質(zhì)分裂包括圍繞新細(xì)胞的赤道周圍形成新細(xì)胞板，然后細(xì)胞壁物質(zhì)在細(xì)胞板位置沉積。5 Meiosis: The basis of sexual reproductionMeiosis is a special form of cell division that takes place in the reproductive organs that produce sex cells. 減數(shù)分裂是細(xì)胞分裂的特殊形式，在產(chǎn)生生殖細(xì)胞的生殖器官中發(fā)生。Like mitosis, it takes place after DNA

35、 replication has occurred and involves two sequential nuclear divisions (meiosis I and meiosis II). These divisions result in four daughter cells, each with half the number of chromosomes of the parent cell. 如有絲分裂，它發(fā)生于DNA 復(fù)制后，包括兩個有順序的核分裂階段（減數(shù)分裂I期和減數(shù)分裂II期）。這些分裂產(chǎn)生4個子細(xì)胞，每一個子細(xì)胞的染色體數(shù)是母細(xì)胞染色體數(shù)的一半。The pheno

36、menon of crossing over during meiosis results in exchanges of genetic information between chromosomes. Hence, the homologous chromosomes distributed to different progeny cells are not identical.減數(shù)分裂交叉現(xiàn)象的出現(xiàn)導(dǎo)致了染色體之間互換遺傳信息。這樣分配到不同后代細(xì)胞中的同源的染色體并不相同。As in mitosis two chromatids exist for each chromosome a

37、t the beginning of prophase I. 象有絲分裂那樣，在分裂前I期的開始，每個染色體含有兩條染色單體。During this phase the homologous chromosomes undergo synapsis or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptinemal complex. 在這一階段，同源的染色體經(jīng)歷了聯(lián)會或配對，這是由蛋白質(zhì)和RNA形成的橋狀結(jié)構(gòu)-聯(lián)會復(fù)合體的出現(xiàn)而發(fā)生的。snpsssynaps

38、is 聯(lián)會The homologous pairs stay together when they align on the metaphase plate. 當(dāng)同源染色體對排列于赤道板上時，（這兩條染色體）同時存在。Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere and move together to one of the two poles of the cell. 然而，與有絲分裂后

39、期不同的是，減數(shù)分裂后期I每個染色體的兩條染色單體在著絲粒處相連，一起移向細(xì)胞的一極。It is this event that results in the halving of the chromosome number in the four daughter cells that result from meiosis.正是由于這一事件導(dǎo)致減數(shù)分裂中4個子細(xì)胞的染色體數(shù)目減半。During telophase I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis (

40、the first nuclear division) follows. 在減數(shù)分裂末期I，核被膜將染色體包裹在細(xì)胞核中，接著多數(shù)種類開始胞質(zhì)分裂（第一次細(xì)胞核分裂）。The second nuclear division begins with metaphase II, in which the chromosomes in each daughter cell again align on a metaphase plate. 第二次細(xì)胞核分裂開始于減數(shù)分裂中期II，每個子細(xì)胞中的染色體重新排列于中期板上。The cenromeres finally divide, and each s

41、ister chromatid moves to one of the poles of the spindle. The next phase is telophase II, followed again by cytokinesis. The result of the entire process is four haploid cells in which parental chromosomes are randomly distributed.著絲粒最終分裂，每個姊妹染色單體移向紡錘體的一極。下一個階段是減數(shù)分裂末期II，然后是胞質(zhì)分裂。整個過程的結(jié)果是4 個單倍體細(xì)胞，親本的染

42、色體被隨機分配。 6) Asexual versus sexual reproductionMitosis and meiosis, respectively, make simple cell division and sexual reproduction possible. Each means of passing on hereditary information has advantages. 有絲分類和減數(shù)分裂分別使簡單的細(xì)胞分裂和有性生殖成為可能。每一種傳遞遺傳信息的方式都有優(yōu)點。In asexual reproduction the parent organism gives

43、 rise to offspring that are genetic clones of the parent. 無性生殖產(chǎn)生后代時克?。ㄍ耆珡?fù)制）了中親本的遺傳信息。The advantages of this type of reproduction are that it preserves the parents successful genetic complement, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction. 這

44、種生殖類型的優(yōu)點是它保存了親本成功的遺傳信息，基本或完全不需要特化出生殖器官，與有性生殖相比更迅速。A major disadvantage of the asexual mode is that a single catastrophic event or disease may destroy an entire population of genetically identical organisms. 無性類型的主要缺點是單一的災(zāi)難性事件或疾病就可以摧毀遺傳信息完全相同的整個種群。A prime benefit of sexual reproduction is that it pro

45、vides genetic variability and a ready mechanism for the elimination of deleterious mutations. It also allows “new” gene forms to arise and spread through populations.有性生殖的主要好處是它能夠提供遺傳的多樣性，具有迅速的清除有害變異的機制。它也使新基因形式出現(xiàn)、增加并擴展到整個種群。Review the former content：1. anaphase n. 分裂后期2. Cell plate n. 細(xì)胞板3. centrom

46、ere n. 著絲粒;著絲點4. chalone n. 抑素5. chromatid n. 染色單體6. chromatin n. 染色質(zhì)7. cytokinesis n. 胞質(zhì)分裂8. diploid n. 二倍體9. haploid n. 單倍體10. histone n. 組蛋白11. Homologous pair n. 同源染色體對12. interphase n. 分裂間期 13. karyotype n. 核型，染色體組型14. meiosis n. 減數(shù)分裂15. Metaphase n. 中期16.metaphase plate n. 中期板，赤道板17.mitosis n.

47、 有絲分裂18.nucleosome n. 生化核小體 19.prophase n. 前期 20.spindle n. 紡錘體21.telophase n. 末期 ExercisesI Key terms: matchingMatch each term on the left with the most appropriate description on the righta. DNA+histoneb. two setsc. X and Yd. one sete. chromosome displayf. set of microtublesg. plant divisionh. gam

48、ete productioni. sequence of cell growth and divisionj. cell divisionk. crossing overl. inhibit cell divisionm. Division of cytoplasmn. single chromosome copyo. positively charged protein1. cytokinesis2. synapsis3. histone4. mitosis5. cell cycle6. chalone7. spindle8. chromatid9. nucleosome10. diploi

49、d 11. meiosis12. cell plate13. sex chromosome14. karyotype15. haploidI Key terms: matchingII. True or falseAutosomes include X and Y chromosomes.2. Cells enter G2 at the end of the S phase.3. Chalones promote cell division.4. The nuclear envelope forms in telophase.5. Plant cells have the most promi

50、nent centrioles.6. Nuclear division may occur without cytokinesis.7. Homologous chromosomes pair in synapsis.8. Chromosomes break at chiasmata(交叉).9. Sexual organisms cannot be cloned.10. All daughter cells are hapoid.kazmtIII CompletionIf human chromsomes are stained on a slide, the resulting displ

51、ay, which is called a karyotype, should contain twenty-two pairs of autosomes and one pair of sex chromosomes.The S in S phase stands for synthesis because during this time DNA is replicated and histones are synthesized.3. Instead of centrioles, most plants and fungi have regions called microtubule

52、organizing centers.During meiosis but not during mitosis , homologous chromosomes pair up in a process of synapsis.Mitosis produces 2 progeny cells, each with a set of diploid chromosomes.III Completion6. Meiosis produces 4 daughter cells, each with a haploid set of haploid chromosomes.7. Microtubul

53、es called centromeric fibers attach to the chromosomes at the kinetochores8.Two chromatids attach to each other at the centromere.IV Multiple choice1. DNA is replicate during the_. A. S phase B. M phase C. G2 phase D. G1 phase E. none of the above2. Members of a chromosome pair collectively make up_

54、. A. a tetrad（四分染色體） B. chromatin C. a homologous pair D. a chromatid E. a nucleosometetrd3. Autosomes represent_. A. all chromosomes found in a normal human cell B. those chromosomes found in egg or sperm cells C. all chromosomes other than the sex chromosomes D. chromosomes pairs with unlike membe

55、rs E. all homologous chromosomes4. In a typical vertebrate the longest phase is_. A. S B. M C. G2 D. G1 E. None of the abovevtbrt脊椎動物5. A normal diploid human cell contains_. A. 46 chromosomes B. 23 chromosomes C. 46 homologous pair of chromosomes D. 20 chromosomes E. 20 pairs of chromosomes and 2 s

56、ex chromosomes6. Male sex chromosomes can never be_. A. haploid B. homlogous C. diploid D. analogous E. duplicated7 During G1,S, and G2 phases, a cell is said to be in _. A. the process of mitosis B. the process of meiosis C. metaphase D. cytokinesis E. interphase8. It is possible for 2 meters of DN

57、A to fit into a human cell 5 micrometers in diameter because_. A. DNA is broken into small fragments B. DNA is wound around histones C. DNA is wound around nonhistone proteins D. Chromosomes are composed of chromatids E. Chromosomes are joined at the centromere 9. The association of a DNA molecule, histones and nonhistone proteins is known as a _. A. nucleosome B. chromosome C. chromatin D. chromatid E. karyotype10. The two dauthter strands of a duplicated chromosome are each known as a _. A. synapse pair B. centromere C. chroma