大學(xué)生物專業(yè)英語lessonth-wenkub.com
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【正文】 College of Life Sciences Hubei Normal University 2022/2/10 47 A prime benefit of sexual reproduction is that it provides geic variability and a ready mechanism for the elimination of deleterious mutations. 有性生殖的首要優(yōu)勢是它提供了遺傳變異和現(xiàn)存排除有害突變的機(jī)制 。 College of Life Sciences Hubei Normal University 2022/2/10 43 6. Asexual versus sexual reproduction 無性和有性生殖 College of Life Sciences Hubei Normal University 2022/2/10 44 Mitosis and meiosis, respectively, make simple cell division and sexual reproduction possible . 有絲分裂和減數(shù)分裂分別可以讓簡單的細(xì)胞分裂和有性生殖成為可能 。 College of Life Sciences Hubei Normal University 2022/2/10 41 During telophase I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis (the first nuclear division) follows. 在末期 I， 核被膜圍繞著細(xì)胞核里的染色體 ， 在大多數(shù)物種里第一次 胞質(zhì)分裂緊隨其后 ． The second nuclear division begins with metaphase II, in which the chromosomes in each daughter cell again align on a metaphase plate． 第二次核分裂開始于分裂中期 II ， 子細(xì)胞中染色體再次重新排列在赤道板上 。 College of Life Sciences Hubei Normal University 2022/2/10 37 College of Life Sciences Hubei Normal University 2022/2/10 38 As in mitosis two chromatids exist for each chromosome at the beginning of prophase I. 正如在有絲分裂中一樣 ， 在前期 I開始的時候每個染色體是以兩個染色單體形式存在 ． College of Life Sciences Hubei Normal University 2022/2/10 39 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 plex. 在這個時期 ， 同源染色體通過蛋白質(zhì)和 RNA組成的聯(lián)會復(fù)合體進(jìn)行聯(lián)會或配對 。 ) Like mitosis, it takes place after DNA replication has occurred and involves two sequential nuclear divisions (meiosis I and meiosis II). 如有絲分裂 ， 它也是發(fā)生在 DNA復(fù)制后 ， 并有兩個連續(xù)的核分裂 。 College of Life Sciences Hubei Normal University 2022/2/10 31 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. 在植物細(xì)胞中 ， 被細(xì)胞壁包圍 ， 胞質(zhì)分裂涉及到 在 正在分裂的細(xì)胞的赤道板上 新的細(xì)胞板的形成 。 College of Life Sciences Hubei Normal University 2022/2/10 27 College of Life Sciences Hubei Normal University 2022/2/10 28 The spindle forms differently in plant and animal cells. 植物和動物細(xì)胞中的紡錘體的形成是不同的。 During anaphase the fibers begin to shorten, and the chromatids begin to move apart. 在分裂后期，纖維開始變短，染色單體分離。 During telophase nuclear envelope begin to form around each set of chromosomes, and division of the cytoplasm takes place. 在分裂末期 ， 在每套染色體周圍形成核膜 ， 細(xì)胞質(zhì)分裂發(fā)生 。 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 ， 每個染色體是由高度濃縮的通過著絲粒連接在一起的兩個染色單體組成
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