📚 IB WJEC Biology: Mitosis - Key Points Review | IB WJEC 生物：有丝分裂 考点精讲
Mitosis is a fundamental process of cell division that guarantees the faithful distribution of duplicated chromosomes into two genetically identical daughter nuclei. For IB and WJEC Biology candidates, examiners consistently test your ability to describe each stage, interpret chromosome behaviour, and relate mitotic errors to disease. This article breaks down every essential point in clear pairings of English and Chinese to help you consolidate the theory and avoid the most frequent mistakes.
有丝分裂是细胞分裂的基本过程，保证复制后的染色体被精确地分配到两个遗传上完全相同的子细胞核中。对于 IB 和 WJEC 生物学考生来说，考官总在反复考核你对各阶段描述、染色体行为解读以及将有丝分裂错误与疾病关联的能力。本文用清晰的中英对照段落拆解每一个必考要点，帮助你巩固理论并避开最常见的失分坑。
1. The Cell Cycle Overview | 细胞周期概述
The cell cycle is the ordered sequence of events that culminate in cell division. It comprises interphase (G₁, S, G₂) and the mitotic phase (mitosis and cytokinesis). Interphase occupies roughly 90% of the cycle; it is not a 'resting' period but a time of intense biosynthetic activity and growth.
细胞周期是导致细胞分裂的有序事件序列。它包括间期（G₁、S、G₂）和有丝分裂期（有丝分裂和胞质分裂）。间期约占整个周期的90%；它并非“休息”期，而是进行旺盛生物合成和生长的时间。
The mitotic phase itself is subdivided into prophase, metaphase, anaphase and telophase, often remembered by the acronym PMAT. Cytokinesis, the physical division of the cytoplasm, usually begins during late telophase and completes the formation of two daughter cells.
有丝分裂期本身可细分为前期、中期、后期和末期，常记作 PMAT。胞质分裂，即细胞质的物理分裂，通常在末期后期开始，最终形成两个子细胞。
2. Interphase: Preparation for Division | 间期：分裂的准备
During G₁ phase, the cell grows, carries out its tissue-specific functions, and synthesises proteins. Key checkpoints assess cell size, nutrient availability, and DNA integrity before committing to replication.
在 G₁ 期，细胞生长、执行组织特异性功能并合成蛋白质。关键的检查点会评估细胞大小、营养供应和 DNA 完整性，然后再决定进入复制阶段。
ire nuclear genome is replicated. As a result, each chromosome now consists of two identical sister chromatids held together by a protein complex called cohesin, with the centromere acting as the primary constriction. Although the DNA content doubles from 2n to 4n, the chromosome number stays the same— remains 46, but each chromosome possesses two chromatids, giving a total of 92 chromatids.
在 S 期（合成期），整个核基因组被复制。因此，每条染色体由两条相同的姐妹染色单体组成，通过一个称为黏连蛋白的蛋白质复合物连接在一起，着丝粒是主缢痕。虽然 DNA 含量从 2n 加倍到 4n，但染色体数目保持不变——在人类细胞中，2n=46 仍然是 46，但每条染色体拥有两条染色单体，总共 92 条染色单体。
During G₂ phase, the cell continues to enlarge and synthesises proteins essential for mitosis, such as tubulin for spindle fibres. The G₂ checkpoint ensures that all DNA has been replicated faithfully before the cell enters prophase.
在 G₂ 期，细胞继续增大并合成有丝分裂所必需的蛋白质，例如构成纺锤丝的微管蛋白。G₂ 检查点确保所有 DNA 被准确复制后，细胞才进入前期。
3. Prophase: Chromosomes Condense | 前期：染色体凝缩
Chromatin fibres coil and fold, becoming visible under the light microscope as distinct, rod-shaped chromosomes. Each chromosome is already duplicated and appears as two sister chromatids connected at the centromere.
染色质纤维螺旋折叠，在光学显微镜下成为可见的、棒状的染色体。此时每条染色体已复制好，表现为两条姐妹染色单体在着丝粒处相连。
The nucleolus disappears, and the nuclear envelope begins to disintegrate into small vesicles, a process driven by phosphorylation of nuclear lamins. In animal cells, the centrosomes (each containing a pair of centrioles) migrate to opposite poles and start organising the mitotic spindle. Microtubules radiate from each centrosome; some, called kinetochore microtubules, will capture the kinetochore protein complexes assembled at the centromeres.
核仁消失，核膜开始分解成小囊泡，这一过程由核纤层蛋白的磷酸化驱动。在动物细胞中，中心体（每个含有一对中心粒）移向两极并开始组织有丝分裂纺锤体。微管从每个中心体放射出来；其中一部分，称为动粒微管，会捕捉组装在着丝粒上的动粒蛋白复合物。
4. Metaphase: Chromosomes Align | 中期：染色体排列
Chromosomes become aligned at the cell's equator, forming the metaphase plate. This alignment results from a push-pull balance of forces exerted by kinetochore microtubules. The sister chromatids face opposite poles, and each kinetochore is attached to microtubules emanating from the pole it faces.
染色体排列在细胞的赤道面，形成中期板。这种排列是动粒微管所施加推拉力量平衡的结果。姐妹染色单体面向相反的两极，且每一个动粒都与其所面向那一极发出的微管相连。
At this stage, chromosomes reach their maximum condensation, making metaphase the ideal phase for karyotyping or counting chromosome numbers. Examiners often ask why a metaphase spread is used: the chromosomes are easily distinguishable and can be paired with their homologues.
此时染色体达到最大程度的凝缩，使得中期成为核型分析或计数染色体数目的理想阶段。考官常问为什么使用中期染色体铺片：因为染色体容易区分，并可配对找到同源染色体。
5. Anaphase: Chromatid Separation | 后期：染色单体分离
Anaphase begins abruptly when the cohesin proteins holding sister chromatids together are cleaved by separase. Once released, sister chromatids are pulled toward opposite poles as kinetochore microtubules shorten through depolymerisation at the plus end.
当连接姐妹染色单体的黏连蛋白被分离酶切割时，后期突然开始。一旦释放，姐妹染色单体即被拉向两极，因为动粒微管通过在正端解聚而缩短。
Simultaneously, non-kinetochore microtubules overlap and push against each other, elongating the cell. By the end of anaphase, a complete set of chromosomes has reached each pole, and the cell is about to divide.
同时，非动粒微管互相重叠并彼此推开，使整个细胞伸长。到后期结束时，完整的一套染色体已达两极，细胞即将分裂。
A crucial exam concept: as soon as sister chromatids separate, each is classified as an individual chromosome. Thus, at anaphase the chromosome number temporarily doubles (e.g., from 46 to 92 in humans) until cytokinesis partitions them into two daughter nuclei.
一个关键的考试概念：姐妹染色单体一旦分开，每条即被视为独立的染色体。因此，在后期染色体数目暂时加倍（例如人类从46条变为92条），直到胞质分裂将它们分配到两个子细胞核中。
6. Telophase and Cytokinesis | 末期与胞质分裂
During telophase, a nuclear envelope reassembles around each cluster of chromosomes using membrane vesicles. The chromosomes decondense back into chromatin, nucleoli reappear, and the mitotic spindle disassembles. Essentially, telophase reverses the events of prophase.
在末期，核膜借助膜囊泡围绕每一簇染色体重新组装。染色体解螺旋回到染色质状态，核仁重新出现，有丝分裂纺锤体解聚。末期基本上逆转了前期的事件。
Cytokinesis usually commences in late anaphase or early telophase. In animal cells, a contractile ring of actin and myosin filaments constricts the plasma membrane, forming a cleavage furrow that deepens until the cell is pinched in two. In plant cells, vesicles derived from the Golgi apparatus align at the equator and fuse to form a cell plate, which matures into a new cell wall and middle lamella.
胞质分裂通常在后期晚期或末期早期开始。在动物细胞中，由肌动蛋白和肌球蛋白丝构成的收缩环收缩质膜，形成分裂沟，逐步加深直至细胞被一分为二。在植物细胞中，来自高尔基体的囊泡在赤道面排列并融合形成细胞板，细胞板成熟后成为新的细胞壁和胞间层。
7. Chromosome Numbers and Ploidy | 染色体数目与倍性
One common examination trap is confusing chromosome number with chromatid number. A diploid human somatic cell entering mitosis has 46 chromosomes, each consisting of two chromatids after S phase. Therefore:
一个常见的考试陷阱是将染色体数与染色单体数混淆。进入有丝分裂的人类二倍体体细胞有46条染色体，S期后每条由两条染色单体组成。因此：
| Stage 阶段 | Chromosome Number 染色体数 | Chromatid Number 染色单体数 | DNA Amount (c) DNA量 |
|---|---|---|---|
| G₁ | 46 | 46 | 2c |
| After S phase S期后 | 46 | 92 | 4c |
| Anaphase 后期 | 92 | 92 | 4c |
| Daughter cell (G₁) 子细胞G₁ | 46 | 46 | 2c |
Mitosis maintains ploidy: if the parent cell is diploid (2n), each daughter nucleus receives exactly the same 2n complement. This genetic constancy is the hallmark of mitotic division.
有丝分裂维持倍性：如果亲代细胞是二倍体（2n），每个子细胞核恰好获得相同的2n组成。这种遗传稳定性是有丝分裂的标志。
8. Mitotic Index and Cancer | 有丝分裂指数与癌症
The mitotic index is defined as
Mitotic Index = (Number of cells in mitosis ÷ Total number of cells) × 100
. It is a reliable indicator of the proliferation rate of a tissue. Tissues with a high mitotic index, such as root tips and early embryos, are commonly used in practical investigations of mitosis.有丝分裂指数定义为：
有丝分裂指数 = (处于有丝分裂的细胞数 ÷ 细胞总数) × 100
。它是组织增殖速率的可靠指标。具有高有丝分裂指数的组织，如根尖和早期胚胎，常用于有丝分裂的实验研究。Cancer arises when the regulatory mechanisms of the cell cycle are disrupted. Oncogenes drive excessive proliferation, while tumour suppressor genes (e.g., p53) normally halt the cycle if DNA damage is detected. Mutations in these genes allow cells to bypass checkpoints, leading to uncontrolled mitosis and tumour formation. Examiners often link mitotic index to cancer diagnosis: a biopsy showing an abnormally high mitotic index may suggest malignancy.
当细胞周期的调控机制被打乱时就会发生癌症。癌基因驱动过度增殖，而抑癌基因（如 p53）通常在检测到DNA损伤时暂停周期。这些基因的突变使细胞绕过检查点，导致不受控的有丝分裂和肿瘤形成。考官经常将有丝分裂指数与癌症诊断关联：活检中显示异常高的有丝分裂指数可能提示恶性。
9. Importance of Mitosis | 有丝分裂的重要性
Mitosis is essential for the growth of multicellular organisms, increasing cell number from a single zygote to trillions of specialised cells. It replaces worn-out or damaged cells in tissues such as skin and gut lining. In certain organisms, mitosis enables asexual reproduction, producing genetically identical offspring (e.g., budding in yeast, vegetative propagation in plants). Crucially, mitosis preserves genetic stability by ensuring that each daughter cell inherits a complete, error-free copy of the genome.
有丝分裂对多细胞生物的生长至关重要，使细胞数量从单个合子增加到数万亿个特化细胞。它替换皮肤、肠道内壁等组织中磨损或受损的细胞。在某些生物中，有丝分裂实现无性生殖，产生遗传上相同的后代（如酵母出芽生殖、植物的营养繁殖）。最关键的是，有丝分裂通过确保每个子细胞继承完整、无错误的基因组拷贝，维持了遗传稳定性。
10. Differences Between Mitosis and Meiosis | 有丝分裂与减数分裂的区别
| Feature 特征 | Mitosis 有丝分裂 | Meiosis 减数分裂 |
|---|---|---|
| Location 发生部位 | Somatic (body) cells 体细胞 | Germline cells → gametes 生殖细胞→配子 |
| Number of divisions 分裂次数 | One 一次 | Two (meiosis I and II) 两次（减数第一次和第二次分裂） |
| Daughter cells produced 子细胞数 | 2, genetically identical 2个，遗传上相同 | 4, genetically varied 4个，遗传上各异 |
| Chromosome number 染色体数 | Maintained (2n → 2n) 维持不变 (2n→2n) | Halved (2n → n) 减半 (2n→n) |
| Homologous pairing 同源配对 | No 无 | Yes, in prophase I 有，在前期I发生 |
| Crossing over 交叉互换 | Absent 不存在 | Occurs in prophase I 发生在前期I |
| Function 功能 | Growth, repair, asexual reproduction 生长、修复、无性生殖 | Sexual reproduction, genetic variation 有性生殖、遗传变异 |
In IB and WJEC exams, you must not only recall these differences but also interpret diagrams of cells at various stages, spotting whether homologous chromosomes are paired or whether chromatids are being separated in an anaphase-like manner to infer mitosis vs. meiosis.
在 IB 和 WJEC 考试中，你不仅要记住这些区别，还必须解读不同阶段细胞的图像，判断是同源染色体配对还是染色单体在以类似后期的方式分离，从而推断有丝分裂还是减数分裂。
11. Regulation of the Cell Cycle | 细胞周期的调控
Progression through the cell cycle is controlled by cyclins and cyclin-dependent kinases (CDKs). The concentration of specific cyclins rises and falls rhythmically, activating CDKs to phosphorylate target proteins that trigger the next phase. For instance, the G₁/S cyclin-CDK complex promotes entry into the S phase.
细胞周期的推进受周期蛋白和周期蛋白依赖性激酶（CDK）调控。特定周期蛋白的浓度有节律地升降，激活 CDK 以磷酸化靶蛋白，从而触发下一阶段。例如，G₁/S 周期蛋白-CDK 复合物促进进入 S 期。
Checkpoints at G₁, G₂, and metaphase pause the cycle if conditions are not optimal. The tumour suppressor protein p53 can activate DNA repair enzymes or, if damage is too severe, trigger apoptosis. Loss-of-function mutations in p53 remove this safeguard, allowing damaged cells to continue dividing—a direct link to tumourigenesis.
G₁、G₂ 和中期的检查点会在条件不理想时暂停周期。抑癌蛋白 p53 可激活 DNA 修复酶，若损伤过于严重则触发细胞凋亡。p53 功能缺失突变移除了这一安全机制，允许受损细胞继续分裂——这与肿瘤发生直接相关。
12. Exam Tips: Common Mistakes | 考试技巧：常见错误
Confusing chromosomes and chromatids: Before anaphase, every chromosome consists of two chromatids. After anaphase separation, each chromatid is a chromosome. Always reread the question to check whether it asks for chromosomes or chromatids at a specific stage.
混淆染色体和染色单体：在后期之前，每条染色体由两条染色单体组成。后期分离之后，每条染色单体即成为一条染色体。一定要重读题目，确认它问的是特定阶段中的染色体数还是染色单体数。
Treating interphase as a resting phase: Interphase is an active period of protein synthesis, organelle duplication, and DNA replication. Credit is often lost when candidates describe it as merely a pause between divisions.
将间期视为休息阶段：间期是蛋白质合成、细胞器复制和 DNA 复制的活跃时期。考生若将其仅描述为分裂之间的暂停，经常会丢分。
Forgetting plant-specific cytokinesis: Stating that a cleavage furrow forms in plant cells is a classic error. Plant cells build a cell plate from Golgi-derived vesicles. Sketch and label diagrams accordingly.
忘记植物特有的胞质分裂：声称植物细胞中形成分裂沟是一个经典错误。植物细胞利用高尔基体来源的囊泡构建细胞板。相应地在图表中画图并标注。
Misidentifying stages in micrographs: Look for the nuclear envelope (present in prophase, absent in metaphase), alignment at the equator (metaphase), separated chromatids moving to poles (anaphase), and two clusters of decondensing chromosomes (telophase). Practice with both animal and plant cells.
在显微照片中错判阶段：观察核膜（前期存在，中期消失）、赤道面排列（
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