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所在平台: Udemy |
课程主页: https://www.udemy.com/course/biology-cell-division/
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这是一门关于细胞分裂的课程，涵盖了细胞生命周期。课程首先介绍了细胞理论，即细胞来源于已存在的细胞，而细胞分裂是实现这一过程的关键。然后，课程详细阐述了细胞周期的两个主要阶段： * **分裂间期（Interphase）**：这是为细胞分裂做准备的阶段，包括G1期（生长和代谢）、S期（DNA复制和染色体复制）和G2期（细胞质生长）。 * **有丝分裂（Mitosis, M phase）**：这是实际的细胞分裂阶段，分为四个时期： * **前期（Prophase）**：染色体凝集，中心体移向两极，核膜和核仁消失，纺锤体开始形成。 * **中期（Metaphase）**：染色体排列在赤道板上。 * **后期（Anaphase）**：着丝点分裂，染色单体移向两极。 * **末期（Telophase）**：染色体到达两极，染色体伸长，核膜和核仁重新出现。 细胞分裂不仅包括细胞核分裂（karyokinesis），还包括细胞质分裂（cytokinesis）。有丝分裂是一种“无损分裂”，确保子细胞与母细胞具有相同的染色体数目。 课程还介绍了**减数分裂（Meiosis）**，这是一种发生在将形成配子的二倍体细胞中的特殊分裂方式。减数分裂是“减数分裂”，因为它可以将染色体数目减半，从而在有性生殖中，当两个配子融合时，可以恢复亲本的染色体数目。减数分裂分为两个阶段： * **减数第一次分裂（Meiosis I）**：同源染色体配对形成二价染色体，并发生交叉互换。减数第一次分裂包括一个长的前期（分为细线期、偶线期、粗线期、复线期和终变期），然后是二价染色体在赤道板上的排列（中期I），同源染色体分离并移向两极（后期I），最后是核膜和核仁的出现（末期I）。此时，每个子细胞的染色体数目是亲本的一半。 * **减数第二次分裂（Meiosis II）**：它类似于有丝分裂，在后期II阶段，姐妹染色单体分离。最终，一次减数分裂产生四个单倍体的细胞。 课程最后通过练习题，引导学习者深入理解细胞周期的各个环节，并对比了有丝分裂和减数分裂的区别与联系，以及它们在生命过程中的重要意义。例如，课程探讨了细胞周期时长、细胞质分裂与核分裂的区别、间期的关键事件、静止期（G0期）、有丝分裂为何是无损分裂、特定事件发生的细胞周期阶段、联会、二价染色体、交叉点、动植物细胞质分裂的差异、减数分裂产生子细胞大小的差异、有丝分裂后期与减数第一次分裂后期的区别、有丝分裂与减数分裂的主要区别以及减数分裂的意义。此外，还提出了诸如“是否能没有S期DNA复制的有丝分裂？”、“是否能没有细胞分裂的DNA复制？”等思考题，并要求分析细胞周期中染色体数目（N）和DNA含量（C）的变化。
SUMMARYAccording to the cell theory, cells arise from preexisting cells. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle. The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase - a period of preparation for cell division, and (ii) Mitosis (M phase) - the actual period of cell division. Interphase is further subdivided into G1 , S and G2. G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication. G2 phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Meiosis is divided into two phases - meiosis I and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase I, the nuclear membrane and nucleolus reappear. Meiosis II is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.EXERCISES1. What is the average cell cycle span for a mammalian cell? 2. Distinguish cytokinesis from karyokinesis. 3. Describe the events taking place during interphase. 4. What is Go (quiescent phase) of cell cycle?5. Why is mitosis called equational division? 6. Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator. (ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place. 7. Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer. 8. How does cytokinesis in plant cells differ from that in animal cells? 9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. 10. Distinguish anaphase of mitosis from anaphase I of meiosis. 11. List the main differences between mitosis and meiosis. 12. What is the significance of meiosis? 13. Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur. 14. Can there be mitosis without DNA replication in ‘S' phase? 15. Can there be DNA replication without cell division? 16. Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell (ii) amount of DNA content (C) per cell
