📚 Meiosis in IGCSE OCR Biology | IGCSE OCR 生物：减数分裂 考点精讲
Meiosis is a fundamental process that produces gametes with half the chromosome number, ensuring genetic continuity and variation in sexually reproducing organisms. In IGCSE OCR Biology, you need to understand the stages, outcomes, and significance of meiosis, as well as how it compares to mitosis. This revision guide covers every key point in detail, with paired English–Chinese explanations to help bilingual learners master the topic.
减数分裂是一个产生染色体数目减半的配子的基本过程，它确保了有性生殖生物的遗传连续性和变异性。在 IGCSE OCR 生物学中，你需要理解减数分裂的阶段、结果与意义，以及它与有丝分裂的区别。本复习指南将详细覆盖每一个考点，并以中英双语解释帮助双语学习者掌握该课题。
1. What is Meiosis? | 什么是减数分裂？
Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically non-identical haploid cells from one diploid parent cell. It occurs only in the reproductive organs to form gametes—sperm and egg cells in animals, pollen and ovules in plants.
减数分裂是一种将染色体数目减半的细胞分裂方式，由一个二倍体亲代细胞产生四个遗传组成不同的单倍体细胞。它只发生在生殖器官中，用以形成配子——动物的精子和卵细胞，植物的花粉和胚珠。
2. Purpose of Meiosis | 减数分裂的目的
The main purposes of meiosis are to produce haploid gametes for sexual reproduction and to create genetic variation among offspring. By halving the chromosome number, fertilisation can restore the diploid number in the zygote. Variation arises from crossing over and independent assortment, which are key events in meiosis.
减数分裂的主要目的是为有性生殖产生单倍体配子，并在后代中制造遗传变异。染色体数目减半后，受精作用可以使合子恢复二倍体数目。变异来源于减数分裂中的关键事件：交叉互换和独立分配。
3. Key Terms: Haploid and Diploid | 关键术语：单倍体与二倍体
Diploid cells (2n) contain two complete sets of chromosomes, one from each parent. In humans, the diploid number is 46. Haploid cells (n) contain a single set of chromosomes; human gametes have 23 chromosomes. Knowing these terms is essential for describing meiosis.
二倍体细胞 (2n) 含有两套完整的染色体，分别来自父本和母本。人类二倍体数目为 46。单倍体细胞 (n) 含有一套染色体；人类配子有 23 条染色体。掌握这些术语是描述减数分裂的基础。
4. Overview of Meiosis: Two Divisions | 减数分裂概览：两次分裂
Meiosis consists of two successive divisions—Meiosis I and Meiosis II—but DNA replicates only once before Meiosis I. Meiosis I separates homologous chromosomes, reducing the chromosome number from 2n to n. Meiosis II separates sister chromatids, similar to mitosis, producing four haploid cells.
减数分裂由两次连续的分裂构成——减数第一次分裂和减数第二次分裂，但 DNA 只在减数第一次分裂前复制一次。减数第一次分裂分离同源染色体，将染色体数从 2n 降至 n。减数第二次分裂分离姐妹染色单体，与有丝分裂相似，产生四个单倍体细胞。
5. Meiosis I: Prophase I – Crossing Over | 减数第一次分裂：前期 I – 交叉互换
During Prophase I, chromosomes condense and homologous chromosomes pair up to form bivalents. Non-sister chromatids of homologous chromosomes can cross over at chiasmata, exchanging segments of DNA. This produces recombinant chromosomes and is a major source of genetic variation.
在前期 I，染色体凝缩，同源染色体配对形成二价体。同源染色体的非姐妹染色单体可在交叉处发生交叉互换，交换 DNA 片段。这产生了重组染色体，是遗传变异的主要来源之一。
6. Meiosis I: Metaphase I – Independent Assortment | 中期 I – 独立分配
Bivalents line up at the equator of the cell, with each pair of homologous chromosomes attached to spindle fibres from opposite poles. The orientation of each bivalent is random—this is independent assortment. It results in different combinations of maternal and paternal chromosomes in the daughter cells.
二价体排列在细胞赤道板上，每对同源染色体与来自相反两极的纺锤丝相连。每个二价体的取向是随机的——这就是独立分配。它使得子细胞中母本和父本染色体出现不同组合。
7. Meiosis I: Anaphase I and Telophase I | 后期 I 和末期 I
In Anaphase I, homologous chromosomes are pulled to opposite poles, while sister chromatids remain attached. This halves the chromosome number. Telophase I forms two haploid nuclei, and cytokinesis typically divides the cytoplasm. The cells are now haploid but still contain double-chromatid chromosomes.
在后期 I，同源染色体被拉向两极，姐妹染色单体仍连在一起。这使得染色体数目减半。末期 I 形成两个单倍体细胞核，胞质分裂通常将细胞质分开。此时细胞为单倍体，但染色体仍由两条姐妹染色单体组成。
8. Meiosis II: The Second Division | 减数第二次分裂
Meiosis II resembles mitosis. In Prophase II, chromosomes recondense. In Metaphase II, chromosomes line up singly on the equator. In Anaphase II, sister chromatids are finally separated to opposite poles. Telophase II and cytokinesis produce four genetically distinct haploid cells.
减数第二次分裂与有丝分裂相似。前期 II 染色体重新凝缩。中期 II 染色体单个排列在赤道板上。后期 II 姐妹染色单体终于被分开，移向两极。末期 II 和胞质分裂产生四个遗传组成不同的单倍体细胞。
9. Chromosome Number Changes | 染色体数量变化
A key diagram to remember is: 2n → (Meiosis I) → n (with double-chromatid chromosomes) → (Meiosis II) → n (single-chromatid chromosomes). The reduction in chromosome number occurs in Meiosis I, while Meiosis II brings the chromosome structure back to a single chromatid per chromosome, ready for fertilisation.
需要记住的关键图示是：2n → (减数第一次分裂) → n（含双姐妹染色单体的染色体）→ (减数第二次分裂) → n（单姐妹染色单体的染色体）。染色体数目的减少发生在减数第一次分裂，而减数第二次分裂将染色体结构恢复为每条染色体一条染色单体，为受精作好准备。
10. Genetic Variation from Meiosis | 减数分裂导致的遗传变异
Two mechanisms in meiosis generate variation: crossing over (recombination of alleles between homologous chromosomes) and independent assortment (random arrangement of bivalents at Metaphase I). Additionally, random fertilisation further increases variation. Together, these processes ensure that no two gametes are genetically identical.
减数分裂中产生变异的两种机制是：交叉互换（同源染色体之间等位基因的重组）和独立分配（中期 I 二价体的随机排列）。此外，随机的受精作用进一步增加了变异。这些过程共同确保了没有任何两个配子在基因上完全相同。
11. Comparison with Mitosis | 与有丝分裂的比较
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| DNA replication | Once before division | Once before Meiosis I |
| Chromosome number of daughter cells | 2n (diploid), identical to parent | n (haploid), half of parent |
| Genetic identity | Daughter cells genetically identical | Daughter cells genetically different |
| Pairing of homologous chromosomes | No pairing | Yes, in Prophase I |
| Crossing over | No | Yes, in Prophase I |
| Number of daughter cells | Two | Four |
| Role | Growth, repair, asexual reproduction | Gamete formation for sexual reproduction |
The table above summarises the essential differences that IGCSE OCR candidates must be able to recall and apply to exam questions. Understanding why meiosis must reduce chromosome number and how it generates variation is critical.
上表总结了 IGCSE OCR 考生必须能够回忆并应用于考试题目的基本区别。理解减数分裂为什么必须减少染色体数目以及它如何产生变异是至关重要的。
12. Gamete Formation in Humans | 人类配子形成
In males, meiosis produces four functional sperm cells from each spermatocyte. In females, meiosis yields one large egg and three small polar bodies, which degenerate. This unequal division ensures the egg retains sufficient cytoplasm to support early development after fertilisation.
在男性中，每个精母细胞通过减数分裂产生四个有功能的精子细胞。在女性中，减数分裂产生一个大卵子和三个小型极体，极体随后退化。这种不等分裂确保卵子保留足够细胞质，以支持受精后的早期发育。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导