【正文】 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(gè)染色體是由高度濃縮的通過(guò)著絲粒連接在一起的兩個(gè)染色單體組成 。 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 43 6. Asexual versus sexual reproduction 無(wú)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(jiǎn)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(kāi)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 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(dòng)物細(xì)胞中的紡錘體的形成是不同的。 Together the G1, S, and G2 phases are called interphase. G1, S, 和 G2一起稱(chēng)為分裂間期 。 Within the nucleus are the chromosomes—tightly coiled strands of DNA and clusters of associated proteins. 細(xì)胞核里的是染色體－－緊密盤(pán)繞成螺旋的 DNA鏈和相關(guān)的成簇蛋白質(zhì) 。 College of Life Sciences Hubei Normal University 2022/2/10 16 The first three include G1, the period of normal metabolism。 College of Life Sciences Hubei Normal University 2022/2/10 25 Mitosis: prophase metaphase anaphase telophase ?chromosome condensation ?mitotic spindle assembly ?nuclear envelope breakdown ?chromosomes align in the central plane ?sister chromatids split and are drawn by the spindle towards the poles Prophase Sister chromatids Prometaphase Metaphase Anaphase Cytokinesis Telophase Interphase Centrosome Microtubule ?spindle microtubules attach to kiochore ?nuclear envelope reforms College of Life Sciences Hubei Normal University 2022/2/10 26 著絲粒（ centromere）：位于染色體縊縮部位，是異染色質(zhì)，是在染色體最后復(fù)制的部位。 The homologous pairs stay together when they align on the metaphase plate. 同源染色體對(duì)一起成線(xiàn)形排列在中期赤道板上 。 College of Life Sciences Hubei Normal University 2022/2/10 46 A major disadvantage of the asexual mode is that a single catastrophic event or