📚 Meiosis | 减数分裂
Meiosis is a specialised form of cell division that halves the chromosome number, producing four genetically non-identical haploid cells from a single diploid parent cell. It is essential for sexual reproduction, generating gametes in animals, spores in plants, and introducing genetic variation through two key mechanisms: crossing over and independent assortment. This article covers the OCR A-Level Biology specification for meiosis, detailing each stage, the sources of variation, comparisons with mitosis, and the consequences of errors such as non-disjunction.
减数分裂是一种特殊的细胞分裂方式，它使染色体数目减半，从一个二倍体亲代细胞产生四个遗传上不同的单倍体细胞。减数分裂对有性生殖至关重要，在动物中形成配子，在植物中产生孢子，并通过交叉互换和独立分配两种关键机制引入遗传变异。本文覆盖OCR A-Level生物大纲中减数分裂的考点，详细讲解各阶段、变异的来源、与有丝分裂的比较以及不分离等错误的后果。
1. Introduction to Meiosis | 减数分裂概述
Meiosis consists of two consecutive divisions, meiosis I and meiosis II, with only one round of DNA replication occurring before meiosis I. The parent cell is diploid (2n), containing two sets of chromosomes. By the end of meiosis, four haploid (n) daughter cells are produced, each with half the genetic material. Unlike mitosis, which produces genetically identical cells for growth and repair, meiosis generates genetic diversity while maintaining the chromosome number across generations when gametes fuse during fertilisation.
减数分裂由连续两次分裂组成，即减数第一次分裂和减数第二次分裂，仅在减数第一次分裂前进行一次DNA复制。亲代细胞是二倍体（2n），含有两套染色体。减数分裂结束时，产生四个单倍体（n）子细胞，每个子细胞拥有一半的遗传物质。与有丝分裂不同，有丝分裂为生长和修复产生遗传上完全相同的细胞，而减数分裂则产生遗传多样性，同时通过配子受精维持世代间染色体数目的恒定。
2. The Stages of Meiosis I | 减数第一次分裂阶段
Meiosis I is the reduction division, where homologous chromosomes are separated. It is divided into prophase I, metaphase I, anaphase I and telophase I. During this division, the chromosome number is halved from diploid to haploid. The key events of meiosis I set the stage for genetic reshuffling. Homologous pairs, each consisting of two sister chromatids, align and then segregate into two daughter nuclei, each receiving one chromosome from each pair.
减数第一次分裂是减数分裂，此时同源染色体发生分离。它分为前期I、中期I、后期I和末期I。在这个过程中，染色体数目由二倍体减半为单倍体。减数第一次分裂的关键事件为遗传重组奠定了基础。每条由两条姐妹染色单体组成的同源染色体对排列后，分离进入两个子细胞核，每个子核得到每对染色体中的一条。
3. Prophase I: Crossing Over | 前期I：交叉互换
Prophase I is the longest stage of meiosis and is subdivided into leptotene, zygotene, pachytene, diplotene and diakinesis. During zygotene, homologous chromosomes pair up in a process called synapsis, forming bivalents held together by the synaptonemal complex. In pachytene, crossing over occurs: non-sister chromatids from homologous chromosomes break and exchange corresponding segments of DNA at points called chiasmata. This recombination produces chromatids with new combinations of alleles, a major source of genetic variation. By diplotene, the synaptonemal complex disassembles, and homologous chromosomes remain connected only at chiasmata.
前期I是减数分裂中最长的阶段，细分为细线期、偶线期、粗线期、双线期和终变期。在偶线期，同源染色体通过联会过程配对，形成由联会复合体连接的二价体。在粗线期发生交叉互换：同源染色体中的非姐妹染色单体断裂并在称为交叉的点上交换相应的DNA片段。这种重组产生了带有新等位基因组合的染色单体，是遗传变异的主要来源。到双线期，联会复合体解体，同源染色体仅在交叉处保持连接。
4. Metaphase I and Independent Assortment | 中期I与独立分配
In metaphase I, bivalents align on the metaphase plate with their centromeres oriented towards opposite spindle poles. The orientation of each homologous pair is random: the maternal and paternal chromosomes of each pair can face either pole independently of other pairs. This independent assortment of chromosomes results in a vast number of possible combinations in the daughter cells. For an organism with a diploid number of 2n, the number of possible gamete combinations from independent assortment alone is 2n (e.g., humans: 223 ≈ 8.4 million). Spindle fibres from the poles attach to the centromeres of each homologous chromosome, preparing for segregation.
在中期I，二价体排列在赤道板上，其着丝粒朝向相反的两极。每条同源染色体对的取向是随机的：母方和父方染色体可以独立于其他染色体对而朝向任意一极。这种染色体的独立分配导致子细胞中出现大量可能的组合。对于一个二倍体数为2n的生物，仅独立分配就可能产生2n种配子组合（例如人类：223 ≈ 840万）。来自两极的纺锤丝附着在各条同源染色体的着丝粒上，为分离做好准备。
5. Anaphase I and Telophase I | 后期I和末期I
During anaphase I, the homologous chromosomes are pulled apart as the spindle fibres shorten. Crucially, sister chromatids remain attached at their centromeres; only the bivalent splits. Each chromosome, still composed of two chromatids, moves to opposite poles. This reductional segregation halves the chromosome number. In telophase I, nuclear envelopes may temporarily re-form around the two haploid sets of chromosomes, and cytokinesis divides the cytoplasm, yielding two daughter cells. In many organisms, meiosis II follows immediately without full decondensation of chromosomes.
在后期I，随着纺锤丝缩短，同源染色体被拉开。关键在于，姐妹染色单体仍在着丝粒处相连；只有二价体分离。每条仍由两条染色单体组成的染色体移向两极。这种减数分离使染色体数目减半。在末期I，核膜可能暂时在两组单倍体染色体周围重新形成，胞质分裂将细胞质分开，产生两个子细胞。在许多生物中，减数第二次分裂紧接着发生，染色体并不完全解螺旋。
6. Meiosis II: The Second Division | 减数第二次分裂
Meiosis II resembles a mitotic division but without prior DNA replication. It separates sister chromatids, resulting in four haploid nuclei. The stages are prophase II, metaphase II, anaphase II and telophase II. In metaphase II, chromosomes align singly on the metaphase plate, with spindle fibres attaching to both sides of the centromere. During anaphase II, the centromere splits, and sister chromatids are pulled to opposite poles, now considered individual chromosomes. Telophase II and cytokinesis produce four genetically distinct haploid cells, each with one copy of each chromosome and a recombined set of alleles.
减数第二次分裂类似有丝分裂，但之前没有DNA复制。它分离姐妹染色单体，最终产生四个单倍体核。阶段包括前期II、中期II、后期II和末期II。在中期II，染色体单独排列在赤道板上，纺锤丝附着在着丝粒的两侧。在后期II，着丝粒分裂，姐妹染色单体被拉向两极，此时它们被视为独立的染色体。末期II和胞质分裂产生四个遗传上不同的单倍体细胞，每个细胞拥有每条染色体的一个拷贝和一套重组的等位基因。
Chromosome number change: 2n → n (meiosis I) → n (meiosis II) | 染色体数目变化：2n → n（减数I）→ n（减数II）
7. Genetic Variation through Meiosis | 减数分裂导致的遗传变异
Two principal mechanisms generate genetic variation during meiosis: crossing over and independent assortment. Crossing over in prophase I recombines linked genes, creating chromatids with new allele combinations. Independent assortment in metaphase I shuffles maternal and paternal chromosomes, producing diverse combinations of unlinked genes. Additionally, random fertilisation multiplies this variation: any gamete can fuse with any gamete from the other parent. For humans, the theoretical number of genetically different zygotes exceeds 70 trillion, ignoring further diversity from crossing over. These processes underpin the variation observed in sexually reproducing populations and are essential for evolution by natural selection.
减数分裂过程中产生遗传变异的两种主要机制是交叉互换和独立分配。前期I的交叉互换重组了连锁基因，创造了带有新等位基因组合的染色单体。中期I的独立分配打乱了母方和父方染色体的组合，产生了非连锁基因的多样组合。此外，随机受精使这种变异倍增：任何一个配子都可以与另一方亲本的任何一个配子融合。对人类而言，即使不考虑交叉互换带来的额外多样性，理论上遗传上不同的受精卵数目也超过70万亿。这些过程支撑了有性生殖群体中观察到的变异，并对自然选择驱动的进化至关重要。
8. Meiosis vs. Mitosis | 减数分裂与有丝分裂的比较
Although both are forms of nuclear division, meiosis and mitosis differ fundamentally in purpose, process and outcomes. The table below summarises the key differences. Mitosis maintains the diploid chromosome number in somatic cells, while meiosis halves it to produce gametes. Meiosis involves two divisions, pairing of homologues, crossing over, and independent assortment, none of which occur in mitosis.
尽管两者都是核分裂的形式，减数分裂和有丝分裂在目的、过程和结果上存在根本区别。下表总结了关键差异。有丝分裂在体细胞中维持二倍体染色体数目，而减数分裂将其减半以产生配子。减数分裂涉及两次分裂、同源染色体配对、交叉互换和独立分配，而这些在有丝分裂中都不发生。
| Feature 特征 |
Meiosis 减数分裂 |
Mitosis 有丝分裂 |
|---|---|---|
| Number of divisions 分裂次数 |
Two 两次 |
One 一次 |
| Chromosome number 染色体数目 |
Halved (2n → n) 减半 |
Maintained (2n → 2n) 维持 |
| Genetic variation 遗传变异 |
High (crossing over, independent assortment) 高（交叉互换，独立分配） |
None (clonal) 无（克隆） |
| Homologous pairing 同源配对 |
Yes (bivalents form) 是（形成二价体） |
No 否 |
| Daughter cells 子细胞 |
Four haploid, genetically distinct 四个单倍体，遗传不同 |
Two diploid, genetically identical 两个二倍体，遗传相同 |
9. Errors in Meiosis: Non-disjunction | 减数分裂错误：不分离
Non-disjunction occurs when chromosomes fail to separate properly during anaphase I or anaphase II. In meiosis I non-disjunction, homologous chromosomes do not segregate, so one daughter cell receives both copies and the other none. In meiosis II non-disjunction, sister chromatids fail to separate, leading to gametes with an extra or missing chromosome. Fertilisation of such gametes results in aneuploidy, an abnormal number of chromosomes. A well-known example is trisomy 21 (Down syndrome), caused by an extra copy of chromosome 21. Other examples include Turner syndrome (monosomy X) and Klinefelter syndrome (XXY). The risk of non-disjunction increases with maternal age, particularly for chromosome 21.
不分离发生在后期I或后期II染色体未能正常分离时。在减数第一次分裂不分离中，同源染色体不分开，导致一个子细胞得到两条染色体，另一个子细胞则没有。在减数第二次分裂不分离中，姐妹染色单体未能分离，形成多一条或少一条染色体的配子。此类配子受精后会导致非整倍体，即染色体数目异常。一个著名的例子是唐氏综合征（21三体），由额外的一条21号染色体引起。其他例子包括特纳综合征（X单体）和克氏综合征（XXY）。不分离的风险随母亲年龄增加，特别是对于21号染色体。
10. Significance of Meiosis | 减数分裂的意义
Meiosis is fundamental to sexual reproduction. It ensures that each gamete receives precisely one copy of each chromosome, so that at fertilisation the diploid number is restored. This maintains a constant chromosome number across generations. Moreover, the genetic variation generated by meiosis and fertilisation provides the raw material for natural selection, enabling populations to adapt to changing environments. In plants, meiosis produces spores that develop into the haploid gametophyte generation, a key feature of alternation of generations. Understanding meiosis also underpins applied topics such as breeding programmes, genetic screening, and the study of chromosomal abnormalities.
减数分裂对性生殖至关重要。它保证每个配子精确地获得每条染色体的一个拷贝，这样在受精时二倍体数目得以恢复，从而维持世代间染色体数目的恒定。此外，减数分裂和受精产生的遗传变异为自然选择提供了原材料，使种群能够适应变化的环境。在植物中，减数分裂产生孢子，孢子发育成单倍体配子体世代，这是世代交替的一个重要特征。理解减数分裂也为育种计划、遗传筛查和染色体异常研究等应用主题奠定了基础。
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