📚 A-Level WJEC Biology: Cell Division – Key Revision Notes | A-Level WJEC 生物：细胞分裂 考点精讲
Cell division is a fundamental process in all living organisms, essential for growth, repair, and reproduction. For WJEC A-Level Biology, a deep understanding of mitosis, meiosis, the cell cycle, and their regulation is required. This article summarises the key concepts, common exam pitfalls, and comparative details you need to master.
细胞分裂是所有生物体的基本过程，对于生长、修复和繁殖至关重要。在 WJEC A-Level 生物学中，需要深入理解有丝分裂、减数分裂、细胞周期及其调控。本文总结了必须掌握的关键概念、常见考试陷阱以及比较细节。
1. The Cell Cycle: An Overview | 细胞周期概述
The cell cycle is the ordered series of events that lead to cell growth and division into two daughter cells. In eukaryotic cells, it consists of interphase (G1, S, G2) and the mitotic (M) phase, which includes mitosis and cytokinesis.
细胞周期是一系列有序事件，导致细胞生长并分裂成两个子细胞。在真核细胞中，它由间期（G1、S、G2 期）和分裂期（M 期）组成，其中 M 期包括有丝分裂和胞质分裂。
Interphase accounts for about 90% of the cell cycle. During this time, the cell grows, carries out its normal metabolic functions, replicates its DNA, and prepares for division. The M phase is relatively short and is when nuclear and cytoplasmic division occur.
间期约占整个细胞周期的 90%。在此期间，细胞生长、执行正常代谢功能、复制 DNA 并准备分裂。M 期相对较短，是核分裂和胞质分裂发生的时期。
Cells that are not actively dividing may exit the cycle and enter a non-dividing state called G0. Neurons and skeletal muscle cells are typical examples of permanent G0 cells.
不活跃分裂的细胞可能退出周期，进入非分裂状态，称为 G0 期。神经元和骨骼肌细胞是永久处于 G0 期的典型例子。
2. Interphase: Preparing for Division | 间期：为分裂做准备
Interphase is divided into three stages: G1, S, and G2. Each stage has specific molecular events that must be completed accurately for successful division.
间期分为三个阶段：G1 期、S 期和 G2 期。每个阶段都有特定的分子事件，必须精确完成才能成功分裂。
- G1 phase: the cell grows in size, synthesises proteins and organelles, and carries out its specialised functions. A key checkpoint at the end of G1 (the restriction point) assesses whether conditions are favourable for division.
- G1 期：细胞体积增大，合成蛋白质和细胞器，并执行其特化功能。G1 期末的关键检查点（限制点）评估条件是否有利于分裂。
- S phase: DNA replication occurs, producing two identical sister chromatids for each chromosome. The centrosome also duplicates.
- S 期：发生 DNA 复制，每条染色体产生两条相同的姐妹染色单体。中心体也进行复制。
- G2 phase: the cell continues to grow and synthesises proteins, including tubulin for spindle fibre formation. A G2 checkpoint ensures all DNA has been replicated without damage.
- G2 期：细胞继续生长并合成蛋白质，包括用于形成纺锤体的微管蛋白。G2 检查点确保所有 DNA 都已复制且无损伤。
3. Stages of Mitosis | 有丝分裂阶段
Mitosis is a continuous process classically divided into four stages: prophase, metaphase, anaphase, and telophase. It produces two genetically identical daughter nuclei.
有丝分裂是一个连续过程，通常划分为四个阶段：前期、中期、后期和末期。它产生两个遗传上相同的子核。
During prophase, chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere. The nucleolus disappears, the nuclear envelope breaks down, and the mitotic spindle begins to form from the centrosomes.
在前期，染色质凝缩为可见的染色体，每条染色体由两条在着丝粒处相连的姐妹染色单体组成。核仁消失，核膜解体，由中心体开始形成有丝分裂纺锤体。
In metaphase, chromosomes align at the metaphase plate (equator) of the cell, guided by spindle fibres attaching to the centromeres. This alignment ensures that each daughter cell will receive one copy of each chromosome.
在中期，染色体在纺锤丝的引导下排列在细胞的赤道板（中期板）上。这种排列确保每个子细胞将获得每条染色体的一个拷贝。
Anaphase begins when the centromeres split, allowing sister chromatids to separate and be pulled to opposite poles by the shortening of spindle fibres. The cell elongates.
后期始于着丝粒分裂，使姐妹染色单体分离，并由缩短的纺锤丝拉向细胞两极。细胞拉长。
In telophase, the separated chromatids decondense, new nuclear envelopes form around each set, and nucleoli reappear. The spindle fibres disassemble.
在末期，分离的染色单体解凝缩，每组染色体周围形成新的核膜，核仁重新出现。纺锤丝解体。
4. Cytokinesis: Dividing the Cytoplasm | 胞质分裂：细胞质的分裂
Cytokinesis is the division of the cytoplasm to form two genetically identical daughter cells. The mechanism differs between animal and plant cells.
胞质分裂是细胞质分裂形成两个遗传相同子细胞的过程。动物细胞和植物细胞的机制不同。
In animal cells, a cleavage furrow forms as a ring of actin and myosin microfilaments contracts around the cell equator, pinching the cell into two.
在动物细胞中，由肌动蛋白和肌球蛋白微丝组成的收缩环在细胞赤道处收缩，形成分裂沟，将细胞缢裂为二。
In plant cells, vesicles derived from the Golgi apparatus move to the equatorial plane and fuse to form a cell plate, which develops into a new cell wall and cell membrane separating the daughter cells.
在植物细胞中，来自高尔基体的囊泡移至赤道面并融合，形成细胞板，进而发育成新的细胞壁和细胞膜，分隔子细胞。
5. Meiosis I: Reductional Division | 减数第一次分裂：减数分裂
Meiosis produces four genetically distinct haploid gametes from one diploid cell. Meiosis I separates homologous chromosomes, reducing the chromosome number by half.
减数分裂由一个二倍体细胞产生四个遗传上不同的单倍体配子。减数第一次分裂分离同源染色体，使染色体数目减半。
Prophase I is further subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis. The key events are synapsis, formation of bivalents, and crossing over between non-sister chromatids at chiasmata, resulting in genetic recombination.
前期 I 进一步细分为细线期、偶线期、粗线期、双线期和终变期。关键事件是联会、二价体形成以及非姐妹染色单体在交叉处发生交叉互换，导致遗传重组。
In metaphase I, bivalents align randomly on the metaphase plate. This independent assortment of maternal and paternal chromosomes is a major source of genetic variation.
在中期 I，二价体随机排列在赤道板上。这种母源和父源染色体的独立分配是遗传变异的主要来源之一。
Anaphase I separates whole chromosomes (still composed of two sister chromatids) to opposite poles, reducing the chromosome number from 2n to n.
后期 I 将完整的染色体（仍由两条姐妹染色单体组成）拉向两极，染色体数目从 2n 减为 n。
Telophase I and cytokinesis produce two haploid daughter cells, which immediately prepare for the second meiotic division.
末期 I 和胞质分裂产生两个单倍体子细胞，它们立即准备进行第二次减数分裂。
6. Meiosis II: Equational Division | 减数第二次分裂：均等分裂
Meiosis II resembles mitosis but without a preceding S phase. It separates sister chromatids, producing four haploid nuclei.
减数第二次分裂类似有丝分裂，但没有之前的 S 期。它分离姐妹染色单体，产生四个单倍体核。
In prophase II, chromosomes recondense, and spindles form in both haploid cells. If nuclear envelopes formed, they break down again.
在前期 II，染色体再次凝缩，并在两个单倍体细胞中形成纺锤体。如果形成了核膜，它们会再次解体。
Metaphase II aligns individual chromosomes at the metaphase plate, with spindle fibres attaching to the centromeres.
中期 II 中，各条染色体排列在赤道板上，纺锤丝附着于着丝粒。
Anaphase II separates sister chromatids at the centromere, pulling them to opposite poles.
后期 II 在着丝粒处分离姐妹染色单体，将它们拉向两极。
Telophase II and cytokinesis result in four genetically non-identical haploid cells, each containing a unique combination of alleles.
末期 II 和胞质分裂产生四个遗传上不同的单倍体细胞，每个都含有独特的等位基因组合。
7. Sources of Genetic Variation | 遗传变异的来源
Meiosis generates genetic diversity through two main mechanisms, which are frequently examined. These ensure that offspring are genetically unique.
减数分裂通过两种主要机制产生遗传多样性，这些机制经常考查。它们确保后代在遗传上是独特的。
Crossing over occurs during prophase I when homologous chromosomes exchange segments of DNA. This produces new combinations of alleles on the same chromosome, known as recombinant chromatids.
交叉互换发生在前期 I，同源染色体交换 DNA 片段。这使同一条染色体上的等位基因产生新的组合，称为重组染色单体。
Independent assortment refers to the random orientation of homologous chromosome pairs on the metaphase I spindle. For humans, with 23 pairs of chromosomes, this alone can produce 2²³ (over 8 million) different gamete combinations.
独立分配是指同源染色体对在中期 I 纺锤体上的随机排列方向。对于人类，有 23 对染色体，仅此一项就能产生 2²³（超过 800 万）种不同的配子组合。
Random fertilisation further multiplies the variation, as any sperm can fuse with any egg, producing an astronomically large number of possible zygote genotypes.
随机受精进一步倍增了变异，因为任何精子都可能与任何卵细胞融合，产生数量极其巨大的可能合子基因型。
8. Mitosis vs. Meiosis: A Comparative Table | 有丝分裂与减数分裂比较表
The table below summarises the key differences between mitosis and meiosis. Use this for quick revision before exams.
下表总结了有丝分裂和减数分裂的主要区别。考试前可用此表快速复习。
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes for sexual reproduction |
| Number of divisions | One | Two (Meiosis I and II) |
| Daughter cells | Two diploid (2n), genetically identical | Four haploid (n), genetically different |
| Homologous pairing | None | Yes, in prophase I |
| Crossing over | No | Yes, during prophase I |
| Chromosome number in daughter cells | Same as parent (2n) | Half of parent (n) |
Now the same table in Chinese:
相同表格的中文版本：
| 特征 | 有丝分裂 | 减数分裂 |
|---|---|---|
| 目的 | 生长、修复、无性繁殖 | 产生配子，用于有性生殖 |
| 分裂次数 | 一次 | 两次（减数第一次和第二次） |
| 子细胞 | 两个二倍体（2n），遗传相同 | 四个单倍体（n），遗传不同 |
| 同源配对 | 无 | 有，在前期 I |
| 交叉互换 | 无 | 有，在前期 I |
| 子细胞染色体数 | 与母细胞相同（2n） | 母细胞的一半（n） |
9. Cell Cycle Regulation and Checkpoints | 细胞周期调控与检查点
The cell cycle is tightly controlled by a network of regulatory proteins to ensure genomic integrity and correct division timing. Failures in this regulation can lead to cancer.
细胞周期由一组调控蛋白网络严格控制，以确保基因组完整性和正确的分裂时机。这种调控失败可能导致癌症。
Cyclins and cyclin-dependent kinases (CDKs) are the key molecules. Cyclins accumulate and are degraded cyclically, activating CDKs which in turn phosphorylate target proteins to drive the cell through checkpoints.
细胞周期蛋白和细胞周期蛋白依赖性激酶（CDK）是关键分子。细胞周期蛋白周期性积累和降解，激活 CDK，后者磷酸化靶蛋白，推动细胞通过检查点。
Three main checkpoints operate: the G1 checkpoint (restriction point) checks for DNA damage and sufficient resources; the G2 checkpoint ensures all DNA is replicated; and the M (spindle assembly) checkpoint verifies all chromosomes are attached to the spindle before anaphase.
三个主要检查点运作：G1 检查点（限制点）检查 DNA 损伤和资源是否充足；G2 检查点确保所有 DNA 已完成复制；M 期（纺锤体组装）检查点验证所有染色体在后期开始前均与纺锤体连接。
The tumour-suppressor protein p53 plays a vital role at the G1 checkpoint. If DNA is damaged, p53 can halt the cycle, activate repair enzymes, or trigger apoptosis if the damage is irreparable.
抑癌蛋白 p53 在 G1 检查点起着至关重要的作用。如果 DNA 受损，p53 可暂停周期、激活修复酶，或在损伤无法修复时触发凋亡。
10. Cancer: Uncontrolled Cell Division | 癌症：失控的细胞分裂
Cancer results from unregulated cell division driven by mutations in proto-oncogenes and tumour-suppressor genes. These mutations accumulate over time, often due to environmental factors or replication errors.
癌症由原癌基因和抑癌基因突变引起的失控细胞分裂所致。这些突变随时间累积，通常由环境因素或复制错误导致。
A mutated proto-oncogene becomes an oncogene, causing excessive cell division even in the absence of growth signals. A classic example is the Ras gene, which encodes a protein involved in growth signal transduction.
突变的原癌基因成为癌基因，即使在缺乏生长信号的情况下也促使细胞过度分裂。典型例子是 Ras 基因，它编码一种参与生长信号转导的蛋白质。
Loss-of-function mutations in tumour-suppressor genes remove the normal brakes on cell division. The p53 gene is the most commonly mutated gene in human cancers; its inactivation allows damaged cells to proceed through the cycle.
抑癌基因的功能丧失突变消除了对细胞分裂的正常制动。p53 基因是人类癌症中最常发生突变的基因；其失活使受损细胞能够继续通过周期。
Cancer cells exhibit several hallmarks, including sustained proliferative signalling, evasion of growth suppressors, resistance to apoptosis, and the ability to invade tissues and metastasise.
癌细胞表现出几个特征，包括持续增殖信号、逃避生长抑制、抵抗凋亡，以及入侵组织和转移的能力。
11. Stem Cells and Differentiation | 干细胞与分化
Stem cells are unspecialised cells that can divide to produce both identical daughter cells (self-renewal) and cells that differentiate into specialised cell types. They are important in development, tissue repair, and medical research.
干细胞是未特化的细胞，能分裂产生相同的子细胞（自我更新）以及分化为特化细胞类型的细胞。它们在发育、组织修复和医学研究中都很重要。
Totipotent stem cells, such as the zygote and early blastomeres, can give rise to all cell types, including the placenta. Pluripotent stem cells (embryonic stem cells) can form any cell of the embryo proper but not extra-embryonic tissues.
全能干细胞，如受精卵和早期卵裂球，能产生所有细胞类型，包括胎盘。多能干细胞（胚胎干细胞）能形成胚胎本身的任何细胞，但不能形成胚外组织。
Multipotent stem cells, found in adult tissues (e.g. bone marrow), can differentiate into a limited range of cell types within a specific lineage. The use of stem cells in regenerative medicine, such as for blood disorders, is a key application.
多能干细胞存在于成体组织（如骨髓）中，能分化为特定谱系内有限范围的细胞类型。干细胞在再生医学中的应用，如治疗血液疾病，是一项关键应用。
Stem cell therapy raises ethical issues, especially the use of embryonic stem cells, which WJEC expects you to discuss in terms of potential benefits versus respect for embryonic life.
干细胞疗法引发伦理问题，特别是胚胎干细胞的使用，WJEC 期望你就能带来的潜在益处与对胚胎生命的尊重两方面进行讨论。
12. Practical Techniques: Observing Mitosis | 实验技术：观察有丝分裂
A common WJEC practical involves preparing a temporary root tip squash to observe and identify the stages of mitosis. Mastering the steps and calculations is essential for exam questions.
WJEC 常见实验包括制备临时根尖压片，以观察和鉴定有丝分裂各阶段。掌握步骤和计算对考试题目至关重要。
The procedure: fix root tips in acid (e.g. 1 M HCl) to hydrolyse cell walls, heat with a stain such as toluidine blue to stain chromosomes, then gently squash under a coverslip to spread cells into a monolayer.
步骤：用酸（如 1 M HCl）固定根尖，水解细胞壁；加热并用甲苯胺蓝等染液染色，使染色体着色；然后在盖玻片下轻轻压片，将细胞铺展为单层。
Under the microscope, you can identify prophase by condensed chromosomes, metaphase by aligned chromosomes, anaphase by separating chromatids, and telophase by two forming nuclei. Interphase cells have a distinct nucleus but no visible chromosomes.
在显微镜下，可通过凝缩的染色体识别前期，通过排列的染色体识别中期，通过分离的染色单体识别后期，通过两个形成的细胞核识别末期。间期细胞有明显的细胞核，但无可见染色体。
The mitotic index can be calculated: (number of cells in mitosis ÷ total number of cells) × 100%. A high index suggests a region of rapid growth,
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