📚 IGCSE WJEC Biology: Mitosis Exam Revision | IGCSE WJEC 生物：有丝分裂 考点精讲
Mitosis is one of the core topics in the WJEC IGCSE Biology specification, forming the foundation for understanding how organisms grow, repair tissues, and reproduce asexually. This article breaks down the entire process step by step, highlights key exam points, and provides clear comparisons to help you secure top marks. By the end, you will be confident in describing the stages, recognising diagrams, and explaining the significance of mitosis in living organisms.
有丝分裂是 WJEC IGCSE 生物学大纲中的核心主题之一，为理解生物体如何生长、修复组织以及进行无性繁殖奠定了基础。本文将逐步拆解整个分裂过程，突出关键考点，并提供清晰的对比，助你稳拿高分。读完本文后，你将能够自信地描述各个阶段、识别相关示意图，并解释有丝分裂在生物体中的重要意义。
1. Introduction to Mitosis | 有丝分裂简介
Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. It is essential for growth, replacement of worn-out cells, and asexual reproduction. In WJEC IGCSE Biology, you are expected to know the sequence of stages (prophase, metaphase, anaphase, telophase) and understand the behaviour of chromosomes during each.
有丝分裂是一种细胞分裂方式，从一个母细胞产生两个遗传上完全相同的子细胞。它对生长、替换衰老细胞以及无性繁殖至关重要。在 WJEC IGCSE 生物学考试中，你需要掌握各阶段的顺序（前期、中期、后期、末期），并理解每个阶段染色体的行为。
Key fact: Mitosis occurs in somatic (body) cells and maintains the diploid chromosome number. In humans, this means each daughter cell receives 46 chromosomes. The process is continuous but is described in discrete stages for convenience.
关键事实：有丝分裂发生在体细胞中，并保持二倍体染色体数目不变。在人类中，这意味着每个子细胞获得 46 条染色体。该过程是连续的，但为了方便描述而将其分为不同阶段。
2. The Cell Cycle | 细胞周期
The cell cycle consists of interphase (about 90% of the time) and the mitotic (M) phase, which includes mitosis and cytokinesis. Interphase is further divided into G₁ (first gap), S (synthesis), and G₂ (second gap) phases. It is during the S phase that DNA replication occurs, ensuring each chromosome consists of two identical sister chromatids held together at the centromere.
细胞周期包括间期（约占 90% 的时间）和有丝分裂期（M 期），后者包括有丝分裂和胞质分裂。间期又细分为 G₁ 期（第一间期）、S 期（合成期）和 G₂ 期（第二间期）。正是 S 期进行 DNA 复制，确保每条染色体由两条相同的姐妹染色单体组成，并由着丝粒连接在一起。
Exam tip: Do not confuse interphase with a ‘resting’ stage. Cells are highly active metabolically during interphase, synthesising proteins, growing, and duplicating organelles. WJEC often asks for the correct order of the cell cycle phases.
考试提示：不要把间期误认为是“休息”阶段。间期细胞代谢非常活跃，进行蛋白质合成、生长和细胞器的复制。WJEC 经常考查细胞周期各阶段的正确顺序。
3. Interphase: Preparation for Division | 间期：分裂的准备
Although interphase is not officially part of mitosis, it is essential to understand because it sets the stage. During G₁, the cell grows and carries out normal functions. In S phase, the DNA is replicated, doubling the chromosomal content. By G₂, the cell checks for errors and completes final protein synthesis needed for division.
虽然间期在严格意义上不属于有丝分裂，但理解它至关重要，因为它为分裂奠定了基础。G₁ 期细胞生长并执行正常功能。S 期 DNA 复制，染色体含量加倍。到 G₂ 期，细胞检查错误，并完成分裂所需的最终蛋白质合成。
After S phase, the cell contains twice the normal amount of DNA. Each chromosome is now composed of two chromatids. Under the microscope, the nucleus appears granular, and the nucleolus is visible. No distinct chromosomes are yet visible.
S 期之后，细胞含有正常 DNA 含量的两倍。每条染色体现在由两条染色单体组成。在显微镜下，细胞核呈现颗粒状，核仁可见。尚未出现明显的染色体结构。
4. Prophase: Chromosomes Condense | 前期：染色体凝缩
Prophase is the first stage of mitosis. Chromatin fibres coil and condense, becoming visible as distinct chromosomes. Each chromosome consists of two sister chromatids joined at the centromere. The nuclear envelope begins to break down, and the nucleolus disappears. In animal cells, centrioles move to opposite poles, and spindle fibres start to form.
前期是有丝分裂的第一个阶段。染色质丝螺旋、浓缩，变成可见的清晰染色体。每条染色体由两条姐妹染色单体构成，在着丝粒处相连。核膜开始解体，核仁消失。在动物细胞中，中心粒向相反的两极移动，纺锤丝开始形成。
WJEC often expects you to label a diagram showing chromosomes condensing and spindle fibres appearing. Remember that in plant cells (like onion root tips), centrioles are absent, but a spindle still forms.
WJEC 通常要求你标注显示染色体凝缩及纺锤丝出现的示意图。请记住，在植物细胞（如洋葱根尖）中，没有中心粒，但纺锤体仍然能够形成。
5. Metaphase: Chromosomes Align | 中期：染色体排列
During metaphase, the chromosomes line up along the metaphase plate (equator) of the cell. The spindle fibres attach to the centromeres of each chromosome, ensuring that sister chromatids are connected to opposite poles. This alignment is crucial for accurate separation later.
中期，染色体排列在细胞的赤道板（中期板）上。纺锤丝连接到每条染色体的着丝粒上，确保姐妹染色单体分别与相反的两极相连。这种排列对于后续的精确分离至关重要。
Metaphase is an excellent time to count chromosomes under the microscope because they are maximally condensed and clearly visible. Many exam questions ask to identify this stage from an image showing a central line of chromosomes.
中期是在显微镜下计数染色体的最佳时机，因为此时染色体高度浓缩且清晰可见。许多考题要求根据图像中染色体排成一列的特征来识别此阶段。
6. Anaphase: Chromatids Separate | 后期：染色单体分离
Anaphase begins when the centromeres split, allowing sister chromatids to be pulled apart by the shortening spindle fibres. Each chromatid (now considered an independent chromosome) moves towards opposite poles of the cell. The cell elongates, and by the end of anaphase, equal sets of chromosomes are gathered at each pole.
后期始于着丝粒分裂，姐妹染色单体被缩短的纺锤丝拉开。每条染色单体（此时可视为独立的染色体）移向细胞相反的两极。细胞被拉长，到后期结束时，相等的染色体组聚集在每一极。
Common misconception: Students sometimes think the chromosome number doubles in anaphase. In fact, the chromosome number temporarily doubles because chromatids separate, but the genetic information remains identical. WJEC may ask why the number appears as 92 in a human cell at this point.
常见误解：学生有时认为染色体数目在后期加倍。事实上，由于染色单体分离，染色体数目暂时加倍，但遗传信息仍然相同。WJEC 可能会问为什么此时人体细胞内染色体数目表现为 92 条。
7. Telophase: Two New Nuclei Form | 末期：两个新核形成
In telophase, the separated chromatids reach the poles and begin to decondense back into chromatin. A new nuclear envelope forms around each set of chromosomes, and the nucleolus reappears. The spindle fibres disassemble. Telophase essentially reverses the events of prophase.
末期，分离的染色单体到达两极并开始解旋回染色质状态。新的核膜围绕每组染色体形成，核仁重新出现。纺锤丝解体。末期基本上是前期事件的逆转过程。
By the end of telophase, two genetically identical nuclei are present within one cell. Cytokinesis usually begins before telophase is fully complete, dividing the cytoplasm.
到末期结束时，一个细胞内形成了两个遗传信息相同的细胞核。胞质分裂通常会在末期完全结束前开始，对细胞质进行分割。
8. Cytokinesis: Division of Cytoplasm | 胞质分裂：细胞质分裂
Cytokinesis is the division of the cytoplasm, which follows mitosis. In animal cells, a cleavage furrow forms, pinching the cell membrane inward until two separate daughter cells are produced. In plant cells, a cell plate forms along the equator, developing into a new cell wall and cell membrane between the two daughter cells.
胞质分裂是有丝分裂之后的细胞质分裂。在动物细胞中，会形成分裂沟，细胞膜向内凹陷，直至产生两个独立的子细胞。在植物细胞中，沿着赤道形成细胞板，进而发育成两个子细胞之间的新细胞壁和细胞膜。
Exam focus: Know the differences in cytokinesis between animal and plant cells. WJEC often includes this in structured questions. Remember that without cytokinesis, multinucleated cells (syncytia) can form, which is unusual in most contexts but seen in some organisms.
考试重点：掌握动物细胞和植物细胞胞质分裂的区别。WJEC 常在结构化试题中包含此考点。记住，如果没有胞质分裂，会形成多核细胞（合胞体），在大多数情况下这并不常见，但在某些生物体中存在。
9. Importance of Mitosis | 有丝分裂的重要性
Mitosis is vital for several biological processes: growth (increasing cell number in multicellular organisms), tissue repair (replacing damaged or dead cells), and asexual reproduction (producing genetically identical offspring in many plants, fungi, and some animals). It ensures genetic stability because the daughter cells are clones of the parent cell.
有丝分裂对多种生物过程至关重要：生长（增加多细胞生物的细胞数量）、组织修复（替换受损或死亡的细胞）和无性繁殖（在许多植物、真菌和某些动物中产生遗传相同的后代）。它确保遗传稳定性，因为子细胞是母细胞的克隆。
In plants, mitosis occurs in meristems (root and shoot tips), leading to primary growth. In animals, stem cells divide by mitosis to produce specialised cells. Cancerous growth results from uncontrolled mitosis, a topic WJEC may link to cell cycle regulation.
在植物中，有丝分裂发生在分生组织（根尖和茎尖），导致初生生长。在动物体内，干细胞通过有丝分裂产生特化细胞。癌变生长源自不受控制的有丝分裂，WJEC 可能会将这一主题与细胞周期调控联系起来。
10. Mitosis vs. Meiosis | 有丝分裂与减数分裂对比
A key comparison in WJEC IGCSE is between mitosis and meiosis. Mitosis produces two diploid daughter cells that are genetically identical, used for growth and repair. Meiosis produces four haploid daughter cells (gametes) that are genetically different, used for sexual reproduction.
WJEC IGCSE 的一个关键对比是有丝分裂和减数分裂。有丝分裂产生两个遗传相同的二倍体子细胞，用于生长和修复。减数分裂产生四个遗传不同的单倍体子细胞（配子），用于有性生殖。
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells produced | 2 | 4 |
| Ploidy of daughter cells | Diploid (2n) | Haploid (n) |
| Genetic variation | None (clones) | High (crossing over, independent assortment) |
| Occurs in | Somatic cells | Germline cells |
You must be able to interpret life cycles and explain where mitosis and meiosis fit. Be clear that meiosis halves the chromosome number, restoring it at fertilisation.
你必须能够解读生命周期，并解释有丝分裂和减数分裂分别发生在哪个环节。要明确减数分裂使染色体数目减半，并在受精时恢复至二倍体。
11. Observing Mitosis in Root Tips | 观察根尖的有丝分裂
The classic practical for WJEC involves staining and observing mitosis in onion or garlic root tips. The actively dividing region (meristem) is found just behind the root cap. After fixation and staining with substances like aceto-orcein or toluidine blue, cells can be viewed under a light microscope.
WJEC 经典实验涉及对洋葱或大蒜根尖进行染色并观察有丝分裂。活跃分裂的区域（分生组织）位于根冠的后方。经过固定并用醋酸地衣红或甲苯胺蓝等物质染色后，可在光学显微镜下观察细胞。
Key method steps: (1) Cut a thin longitudinal section of the root tip. (2) Place on a slide and add a drop of stain. (3) Warm gently to intensify staining. (4) Squash the sample under a coverslip to spread cells into a single layer. (5) Scan for mitotic stages. You should be able to calculate the mitotic index: (number of cells in mitosis ÷ total number of cells) × 100.
关键方法步骤：（1）切取根尖的薄纵切片。（2）置于载玻片上，加一滴染液。（3）温和加热以增强着色效果。（4）盖上盖玻片后压片，使细胞分散成单层。（5）扫描寻找有丝分裂各阶段。你应能够计算有丝分裂指数：（处于分裂期的细胞数 ÷ 细胞总数）× 100。
Common improvement tips: use the terminal few millimetres of the root tip, avoid over-squashing, and focus using the fine focus knob. Remember to explain why root tips are used (contain meristematic cells undergoing rapid division).
常见改进要点：使用根尖末端几毫米的部分，避免过度压片，并使用细准焦螺旋调焦。记得解释为何使用根尖（因为它们含有快速分裂的分生组织细胞）。
12. Common Exam Questions and Tips | 常见考题与备考建议
WJEC frequently asks students to identify stages from diagrams, describe chromosome behaviour, compare mitosis and meiosis, or interpret a graph of DNA content over time. You may also get data-based questions on mitotic index calculations and practical evaluation.
WJEC 经常要求学生根据示意图识别有丝分裂各阶段、描述染色体行为、比较有丝分裂与减数分裂，或解读 DNA 含量随时间变化的图表。你还可能遇到基于数据的题目，涉及有丝分裂指数计算和实验评价。
Top revision tips: (1) Draw and label each stage multiple times, emphasising spindle fibres, centromeres, and chromosome shapes. (2) Create a story mnemonic: ‘I Passed My Anatomy Test’ (Interphase, Prophase, Metaphase, Anaphase, Telophase). (3) Practice calculating mitotic indices with sample data. (4) Always use the terms ‘chromatid’ and ‘chromosome’ precisely – a chromatid becomes a chromosome only after separation.
考前复习秘诀：（1）反复绘制并标注每个阶段，突出纺锤丝、着丝粒和染色体的形态。（2）编一个记忆口诀，例如“间前中后末”。（3）用样题数据练习计算有丝分裂指数。（4）务必精确使用“染色单体”和“染色体”这两个术语——染色单体只有在分离之后才被称为染色体。
Finally, remember that mitosis is a continuous process, but describing stages helps us understand the key events. Be prepared to explain what happens at each transition, such as prophase to metaphase, and relate the process to its roles in living organisms.
最后，请记住有丝分裂是一个连续的过程，但分阶段描述有助于我们理解关键事件。准备好解释每个过渡阶段（例如由前期进入中期）发生的变化，并将该过程与其在生物体中的作用联系起来。
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