2. MeiosisMeiosis 减数分裂

4. Learning Objectives Definition of MeiosisDefinition of Meiosis Processes of MeiosisProcesses of Meiosis Significance of MeiosisSignificance of Meiosis

5. 减数分裂 (reduction division) 是有性生 殖个体性成熟后，形成生殖细胞过程中 发生的一种特殊的细胞分裂方式。 减数分裂的主要特点是 DNA 复制 1 次， 细胞连续分裂两次，结果形成了染色体 数目减半的生殖细胞。

6. Phases of Meiosis First DivisionFirst Division Prophase I Metaphase I Anaphase I Telophase I Second DivisionSecond Division Prophase II Metaphase II Anaphase II Telophase II

7. 减数分裂的过程 减数第一次分裂 细线期 偶线期 前期Ⅰ 粗线期 双线期 终变期 中期Ⅰ 后期Ⅰ 末期Ⅰ 减数第二次分裂 前期Ⅱ 中期Ⅱ 后期Ⅱ 末期Ⅱ

8. First division : The result of this cell division is a reduction of the number of chromosomes per cell.

11. Interphase The DNA is replicated into two identical copies, just as in mitosis. During interphase, the DNA is not visible. The only thing you could see in a microscope is the nucleolus, an area of intense RNA synthesis.

12. Prophase is quite different to the mitotic prophase; the chromosomes become visible. the chromosomes become visible. They look very long, as they are not yet totally condensed. Their ends are linked to one pole of the nucleus. They look very long, as they are not yet totally condensed. Their ends are linked to one pole of the nucleus. The chromosomes that belong to the same pair come next to each other. The chromosomes that belong to the same pair come next to each other. The chromosomes become thicker and shorter, as they condense more and more. They are linked to each other in pairs. The chromosomes become thicker and shorter, as they condense more and more. They are linked to each other in pairs. At this stage, the 2 chromatids of each chromosome become visible. At this stage, the 2 chromatids of each chromosome become visible. The chromosomes then start to move away from each other, but remain linked at points called chiasmata. The chromosomes then start to move away from each other, but remain linked at points called chiasmata. At this stage, some genetic material can be exchanged between chromosomes : it is now that crossing-over may take place. At this stage, some genetic material can be exchanged between chromosomes : it is now that crossing-over may take place.

20. Metaphase I : the pairs of chromosomes (tetrads) are situated at the equator of the cell. At this stage, the nuclear membrane has disappeared. The centromeres are orientated toward the poles of the cell, and the chromosomes are fully condensed.

21. Anaphase I : the chromosomes (two chromatids each) migrate toward opposite poles. There is a separation of chromosomes, and not of chromatids as in mitosis. Each pole will receive one set of chromosomes of 2 chromatids.

22. The telophase is very short, and often mistaken with prophase II. There is no time for the formation of a new cell membrane, nor for duplication of DNA. The next step starts straight away.

23. Cell division II The second division conserves the number of chromosomes but divides the chromatids.

24. Prophase II: it is very short. Everything is ready. The two centrioles migrate away from each other, and a network of microtubules forms in each daughter cell. The two networks are parallel to each other, perpendicular to the previous one.

25. Metaphase II : the chromosomes are situated on the equator. We therefore see two equators

26. Anaphase II : the chromatids, although present at the first cell division, only separate now. They migrate toward the two opposite poles of the cell.

27. Telophase II: the reconstitution of four daughter cells starts and finishes.

29. 减数分裂的意义 1. 产生单倍体, 单倍体结合成二倍体, 保持人类染色体 数目的相对稳定和人类各种形状在世代遗传中保持 相对稳定。 2. 同源染色体的分离是分离律的细胞学基础。 3. 非同源染色体之间可以随机组合进入同一生殖细胞, 是自由组合律的细胞学基础。 4. 位于同一条染色体上的许多基因都相互连锁, 经过 减数分裂一起进入一个生殖细胞, 同时同源染色体 间的部分交换产生新的连锁关系. 这是连锁与互换 律的细胞学基础。 5. 为人类各种变异提供了细胞学 基础。 6. 减数分裂异常是导致染色体病的重要原因之一。

30. Conclusion : in most cases, the division results in spores or gametes. The cell has given rise to four daughter cells, each with a different set of chromosomes. Each has a set of chromosomes, and will meet another set of chromosomes in another cell in order to constitute a fertilised egg which will give a fully working new individual (plant or animal). Maybe you could think of all the different processes that led to variation in the meiosis...

31. how many chromosomes has the cell? how many pairs?

32. 减数分裂与有丝分裂比较