📚 Meiosis and the Formation of Gametes | 减数分裂与配子的形成
Meiosis is a special type of cell division that produces gametes – sperm and egg cells in animals, and pollen and ovule cells in flowering plants. It halves the chromosome number of a parent cell, which is why it is called a reduction division. This process also creates genetic variation between offspring.
减数分裂是一种特殊的细胞分裂方式，用于产生配子——动物的精子和卵细胞，以及开花植物的花粉和胚珠细胞。它使母细胞的染色体数目减半，因此也被称为“减数分裂”。这一过程还会造成后代之间的遗传差异。
1. Chromosome Numbers: Haploid and Diploid | 染色体数目：单倍体与二倍体
All normal body cells are diploid, meaning they contain two sets of chromosomes, one inherited from each parent. In humans, a diploid cell has 46 chromosomes. This is written as 2n, where n is the number of chromosome pairs.
所有正常体细胞都是二倍体，即它们含有两套染色体，一套来自父亲，一套来自母亲。人类体细胞有46条染色体，记作2n，其中n表示染色体对数。
Gametes are haploid, meaning they contain only one set of chromosomes. In humans, sperm and egg cells each have 23 chromosomes, written as n. When a sperm fuses with an egg during fertilisation, the diploid number is restored.
配子是单倍体，即只含有一套染色体。人类的精子和卵细胞各有23条染色体，记作n。当精子与卵细胞结合时，二倍体数目得以恢复。
Human: 2n = 46 → n = 23
2. Where Does Meiosis Occur? | 减数分裂发生在哪里？
Meiosis only takes place in the reproductive organs. In most animals, it occurs in the testes to produce sperm and in the ovaries to produce egg cells. In flowering plants, meiosis happens inside the anthers to make pollen grains and in the ovules to make embryo sacs.
减数分裂只发生在生殖器官中。在大多数动物体内，睾丸中的减数分裂产生精子，卵巢中的减数分裂产生卵细胞。在开花植物中，花药内发生减数分裂形成花粉粒，胚珠内发生减数分裂形成胚囊。
Because meiosis is limited to these organs, all other cells of the body divide by mitosis for growth and repair.
由于减数分裂仅限于这些器官，身体的其他细胞都通过有丝分裂进行生长和修复。
3. Overview of the Process | 减数分裂过程概述
Before meiosis begins, DNA replicates during the S phase of interphase. Each chromosome now consists of two identical sister chromatids joined by a centromere. Meiosis involves two consecutive divisions: meiosis I and meiosis II.
在减数分裂开始前，DNA在间期S期进行复制。每条染色体此时由两个相同的姐妹染色单体在着丝粒处相连。减数分裂包括连续两次分裂：减数第一次分裂（减数分裂Ⅰ）和减数第二次分裂（减数分裂Ⅱ）。
In meiosis I, homologous chromosomes (one from each parent) pair up, exchange genetic material, and then separate. This halves the chromosome number. In meiosis II, sister chromatids separate, similar to mitosis. The final result is four haploid daughter cells, each genetically distinct from the parent cell.
在减数第一次分裂中，同源染色体（一条来自父方、一条来自母方）配对、交换遗传物质，然后分开，使染色体数目减半。在减数第二次分裂中，姐妹染色单体分开，类似于有丝分裂。最终形成四个单倍体子细胞，每个细胞在遗传上与母细胞不同。
4. Stages of Meiosis I | 减数第一次分裂各阶段
Prophase I: The chromosomes condense and become visible. Homologous chromosomes pair up, forming tetrads. During this stage, non-sister chromatids may exchange segments in a process called crossing over.
前期Ⅰ：染色体螺旋化并变得可见。同源染色体配对，形成四分体。在此阶段，非姐妹染色单体之间可能交换片段，这一过程称为交叉互换。
Metaphase I: The tetrads line up at the equator of the cell. The random orientation of each homologous pair is called independent assortment, which leads to genetic variation.
中期Ⅰ：四分体排列在细胞赤道板上。每一对同源染色体的随机排列称为自由组合，这导致了遗传变异。
Anaphase I: Homologous chromosomes are pulled to opposite poles. The chromosome number is halved because each pole receives only one set of chromosomes.
后期Ⅰ：同源染色体被拉向细胞两极。由于每极只获得一套染色体，染色体数目减半。
Telophase I and Cytokinesis: The cell divides into two cells. Each daughter cell now has haploid chromosome number, but each chromosome still consists of two chromatids.
末期Ⅰ和胞质分裂：细胞分裂成两个子细胞。每个子细胞现在拥有单倍体染色体数目，但每条染色体仍包含两条染色单体。
5. Stages of Meiosis II | 减数第二次分裂各阶段
Prophase II: A new spindle forms. There is no further DNA replication because the chromosomes are already doubled.
前期Ⅱ：新的纺锤体形成。不再进行DNA复制，因为染色体已经复制过。
Metaphase II: Chromosomes line up at the equator individually, with their sister chromatids attached to opposite poles.
中期Ⅱ：染色体在赤道板上排列，姐妹染色单体分别连接向两极的纺锤丝。
Anaphase II: Sister chromatids are separated and move to opposite poles. Each chromatid becomes a single chromosome.
后期Ⅱ：姐妹染色单体分开并移向两极，每条染色单体成为一条染色体。
Telophase II: The cells divide again, producing four haploid gametes in total, each with a single set of chromosomes.
末期Ⅱ：细胞再次分裂，最终共产生四个单倍体配子，每个配子含一套染色体。
6. The Significance of Meiosis: Genetic Variation | 减数分裂的意义：遗传变异
Meiosis is essential for sexual reproduction because it maintains a constant chromosome number from generation to generation. It also increases genetic variation through two key mechanisms.
减数分裂对于有性生殖至关重要，因为它能保持世代之间染色体数目的恒定。它还通过两个关键机制增加遗传变异。
Crossing over: During prophase I, non-sister chromatids exchange equivalent portions of DNA. This creates new combinations of alleles on the same chromosome.
交叉互换：在前期Ⅰ，非姐妹染色单体会交换等长的DNA片段，从而在同一染色体上形成新的等位基因组合。
Independent assortment: During metaphase I, homologous pairs line up randomly at the equator. The way each pair aligns is independent of others, so gametes receive different mixtures of maternal and paternal chromosomes.
自由组合：在中期Ⅰ，同源染色体随机排列在赤道板上。每对同源染色体的取向彼此独立，因此配子会获得母源和父源染色体的不同组合。
These mechanisms mean that no two gametes are genetically identical, which is why siblings (except identical twins) look different from each other.
这些机制意味着没有两个配子在遗传上是完全相同的，这也是为什么兄弟姐妹（同卵双胞胎除外）看起来各不相同。
7. Meiosis vs Mitosis | 减数分裂与有丝分裂的比较
Mitosis and meiosis are both forms of cell division, but they serve different purposes and have different outcomes. The table below summarises the main differences.
有丝分裂和减数分裂都是细胞分裂的形式，但它们的目的和结果不同。下表总结了主要区别。
| Feature | Mitosis | Meiosis |
| Purpose | Growth and repair | Production of gametes |
| Location | All body cells | Published by TutorHao | IGCSE Biology Revision Series | aleveler.com
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