📚 Mitosis – CCEA A-Level Biology Exam Revision | 有丝分裂 – CCEA A-Level 生物考试精讲
Mitosis is the process of nuclear division that produces two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus. It is essential for growth, repair and asexual reproduction in eukaryotic organisms. For CCEA A-Level Biology, you must be able to describe the stages in detail, explain the behaviour of chromosomes and chromatids, and link mitosis to the cell cycle and cancer.
有丝分裂是产生两个遗传上完全相同的子细胞核的核分裂过程，每个子核所含染色体数目与亲代核相同。它对真核生物的生长、修复和无性繁殖至关重要。在 CCEA A-Level 生物学考试中，你必须能够详细描述各个阶段，解释染色体和染色单体的行为，并将有丝分裂与细胞周期和癌症联系起来。
1. The Cell Cycle and Interphase | 细胞周期与间期
Before mitosis begins, the cell spends most of its life in interphase, which is divided into G₁, S and G₂ phases. During G₁ (gap 1), the cell grows, synthesises proteins and carries out its normal metabolic functions. Many cells permanently exit the cycle at this point and enter a resting state called G₀.
在有丝分裂开始前，细胞大部分时间处于间期，间期可细分为 G₁ 期、S 期和 G₂ 期。在 G₁ 期（第一间期），细胞生长、合成蛋白质并进行正常的代谢活动。许多细胞会在此时永久退出细胞周期，进入称为 G₀ 的静息状态。
The S phase (synthesis) is the period when DNA replication occurs. Each chromosome is faithfully duplicated, forming two identical sister chromatids held together by a protein complex called cohesin at the centromere. At the end of S phase, the cell contains exactly twice the amount of DNA (4C vs 2C in G₁).
S 期（合成期）是 DNA 复制发生的时期。每条染色体被精确复制，形成两条由黏连蛋白在着丝粒处连接在一起的、完全相同的姐妹染色单体。S 期结束时，细胞内的 DNA 含量恰好是 G₁ 期（2C）的两倍（4C）。
G₂ (gap 2) is a second growth phase in which the cell continues to enlarge, synthesises proteins needed for division, and checks the replicated DNA for errors. By the end of interphase, the chromosomes are still long, thin threads that are not visible under the light microscope.
G₂ 期（第二间期）是第二个生长期，细胞继续增大，合成分裂所需的蛋白质，并检查复制后的 DNA 是否存在错误。间期结束时，染色体仍为长而细的丝状结构，在光学显微镜下不可见。
2. Prophase | 前期
Prophase is the first stage of mitosis. Chromatin fibres begin to coil and condense, becoming visible as distinct chromosomes, each consisting of two sister chromatids joined at the centromere. The nucleolus shrinks and disappears.
前期是有丝分裂的第一个阶段。染色质纤维开始螺旋化并凝缩，变为可见的独立染色体，每条染色体由两条在着丝粒处相连的姐妹染色单体组成。核仁逐渐缩小并消失。
The two centrosomes, which were duplicated during interphase, migrate to opposite poles of the cell. From each centrosome, microtubules start to polymerise and form the mitotic spindle. Microtubules that will attach to chromosomes are called kinetochore microtubules.
在间期完成复制的两个中心体移向细胞两极。从每个中心体开始，微管聚合形成有丝分裂纺锤体。那些将要连接染色体的微管称为动粒微管。
In late prophase (often termed prometaphase in some textbooks), the nuclear envelope breaks down into small vesicles. This breakdown allows spindle microtubules to access the chromosomes. Kinetochore proteins assemble at the centromere of each chromatid.
在前期末（有些教材称为前中期），核膜解体成小囊泡。核膜的瓦解使纺锤体微管得以接触染色体。每条染色单体的着丝粒区域组装出动粒蛋白复合体。
3. Metaphase | 中期
During metaphase, the chromosomes become fully condensed and are moved by the spindle fibres to the equatorial plane (metaphase plate) of the cell. Kinetochore microtubules from opposite poles attach to the kinetochores of sister chromatids, exerting tension that aligns each chromosome at the cell’s midpoint.
在中期，染色体达到最大程度的凝缩，并被纺锤丝牵引至细胞的赤道面（中期板）。来自两极的动粒微管分别与姐妹染色单体的动粒相连，所产生的张力使每条染色体排列在细胞的中央平面。
The alignment ensures that when sister chromatids separate in the next stage, each daughter nucleus receives one identical copy of each chromosome. This is the stage at which chromosome number is most easily counted under a microscope.
这种排列保证了在下一阶段姐妹染色单体分离时，每个子细胞核都获得每条染色体的一份相同副本。中期是显微镜下最便于计数染色体数目的阶段。
4. Anaphase | 后期
Anaphase begins abruptly when the cohesin proteins holding sister chromatids together are cleaved by the enzyme separase. This allows the centromeres to divide, and each sister chromatid is pulled towards opposite poles of the cell.
后期突然开始，此时连接姐妹染色单体的黏连蛋白被分离酶切断。这使着丝粒一分为二，每条姐妹染色单体分别被拉向细胞两极。
As the kinetochore microtubules shorten by depolymerisation at the kinetochore ends, the separated chromatids – now individual chromosomes – move poleward. At the same time, non‑kinetochore microtubules elongate, pushing the two poles further apart and causing the cell to elongate.
动粒微管通过在动粒端的解聚而缩短，使得分离后的染色单体（此时已成为独立的染色体）向两极移动。与此同时，非动粒微管伸长，将两极进一步推开，使细胞拉长。
Because each chromatid now becomes a distinct chromosome, the chromosome number in the cell temporarily doubles (for a human cell, from 2n=46 to 4n=92) until cytokinesis separates the cytoplasm.
由于每条染色单体现在都成为一条独立的染色体，细胞内的染色体数目暂时加倍（以人类细胞为例，从 2n=46 变为 4n=92），直到胞质分裂将细胞质分开。
5. Telophase | 末期
Telophase essentially reverses the events of prophase. Once the chromosomes have reached the poles, they begin to decondense and return to their extended, thread‑like chromatin form. A new nuclear envelope re-forms around each cluster of chromosomes from fragments of the old nuclear envelope and endoplasmic reticulum.
末期基本上是前期的逆转。染色体到达两极后，开始解旋并恢复为伸展的、丝状的染色质形态。由旧核膜碎片和内质网重新形成核膜，包裹每一组染色体。
The nucleoli reappear, and the mitotic spindle disassembles. At this point, two genetically identical daughter nuclei are present within a single cell, marking the end of nuclear division.
核仁重新出现，有丝分裂纺锤体解体。此时，单个细胞内含有两个遗传上完全相同的子细胞核，标志着核分裂的结束。
6. Cytokinesis – Animal vs Plant Cells | 胞质分裂——动物细胞与植物细胞的比较
Cytokinesis is the division of the cytoplasm, usually beginning in late anaphase or telophase. The mechanism differs significantly between animal and plant cells because of the rigid plant cell wall.
胞质分裂是细胞质的分裂，通常始于后期末或末期。由于植物细胞具有坚硬的细胞壁，动物细胞和植物细胞的分裂机制显著不同。
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| 分裂方式 | Cleavage furrow formed by a contractile ring of actin and myosin filaments | Cell plate formation from Golgi‑derived vesicles containing pectin and cellulose |
| 机制 | Ring contracts and pinches the cell into two | Vesicles fuse at the equatorial plane, forming a new cell wall between daughter cells |
| 细胞骨架参与 | Actin and myosin microfilaments | Phragmoplast microtubules guide vesicles to the division plane |
英文要点： In animal cells, a cleavage furrow forms when a contractile ring composed of actin and myosin filaments tightens around the cell equator. This ring progressively deepens until the parent cell is split into two separate daughter cells.
中文要点： 在动物细胞中，由肌动蛋白和肌球蛋白丝组成的收缩环在细胞赤道处形成分裂沟。该环逐渐收紧，直至母细胞被分割为两个独立的子细胞。
In plant cells, vesicles derived from the Golgi apparatus accumulate at the equator and fuse to form a cell plate. This plate expands outwards, and its contents mature into a new middle lamella and primary cell wall, separating the two daughter cells.
在植物细胞中，来自高尔基体的囊泡在赤道面聚集并融合，形成细胞板。细胞板向外扩展，其内含物最终发育为新的胞间层和初生细胞壁，将两个子细胞分开。
7. Chromosome Number and DNA Content Changes | 染色体数目与DNA含量的变化
A common exam question asks you to describe or graph the changes in chromosome number and DNA mass per cell during the cell cycle. Assume a diploid organism with 2n chromosomes and a DNA mass of 2C in G₁.
考试中常见的一道题目是要求描述或画出细胞周期中染色体数目和单个细胞DNA质量的变化。假设一个二倍体生物，染色体数为 2n，G₁ 期DNA质量为 2C。
DNA content: G₁ = 2C → after S phase = 4C → after mitosis and cytokinesis = 2C
DNA 含量：G₁ = 2C → S期后 = 4C → 有丝分裂和胞质分裂后 = 2C
Chromosome number (per cell): G₁ = 2n → S & G₂ = 2n (each chromosome now has 2 chromatids) → early anaphase = 4n → telophase & cytokinesis = 2n
染色体数目（每个细胞）：G₁ = 2n → S 和 G₂ = 2n（每条染色体此时含两条染色单体）→ 后期初 = 4n → 末期及胞质分裂后 = 2n
Note that during S and G₂ phases the chromosome number remains 2n because the count is defined by the number of centromeres, not the number of chromatids. The DNA mass doubles from 2C to 4C. At anaphase, centromere splitting momentarily doubles the chromosome count to 4n until the cytoplasm divides.
请注意，在 S 期和 G₂ 期染色体数目仍为 2n，因为染色体数目由着丝粒数界定，而非染色单体数。DNA 质量则从 2C 加倍至 4C。在后期，着丝粒分裂使染色体数目暂时加倍为 4n，直至细胞质分开。
8. Significance of Mitosis | 有丝分裂的重要意义
Mitosis achieves genetic stability: each daughter nucleus receives an exact copy of the parent cell’s genetic information. This is essential for growth, where an organism increases in cell number, and for the replacement of damaged or worn‑out cells, such as skin cells or red blood cells.
有丝分裂实现了遗传的稳定性：每个子细胞核都获得亲代细胞遗传信息的精确副本。这对生物体的生长（细胞数量增加）以及更换受损或衰老的细胞（如表皮细胞或红细胞）都至关重要。
Asexual reproduction in many eukaryotes, such as budding in yeast or vegetative propagation in plants, relies entirely on mitosis. Mitosis also allows for cloning and regeneration, for example in starfish regenerating lost arms.
许多真核生物的无性繁殖，如酵母的出芽生殖或植物的营养繁殖，完全依赖有丝分裂。有丝分裂还使得克隆和再生成为可能，例如海星再生失去的腕足。
9. Control of the Cell Cycle | 细胞周期的调控
The cell cycle is tightly regulated by a series of molecular checkpoints. The three main checkpoints occur at G₁/S, G₂/M and during metaphase (the spindle assembly checkpoint). These ensure that the cell only proceeds if conditions are favourable and if previous steps have been completed accurately.
细胞周期受到一系列分子检查点的严格调控。三个主要的检查点位于 G₁/S 转换点、G₂/M 转换点以及中期阶段（纺锤体组装检查点）。它们确保细胞仅在条件适宜且前一步准确完成后才继续前进。
Progression through checkpoints is driven by cyclin‑dependent kinases (CDKs) whose activity depends on binding to cyclin proteins. The concentration of specific cyclins rises and falls at specific stages, activating CDKs. For example, the maturation promoting factor (MPF), a cyclin‑CDK complex, triggers entry into mitosis by phosphorylating proteins that promote mitotic events.
通过检查点的进程由周期蛋白依赖性激酶（CDK）驱动，CDK 的活性依赖于与周期蛋白（cyclin）结合。特定周期蛋白的浓度在不同阶段有规律地升降，从而激活 CDK。例如，成熟促进因子（MPF）是一种 cyclin‑CDK 复合物，通过磷酸化促进有丝分裂事件的蛋白质，触发细胞进入有丝分裂。
If DNA damage is detected, the tumour suppressor protein p53 can halt the cycle at G₁/S by initiating transcription of a CDK inhibitor, allowing time for repair or triggering apoptosis. Failure of checkpoints can lead to uncontrolled division.
如果检测到 DNA 损伤，肿瘤抑制蛋白 p53 能够通过启动 CDK 抑制因子的转录，将细胞周期阻滞在 G₁/S 检查点，为修复争取时间，或引发细胞凋亡。检查点失效可导致细胞分裂失控。
10. Mitosis and Cancer | 有丝分裂与癌症
Cancer is fundamentally a disease of uncontrolled cell division. Mutations in proto‑oncogenes or tumour suppressor genes disrupt the normal regulatory mechanisms of the cell cycle, allowing cells to bypass checkpoints and divide indefinitely, forming a tumour.
癌症本质上是一种细胞分裂失控的疾病。原癌基因或肿瘤抑制基因发生突变，会破坏细胞周期的正常调控机制，使细胞无视检查点而无限增殖，形成肿瘤。
Understanding the molecular biology of mitosis has enabled the development of chemotherapeutic drugs that target dividing cells. For instance, taxol stabilises microtubules and prevents the spindle fibres from depolymerising, arresting cells in metaphase. Vinca alkaloids, on the other hand, inhibit microtubule polymerisation, blocking spindle formation.
对有丝分裂分子生物学的理解，推动了靶向分裂细胞的化疗药物的发展。例如，紫杉醇能稳定微管，阻止纺锤丝解聚，使细胞停滞在中期；而长春花生物碱则抑制微管聚合，阻断纺锤体形成。
11. Common Exam Mistakes and Top Tips | 常见考试错误与高分技巧
Mixing up chromosome and chromatid: A chromosome is a single DNA molecule, but after S phase it consists of two sister chromatids. Count chromosomes by centromeres – not by the number of visible arms. When centromeres divide in anaphase, the chromatid count becomes zero and chromosome number doubles.
混淆染色体与染色单体： 染色体是单个 DNA 分子，但在 S 期后由两条姐妹染色单体组成。计算染色体数目时要看着丝粒数，而不是看可见臂的数量。在后期着丝粒分裂后，染色单体不再存在，染色体数目加倍。
Forgetting to link structures to stages: Always state clearly where specific structures appear or disappear – e.g. the spindle forms in prophase, the nucleolus disappears in prophase and reappears in telophase, the nuclear envelope breaks down in late prophase and re‑forms in telophase. These details earn marks.
忘记将结构与阶段联系起来： 一定要清楚地说明特定结构出现或消失的时期——比如纺锤体在前期形成，核仁在前期消失、在末期重现，核膜在前期末解体、在末期重建。这些细节都是得分点。
Diagrams and data: You may be asked to identify stages from micrographs or interpret data showing DNA content per cell. Practise sketching the stages and labelling centromeres, chromatids, spindle fibres, and the metaphase plate. Always use the terms ‘chromosome’ and ‘chromatid’ precisely.
图表与数据： 你可能会被要求从显微照片中辨认阶段，或解读显示每个细胞 DNA 含量的数据。练习绘制各阶段简图并标注着丝粒、染色单体、纺锤丝和中期板。始终精确使用“染色体”和“染色单体”这两个术语。
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