📚 Cell Division in IB and WJEC Biology | IB 和 WJEC 生物：细胞分裂考点精讲
Cell division is the process by which a parent cell divides into two or more daughter cells, serving as the basis for growth, repair, and reproduction in living organisms. In both IB and WJEC specifications, you are expected to understand the detailed stages of mitosis and meiosis, the regulation of the cell cycle, and the significance of these processes in genetic variation and disease. This article will break down every key concept with clear explanations and paired bilingual content to help you master the topic.
细胞分裂是亲代细胞分裂成两个或多个子细胞的过程，是生物体生长、修复和繁殖的基础。在 IB 和 WJEC 考试大纲中，你需要详细理解有丝分裂和减数分裂的各个阶段、细胞周期的调控，以及这些过程在遗传变异和疾病中的意义。本文将用清晰的双语对照逐一解析每个核心概念，帮助你彻底掌握这一主题。
1. The Cell Cycle and Its Phases | 细胞周期及其分期
The cell cycle consists of interphase (G1, S, G2) and the mitotic phase (M phase), which includes mitosis and cytokinesis. Interphase is not a resting stage; it is a period of intense metabolic activity where the cell grows and DNA is replicated.
细胞周期包括间期（G1 期、S 期、G2 期）和有丝分裂期（M 期），后者包含有丝分裂和胞质分裂。间期并非休息阶段，而是细胞生长和 DNA 复制的代谢旺盛期。
During G1 (Gap 1), the cell synthesises proteins and organelles, increasing in size. At the G1 checkpoint, the cell assesses whether conditions are favourable for division. If not, it may enter a non-dividing state called G0.
在 G1 期（第一个间隔期），细胞合成蛋白质和细胞器，体积增大。在 G1 检查点，细胞评估分裂条件是否有利；若不利，细胞可能进入不分裂的 G0 期。
The S (synthesis) phase is when DNA replication occurs, producing two identical copies of each chromosome, called sister chromatids, held together at the centromere. The centrosome also duplicates.
S 期（合成期）是 DNA 复制发生的阶段，产生每条染色体的两个相同拷贝，称为姐妹染色单体，由着丝粒相连。中心体也在此期复制。
In G2 (Gap 2), the cell continues to grow and synthesises proteins necessary for mitosis, such as tubulin for spindle fibres. The G2 checkpoint ensures all DNA has been replicated without damage.
在 G2 期（第二个间隔期），细胞继续生长并合成有丝分裂所需的蛋白质，如组成纺锤丝的微管蛋白。G2 检查点确保所有 DNA 已准确复制且无损伤。
2. Mitosis: An Overview | 有丝分裂概述
Mitosis is the division of the nucleus that produces two genetically identical daughter nuclei. It is essential for growth, tissue repair, and asexual reproduction. The process is divided into prophase, metaphase, anaphase, and telophase (PMAT).
有丝分裂是细胞核的分裂，产生两个遗传上完全相同的子细胞核，对生长、组织修复和无性繁殖至关重要。该过程分为前期、中期、后期和末期（PMAT）。
Although mitosis is a continuous process, it is helpful to describe it in stages. Cytokinesis, the division of the cytoplasm, usually overlaps with telophase.
虽然有丝分裂是连续的过程，但分阶段描述更便于理解。胞质分裂（细胞质的分裂）通常与末期重叠发生。
3. Prophase and Metaphase | 前期和中期
In prophase, chromatin condenses into visible chromosomes, each consisting of two sister chromatids. The nuclear envelope breaks down, and the nucleolus disappears. The two centrosomes migrate to opposite poles of the cell, and spindle fibres begin to form.
在前期，染色质凝缩成可见的染色体，每条染色体包含两个姐妹染色单体。核膜解体，核仁消失。两个中心体向细胞两极移动，纺锤丝开始形成。
In metaphase, chromosomes align along the metaphase plate (the equator of the cell). Each chromosome is attached to spindle fibres from both poles via its kinetochore, a protein structure at the centromere. This alignment ensures accurate segregation.
在中期，染色体排列在赤道板（细胞的中央平面）上。每条染色体通过着丝粒上的动粒（着丝点）蛋白结构分别连接来自两极的纺锤丝。这种排列保证了分离的精确性。
4. Anaphase, Telophase, and Cytokinesis | 后期、末期和胞质分裂
During anaphase, the sister chromatids are separated and pulled to opposite poles as the spindle fibres shorten. Each chromatid is now considered an individual chromosome. The cell elongates, and the poles move further apart.
在后期，姐妹染色单体分离，并随着纺锤丝缩短被拉向两极。此时每条染色单体成为独立的染色体。细胞伸长，两极间距增大。
Telophase sees the arrival of chromosomes at the poles, where they decondense back into chromatin. New nuclear envelopes form around each set of chromosomes, and nucleoli reappear. The spindle fibres disassemble.
在末期，染色体到达两极，并解螺旋恢复为染色质。每组染色体周围形成新的核膜，核仁重新出现。纺锤丝解体。
Cytokinesis differs between animal and plant cells. In animal cells, a cleavage furrow forms and pinches the cell into two. In plant cells, a cell plate forms at the equator, eventually becoming a new cell wall.
胞质分裂在动植物细胞中不同。动物细胞通过形成分裂沟将细胞缢裂为二；植物细胞在赤道板处形成细胞板，最终发育成新的细胞壁。
5. Regulation of the Cell Cycle: Cyclins and CDKs | 细胞周期的调控：周期蛋白和 CDK
The cell cycle is tightly controlled by a family of proteins called cyclins and cyclin-dependent kinases (CDKs). Cyclin levels fluctuate during the cycle, while CDK levels remain relatively constant. When a cyclin binds to its corresponding CDK, the complex becomes active and phosphorylates target proteins that drive the cell past checkpoints.
细胞周期受到周期蛋白和周期蛋白依赖性激酶（CDK）的严密调控。周期蛋白的水平在周期中波动，而 CDK 水平相对恒定。当周期蛋白与相应 CDK 结合后，该复合物被激活，能磷酸化靶蛋白，推动细胞通过检查点。
For instance, the G1/S cyclin–CDK complex pushes the cell from G1 into S phase, while the M-phase cyclin–CDK triggers the events of mitosis. The anaphase-promoting complex (APC/C) marks securin for destruction, allowing separase to cleave cohesin and trigger chromatid separation.
例如，G1/S 期周期蛋白–CDK 复合物推动细胞从 G1 期进入 S 期，而 M 期周期蛋白–CDK 触发有丝分裂事件。后期促进复合物（APC/C）标记保障蛋白（securin）予以降解，使分离酶能够切割粘连蛋白，从而触发染色单体分离。
6. Meiosis: Reduction Division | 减数分裂：减数性分裂
Meiosis is a specialised type of cell division that produces gametes (sperm and egg cells) with half the chromosome number. It involves two consecutive divisions, meiosis I and meiosis II, resulting in four haploid cells. Meiosis introduces genetic variation through independent assortment and crossing over.
减数分裂是一种特殊的分裂方式，产生染色体数目减半的配子（精子和卵细胞）。它包括两次连续分裂，即减数第一次分裂和减数第二次分裂，最终形成四个单倍体细胞。减数分裂通过自由组合和交叉互换引入遗传变异。
Unlike mitosis, meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids, mirroring a mitotic division.
与有丝分裂不同，减数第一次分裂分离同源染色体，而减数第二次分裂分离姐妹染色单体，类似于有丝分裂。
7. Prophase I and Crossing Over | 前期 I 和交叉互换
Prophase I is a lengthy and complex stage divided into leptotene, zygotene, pachytene, diplotene, and diakinesis. During zygotene, homologous chromosomes pair up in a process called synapsis, forming bivalents (tetrads). The synaptonemal complex holds them together.
前期 I 是一个漫长而复杂的阶段，可细分为细线期、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配对，形成二价体（四分体）。联会复合体将它们维系在一起。
Crossing over occurs during pachytene, where non-sister chromatids of homologous chromosomes break and exchange corresponding segments at points called chiasmata. This recombination creates new combinations of alleles, a key source of genetic variation.
交叉互换发生在粗线期，同源染色体的非姐妹染色单体在称为交叉点的位置断裂并交换相应片段。这种重组创造了新的等位基因组合，是遗传变异的关键来源。
8. Metaphase I to Telophase I | 中期 I 至末期 I
In metaphase I, bivalents align along the metaphase plate, with spindle fibres attaching to the kinetochores of each homologous chromosome from opposite poles. The random orientation of each bivalent is called independent assortment, generating 2n possible combinations (where n is the haploid number).
在中期 I，二价体排列在赤道板上，纺锤丝从两极分别连接到每个同源染色体的动粒上。每个二价体的随机取向称为自由组合，能产生 2n 种可能的组合（n 为单倍体染色体数）。
Anaphase I sees homologous chromosomes separated and pulled to opposite poles, while sister chromatids remain attached at the centromere. This reductional division halves the chromosome number.
在后期 I，同源染色体分离并被拉向两极，姐妹染色单体仍由着丝粒相连。这种减数性分裂使染色体数目减半。
Telophase I and cytokinesis produce two haploid cells, but each chromosome still consists of two sister chromatids. Often, nuclei do not fully reform, and the cells proceed directly to meiosis II.
末期 I 和胞质分裂产生两个单倍体细胞，但每条染色体仍由两个姐妹染色单体组成。核膜通常不重新完全形成，细胞直接进入减数第二次分裂。
9. Meiosis II and the Final Result | 减数第二次分裂及最终结果
Meiosis II is essentially a mitotic division without DNA replication. In prophase II, chromosomes condense again; in metaphase II, they align on the metaphase plate; in anaphase II, sister chromatids separate; and in telophase II, nuclei reform. Cytokinesis follows, yielding four haploid cells.
减数第二次分裂本质上是一次没有 DNA 复制的有丝分裂。在前期 II，染色体再次凝缩；在中期 II，染色体排列在赤道板；在后期 II，姐妹染色单体分离；在末期 II，核膜重新形成。随后进行胞质分裂，产生四个单倍体细胞。
The four resulting cells are genetically distinct due to crossing over and independent assortment. In animals, these cells develop into gametes; in plants, they form spores.
由于交叉互换和自由组合，所产生的四个细胞在遗传上各不相同。在动物中，这些细胞发育为配子；在植物中则形成孢子。
10. Meiosis and Genetic Variation | 减数分裂与遗传变异
Genetic variation arises from three main mechanisms in meiosis: crossing over during prophase I, independent assortment during metaphase I, and the random fusion of gametes during fertilisation. These processes contribute to the diversity within a species and are fundamental to evolution.
减数分裂中的遗传变异主要源自三种机制：前期 I 的交叉互换、中期 I 的自由组合，以及受精时配子的随机融合。这些过程促进了物种内部的多样性，是进化的基础。
Crossing over can occur multiple times along the same chromosome, giving rise to recombinant chromatids that carry a mix of maternal and paternal alleles. Independent assortment means that the maternal and paternal chromosomes are distributed randomly to daughter cells.
交叉互换在同一条染色体上可多次发生，产生同时携带母本和父本等位基因的重组染色单体。自由组合意味着母源和父源染色体随机分配到子细胞中。
The number of possible gamete combinations is 223 in humans (over 8 million), not accounting for additional variation from crossing over. When random fertilisation is included, the potential genetic diversity is enormous.
在人类中，配子组合的可能数为 223（超过八百万），这还没有计入交叉互换带来的额外变异。如果再加上随机受精，潜在的遗传多样性极为庞大。
11. Errors in Cell Division: Non-disjunction and Cancer | 细胞分裂错误：不分离与癌症
Non-disjunction is the failure of chromosomes to separate properly during anaphase of meiosis I or II, resulting in gametes with an abnormal number of chromosomes (aneuploidy). For example, trisomy 21 (Down syndrome) is caused by an extra copy of chromosome 21.
不分离是指减数第一次或第二次分裂后期染色体未能正常分开，导致配子染色体数目异常（非整倍体）。例如，21 三体综合征（唐氏综合征）就是由多了一条 21 号染色体引起的。
Non-disjunction can also occur in mitosis, leading to mosaicism. Age of the mother is a risk factor for meiotic non-disjunction in humans.
不分离也可能发生在有丝分裂中，导致嵌合体。母亲年龄是人类减数分裂不分离的一个风险因素。
Uncontrolled cell division can lead to cancer. Mutations in proto-oncogenes (which code for proteins that promote cell division) can turn them into oncogenes, while mutations in tumour suppressor genes (like p53, which normally halt the cycle or trigger apoptosis) remove critical brakes on division. The cell cycle checkpoints fail, and tumours develop.
细胞分裂失控可导致癌症。原癌基因（编码促进细胞分裂的蛋白）突变可转变为癌基因，而肿瘤抑制基因（如 p53，通常能暂停细胞周期或触发凋亡）的突变则移除了抑制分裂的关键刹车。细胞周期检查点失效，导致肿瘤形成。
12. Comparing Mitosis and Meiosis | 有丝分裂与减数分裂比较
| Feature / 特征 | Mitosis / 有丝分裂 | Meiosis / 减数分裂 |
|---|---|---|
| Number of divisions / 分裂次数 | One / 一次 | Two / 两次 |
| Daughter cell chromosome number / 子细胞染色体数目 | Diploid (2n) – identical to parent / 二倍体 (2n) – 与亲代相同 | Haploid (n) – half the parent / 单倍体 (n) – 亲代的一半 |
| Genetic variation / 遗传变异 | None (clones) / 无（克隆） | High – crossing over, independent assortment / 高 – 交叉互换、自由组合 |
| Homologous pairing / 同源配对 | No / 无 | Yes, in prophase I / 有，发生在前期 I |
| Function / 功能 | Growth, repair, asexual reproduction / 生长、修复、无性繁殖 | Gamete formation, genetic diversity / 配子形成、遗传多样性 |
Understanding these differences is critical for both IB and WJEC exam questions, which often ask you to compare and contrast the processes or explain the significance of each.
理解这些差异对 IB 和 WJEC 考试至关重要，题目经常要求你比较和对比这两种过程，或解释各自的意义。
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