大学生物专业英语lessonth

Lesson threeCellular reproduction: mitosis and meiosis repository n.贮藏室, 智囊团, 知识库, 仓库chromosome n.生物染色体histone n.生化组蛋白nucleosome n.生化核小体chromatin n.核染质, 染色质pictorial adj.图示的 n.画报karyotype n.生物 染色体组型homologous pair 同源染色体对autosome n.正染色体, 常染色体 diploid adj.双重的, n.二倍体haploid n.生物单倍体 adj.单一的immortal adj.不朽的Chalone n.生化抑素prophase n. (细胞分裂)前期chromatid n.生物染色单体centromere n.生着丝点, 着丝粒metaphase n.中期anaphase n.生(细胞分裂的)后期 Telophase n.生(细胞有丝分裂的)末期kinetochore n.生动粒, 着丝粒, 着丝点cytokinesis n.细胞浆移动, 原浆移动pinching v.收聚cell plate 细胞板mitosis n.有丝分裂meiosis n.减数分裂, 成熟分裂 Halving 对分, 二等分, 减半Synapsis 联会synaptinemal complex 联会复合物Asexual adj.无性的, 无性生殖的catastrophic adj.悲惨的, 灾难的Elimination n.排除, 除去, 消除, 消灭 1. The nucleus and chromosomes The cell nucleus is the main repository of genetic information. 细胞核是贮藏遗传信息的主要场所。Within the nucleus are the chromosomestightly coiled strands of DNA and clusters of associated proteins.细胞核里的是染色体紧密盘绕成螺旋的DNA链和相关的成簇蛋白质。 Long stretches of the continuous DNA molecule wind around these clusters of proteins, or histones, forming beadlike complexes known as nucleosomes. 长链DNA分子缠绕着成簇的蛋白质，或组蛋白，形成珠链状的核小体。 More coiling and supercoiling produces a dense chromosome structure. 进一步的螺旋和超螺旋形成致密的染色体组结构。Each long strand of DNA combine with histones and nonhistone proteins to make up the substance chromatin.每个长链DNA与组蛋白和非组蛋白一起构成染色质物质。 A pictorial display of an organisms chromosomes in the coiled, condensed state is known as a karyotype.生物体中的致密的超螺旋状态的染色体的表型称为核型Karyotype reveal that in most cells all but sex chromosomes are present as two copies, referred to as homologous pairs. 表型显示：在大多数细胞里，除了性染色体外，其它的染色体都是成对出现的，称之为同源染色体对。 Non-sex chromosomes are called autosom es. 非性染色体称常染色体。O rganisms whose cells contain two sets of parental chromosomes are called diploid; those with cells containing a single set of parental chromosomes are called haploid.细胞里含有两套亲本染色体的生物体称二倍体；细胞里含有单套染色体的生物体称单倍体。 2. The cell cycle细胞周期 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.细胞周期遵循特定的程序：细胞生长，分裂准备，分裂成2个子细胞，每个子细胞再循环。 Such cycling in effect makes single-celled organisms immortal. 此循环可以有效的使得单细胞生物永生。 Many cells in multicellular organisms, including animal muscle and nerve cells, either slow the cycle or break out of it altogether.多细胞生物中的许多细胞，包括动物肌肉和神经细胞，要么减慢周期速度，要么一起脱离细胞周期。The normal cell cycle consists of four phases. 正常细胞周期由4个时期组成。 The first three include G 1, the period of normal metabolism; S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized; and G 2, a brief period of metabolism and additional growth. 前三个时期包括G 1,正常新陈代谢时期；S期,生物分子的合成继续，同时，DNA复制，组蛋白合成； G 2 期，新陈代谢和进一步生长的短暂时期。Together the G 1, S, and G 2 phases are called interphase. G 1, S, 和G 2一起称为分裂间期。 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. 细胞周期的第四个时期是M期，有丝分裂期，复制的染色体浓缩，移动并细胞分裂。 It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as chalones.据称是细胞质中的物质与外来的激活因子和抑制因子如抑素等一起控制了细胞周期。 3. Mitosis: partitioning the hereditary material有丝分裂：遗传物质的分离 Biologists divide the mitotic cycle into four phases. 生物学家将有丝分裂划分为4个阶段。At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. 分裂前期，每个染色体是由高度浓缩的通过着丝粒连接在一起的两个染色单体组成。(分裂前期，高度浓缩的两个染色单体通过着丝粒连接在一起形成染色体) As prophase ends and metaphase begins, the condensed chromosomes become associated with the spindle.分裂前期结束，紧接着是分裂中期，浓缩的染色体与纺锤体相连。(在分裂前期后期和分裂中期前期，浓缩的染色体与纺锤体相连)Eventually the chromosomes become arranged in a plane (called the metaphase plate) at a right angle to the spindle fiber.最后染色体在纺锤丝的牵引下以正确的角度排列在赤道板上。 Next, during anaphase, the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell.在分裂后期，两个姊妹染色单体分离，分别被拉向细胞两极。 During telophase nuclear envelope begin to form around each set of chromosomes, and division of the cytoplasm takes place. 在分裂末期，在每套染色体周围形成核膜，细胞质分裂发生。 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.在有丝分裂过程中，纺锤体微管在确保染色单体配对和在适当时间以正确方向进行分离中发挥了关键性的作用。 Each half of the spindle forms as microtubules extend from each pole of a dividing cell to the region of the metaphase plate.微管从正在分裂的细胞两极向赤道板延伸，就形成了纺锤体的一半。 During prophase, other microtubules, the centromeric fiber, extend outward from the spindle poles to structures on the chromosomes called kinetochores. 在分裂前期，其它微管，着丝粒纤维从纺锤体的两极延伸到染色体的动粒区域。During anaphase the fibers begin to shorten, and the chromatids begin to move apart.在分裂后期，纤维开始变短，染色单体分离。 Mitosis: prophasemetaphaseanaphasetelophase chromosome condensationmitotic spindle assemblynuclearenvelopebreakdown chromosomes alignin the central planesister chromatids split and are drawn by the spindle towardsthe polesProphase Sister chromatidsPrometaphaseMetaphaseAnaphaseCytokinesis TelophaseInterphase CentrosomeMicrotubule spindle microtubulesattach to kinetochorenuclear envelopereforms 着丝粒（centromere）：位于染色体缢缩部位，是异染色质，是在染色体最后复制的部位。着丝粒在染色体上的位置是确定的。动粒（kinetochore）：位于着丝粒外侧的蛋白质复合体。 The spindle forms differently in plant and animal cells. 植物和动物细胞中的纺锤体的形成是不同的。In animal it is associated with centriole, while in plant and fungal cells spindle formation is associated with regions called microtubule organizing centers. 在动物中，纺锤体的形成与中心粒相关，而在植物和真菌细胞中，纺锤体的形成与微管组织中心相关。 4. Cytokinesis: Partitioning the cytoplasm胞质分裂：细胞质分离 The division of the cell cytoplasm at the end of mitosis is called cytokinesis. 在有丝分裂结束时，细胞质的分裂称为胞质分裂。In animal cells it takes place as a ring of actin filaments contracts around the cell equator, pinching the cell in two. 在动物细胞中，环形的肌动蛋白纤维围绕赤道板收缩，将细胞一分为二。 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.在植物细胞中，被细胞壁包围，胞质分裂涉及到在正在分裂的细胞的赤道板上新的细胞板的形成。 (在植物细胞中，在赤道板形成新的细胞板。 )Cell wall material is then deposited in the region of the cell plate.然后，细胞壁材料在细胞板区域沉积 Operation of the Contractile Ringduring Cytokinesis 5. Meiosis: the basis of sexual reproduction减数分裂：有性生殖的基础 Meiosis is a special form of cell division that takes place in the reproductive organs that produce sex cells. 减数分裂是发生在产生性细胞的生殖器官中的一种特殊的分裂形式。(减数分裂是性细胞分裂的特殊形式。 )Like mitosis, it takes place after DNA replication has occurred and involves two sequential nuclear divisions (meiosis I and meiosis II). 如有丝分裂，它也是发生在DNA复制后，并有两个连续的核分裂。 These division results in four daughter cells, each with half the number of chromosomes of the parent cell. 减数分裂产生4个子细胞，分别含有亲本一半的染色体数。The phenomenon of crossing over during meiosis results in exchange of genetic information between chromosomes. 在减数分裂中的交换现象导致染色体之间的遗传信息交换。 H ence, the homologous chromosome distributed to different progeny cells are not identical.因此，分配到不同子细胞中的同源染色体是不相同的。 As in mitosis two chromatids exist for each chromosome at the beginning of prophase I. 正如在有丝分裂中一样，在前期I开始的时候每个染色体是以两个染色单体形式存在 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.在这个时期，同源染色体通过蛋白质和RNA组成的联会复合体进行联会或配对。The homologous pairs stay together when they align on the metaphase plate.同源染色体对一起成线形排列在中期赤道板上。 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. 但是，与有丝分裂后期不同的是，在后期I，每组染色体的两个染色单体连接在着丝点上并一起移向细胞两极的一极。It is this event that results in the halving of the chromosome number in the four daughter cells that result from meiosis.由此而导致4个子细胞中染色体数减半。 During telophase I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis (the first nuclear division) follows. 在末期I，核被膜围绕着细胞核里的染色体，在大多数物种里第一次胞质分裂紧随其后 The second nuclear division begins with metaphase II, in which the chromosomes in each daughter cell again align on a metaphase plate第二次核分裂开始于分裂中期II ，子细胞中染色体再次重新排列在赤道板上。 The centromeres finally divide, and each sister chromatid moves to one of the poles of the spindle.着丝粒最终分离，每个姊妹染色单体移向两极。The next phase is telophase II, followed again by cytokinesis.下一个时期是末期II，紧接着是第二次胞质分裂。 The result of the entire process is four haploid cells in which parental chromosomes are randomly distributed. 整个减数分裂过程，产生4个亲本染色体随机分配的单倍体细胞。 6. Asexual versus sexual reproduction无性和有性生殖 Mitosis and meiosis, respectively, make simple cell division and sexual reproduction possible .有丝分裂和减数分裂分别可以让简单的细胞分裂和有性生殖成为可能。 Each means of passing on hereditary information has advantages.每种传递遗传信息的方式各有优势。 In asexual reproduction the parent organism gives rise to offspring that are genetic clones of the complement, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction.在无性生殖中，子细胞就是亲本的克隆，保留了亲本的遗传信息，亲本不需要生殖器官的特化，比有性生殖快的多。 A major disadvantage of the asexual mode is that a single catastrophic event or disease may destroy an entire population of genetically identical organisms. 无性生殖的一个主要的劣势是，一个简单灾难性事件或疾病都可能摧毁遗传上一致的整个种群。 A prime benefit of sexual reproduction is that it provides genetic variability and a ready mechanism for the elimination of deleterious mutations. 有性生殖的首要优势是它提供了遗传变异和现存排除有害突变的机制。 It also allows “new” gene forms to arise and spread through populations.它也可以产生新的基因并在种群中传递(蔓延 )。