Mitosis in IGCSE OCR Biology | IGCSE OCR 生物：有丝分裂 考点精讲

📚 Mitosis in IGCSE OCR Biology | IGCSE OCR 生物：有丝分裂 考点精讲

Mitosis is a fundamental process of cell division that produces two genetically identical daughter cells from a single parent cell. For IGCSE OCR Biology students, understanding mitosis is essential, as it underpins growth, repair, and asexual reproduction. This article breaks down every key concept, stage, and exam requirement so you can approach questions with confidence.

有丝分裂是细胞分裂的基本过程，从单个母细胞产生两个遗传上完全相同的子细胞。对于学习IGCSE OCR生物学的学生来说，理解有丝分裂至关重要，因为它是生长、修复和无性生殖的基础。本文将拆解每一个关键概念、阶段以及考试要求，帮助你自信应对考题。

1. Overview and Importance of Mitosis | 有丝分裂概述与重要性

Mitosis is a type of nuclear division that maintains the chromosome number. In humans, body cells have 46 chromosomes (the diploid number, 2n), and after mitosis each daughter cell still has 46 chromosomes.

有丝分裂是一种维持染色体数目的核分裂方式。在人体中，体细胞含有46条染色体（二倍体数，2n），有丝分裂后每个子细胞仍然有46条染色体。

The main roles of mitosis are growth of an organism, replacement of worn-out or damaged cells, and asexual reproduction in certain organisms, such as yeast or plants producing runners.

有丝分裂的主要作用是生物体的生长、替换衰老或受损的细胞，以及某些生物（如酵母或产生匍匐茎的植物）的无性生殖。

Mitosis produces clones of the parent cell, meaning there is no genetic variation introduced – unlike in meiosis. This is vital for maintaining the same genetic information in all body cells.

有丝分裂产生母细胞的克隆体，意味着不会引入遗传变异——这与减数分裂不同。这对于在全身细胞中保持相同的遗传信息至关重要。

2. The Cell Cycle and Interphase | 细胞周期与间期

Mitosis is only one part of the cell cycle. The cell cycle consists of interphase and the mitotic phase. Interphase is often mistakenly called the ‘resting phase’, but it is actually a period of intense activity and preparation.

有丝分裂只是细胞周期的一部分。细胞周期包括间期和有丝分裂期。间期常常被错误地称为“静止期”，但实际上它是一个活跃活动和准备的时期。

Interphase can be divided into three stages: G1 (first gap), S (synthesis of DNA), and G2 (second gap). During G1, the cell grows and new organelles are made. In S phase, DNA replication occurs, so each chromosome becomes two identical sister chromatids held together by a centromere.

间期可分为三个阶段：G1（第一间隙期）、S（DNA合成期）和G2（第二间隙期）。在G1期，细胞生长并制造新的细胞器。在S期，进行DNA复制，因此每条染色体变成由着丝粒连接的两条相同姐妹染色单体。

In G2, the cell continues to grow and synthesises proteins needed for division. By the end of interphase, the DNA is in the form of loosely coiled chromatin, which cannot be seen clearly under a light microscope.

在G2期，细胞继续生长并合成分裂所需的蛋白质。间期结束时，DNA以松散缠绕的染色质形式存在，在光学显微镜下无法清晰看到。

A useful mnemonic: ‘I Put My Alligator To College’ stands for Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis. Just remember that interphase is not actually part of mitosis itself!

一个有用的记忆口诀：“我把我鳄鱼送去大学”对应的英文缩写是间期、前期、中期、后期、末期和胞质分裂。但要记住，间期实际上不属于有丝分裂本身！

3. Prophase | 前期

Prophase is the first stage of mitosis. The chromatin condenses and coils up tightly to become visible chromosomes. Under a microscope, each chromosome is seen as two chromatids joined at the centromere.

前期是有丝分裂的第一个阶段。染色质凝缩并紧密缠绕，变成可见的染色体。在显微镜下，每条染色体看起来是由着丝粒连接的两条染色单体。

The nuclear envelope (membrane) breaks down, and the nucleolus disappears. Spindle fibres begin to form from the centrosomes, which move to opposite poles of the cell.

核膜分解，核仁消失。纺锤丝从中心体开始形成，中心体移向细胞的两极。

In animal cells, centrioles are involved in organising the spindle, but plant cells lack centrioles and still manage to form spindle fibres.

在动物细胞中，中心粒参与组织纺锤体，但植物细胞没有中心粒，却仍然能够形成纺锤丝。

4. Metaphase | 中期

During metaphase, the chromosomes line up along the equator (middle) of the cell, often called the metaphase plate. This arrangement ensures that each daughter cell will receive one copy of each chromosome.

在中期，染色体排列在细胞的赤道板（中间）上，常被称为中期板。这种排列确保每个子细胞将获得每条染色体的一个拷贝。

The spindle fibres attach to the centromeres of each chromosome. For each chromosome, one spindle fibre comes from one pole and another from the opposite pole, creating tension that holds the chromosomes in place.

纺锤丝附着在每条染色体的着丝粒上。对于每条染色体，一根纺锤丝来自一极，另一根来自对极，产生张力将染色体固定在位。

At this stage, chromosomes are at their most condensed and are easiest to observe. This is why a root tip squash often shows many cells in metaphase when counting chromosomes.

在这一阶段，染色体最为浓缩，最适合观察。这就是为什么根尖压片在数染色体时常常显示许多处于中期的细胞。

5. Anaphase | 后期

Anaphase begins when the centromeres split. The spindle fibres shorten, pulling the now-separated sister chromatids (now called daughter chromosomes) towards opposite poles of the cell.

后期从着丝粒分裂开始。纺锤丝缩短，将此时已分开的姐妹染色单体（现在称为子染色体）拉向细胞的两极。

Importantly, anaphase ensures that each pole receives an identical set of chromosomes. If a cell originally had 2n = 46, then 46 chromosomes move to one pole and 46 to the other.

重要的是，后期确保每一极得到一组完全相同的染色体。如果细胞原本是2n=46，那么46条染色体移向一极，另46条移向另一极。

A common misconception is that the chromosome number doubles during anaphase; in fact, the number of separate DNA molecules doubles, but each pole receives a complete diploid set, not a doubled set.

一个常见的误解是染色体数目在后期加倍；实际上，独立DNA分子的数量加倍了，但每一极得到的是一个完整的二倍体组，而不是加倍后的组。

6. Telophase and Cytokinesis | 末期与胞质分裂

In telophase, the spindle fibres disintegrate. A new nuclear envelope forms around each set of chromosomes at the two poles. The chromosomes decondense back into chromatin, becoming invisible under the light microscope.

在末期，纺锤丝解体。新的核膜在每一组染色体周围形成，位于两极。染色体解凝缩回染色质，在光学显微镜下变得不可见。

The nucleolus reappears in each new nucleus. Telophase is essentially the reverse of prophase. Mitosis (nuclear division) is now complete, but the cell still needs to divide its cytoplasm.

核仁在每个新核中重新出现。末期本质上是前期的逆转。有丝分裂（核分裂）现在完成了，但细胞还需要分裂细胞质。

Cytokinesis is the division of the cytoplasm. In animal cells, a cleavage furrow forms and pinches the cell into two. In plant cells, a cell plate forms at the equator and develops into a new cell wall and cell membrane, separating the two daughter cells.

胞质分裂是细胞质的分裂。在动物细胞中，形成一个分裂沟，将细胞缢裂为二。在植物细胞中，在赤道板处形成细胞板，发展成新的细胞壁和细胞膜，将两个子细胞分开。

The outcome of mitosis and cytokinesis is two genetically identical daughter cells, each with the same chromosome number as the parent cell.

有丝分裂和胞质分裂的结果是两个遗传上完全相同的子细胞，每个子细胞的染色体数目与母细胞相同。

7. Differences between Animal and Plant Cell Mitosis | 动植物细胞有丝分裂的差异

Although the stages of mitosis are very similar in all eukaryotic cells, there are a few important differences that OCR examiners love to test.

尽管有丝分裂的阶段在所有真核细胞中非常相似，但有一些重要差异是OCR考官喜欢考查的。

Feature / 特征	Animal Cells / 动物细胞	Plant Cells / 植物细胞
Centrioles / 中心粒	Present, help organise spindle / 存在，帮助组织纺锤体	Absent, spindle still forms / 不存在，纺锤体仍能形成
Cytokinesis / 胞质分裂	Cleavage furrow pinches the cell / 分裂沟将细胞缢裂	Cell plate forms, then new cell wall / 形成细胞板，随后形成新细胞壁
Spindle shape / 纺锤体形状	Aster rays may extend from centrioles / 可从中发散出星射线	No asters, spindle is barrel-shaped / 无星体，纺锤体呈桶形

Remember that plant cells have a rigid cell wall, so they cannot pinch inwards; instead they build a new structure from the inside out.

要记住，植物细胞有坚硬的细胞壁，所以它们不能向内缢裂；它们是由内向外构建新的结构。

8. Mitosis and Cancer | 有丝分裂与癌症

Cancer occurs when the cell cycle becomes uncontrolled. Mitosis repeats indefinitely without the normal checkpoints, producing a mass of abnormal cells called a tumour.

癌症发生在细胞周期失控之时。有丝分裂无限重复，没有正常的检查点，产生一团异常的细胞，称为肿瘤。

Genes that regulate the cell cycle can mutate due to various factors, such as UV radiation, chemicals in tobacco smoke, or viral infections. When proto-oncogenes become oncogenes, or tumour suppressor genes are inactivated, the cell loses control over division.

调节细胞周期的基因可能因各种因素突变，如紫外线辐射、烟草烟雾中的化学物质或病毒感染。当原癌基因变成癌基因，或者抑癌基因失活时，细胞就失去了对分裂的控制。

Malignant tumour cells can spread via the blood or lymph to form secondary tumours (metastasis). Treatments like chemotherapy often target rapidly dividing cells, interfering with spindle fibre formation or DNA replication, which affects all quickly dividing cells (including hair follicles and gut lining, causing side effects).

恶性肿瘤细胞可以通过血液或淋巴扩散，形成继发性肿瘤（转移）。像化疗这样的治疗通常针对快速分裂的细胞，干扰纺锤丝形成或DNA复制，这会影响到所有快速分裂的细胞（包括毛囊和肠壁，引起副作用）。

9. Comparison of Mitosis and Meiosis | 有丝分裂与减数分裂的比较

GCSE students must be able to distinguish between mitosis and meiosis clearly. Mitosis produces two diploid cells; meiosis produces four haploid gametes.

GCSE学生必须能够清楚地区分有丝分裂与减数分裂。有丝分裂产生两个二倍体细胞；减数分裂产生四个单倍体配子。

Mitosis involves one division; meiosis involves two successive divisions (meiosis I and II). In meiosis, homologous chromosomes pair up and crossing over occurs, introducing genetic variation. Mitosis does not involve pairing or crossing over.

有丝分裂涉及一次分裂；减数分裂涉及两次连续分裂（减数第一次和减数第二次）。在减数分裂中，同源染色体配对并发生交叉互换，引入遗传变异。有丝分裂不涉及配对或交叉互换。

Mitosis produces genetically identical cells; meiosis produces genetically different cells. This is why mitosis is used for growth and repair, while meiosis is essential for sexual reproduction.

有丝分裂产生遗传上相同的细胞；减数分裂产生遗传上不同的细胞。这就是为什么有丝分裂用于生长和修复，而减数分裂对于有性生殖至关重要。

Mitosis / 有丝分裂	Meiosis / 减数分裂
2 daughter cells / 两个子细胞	4 daughter cells / 四个子细胞
Diploid (2n) → Diploid (2n)	Diploid (2n) → Haploid (n)
One division / 一次分裂	Two divisions / 两次分裂
No pairing of homologous chromosomes / 同源染色体不配对	Homologous chromosomes pair up / 同源染色体配对
No crossing over / 无交叉互换	Crossing over occurs / 发生交叉互换
Genetically identical daughter cells / 遗传上相同的子细胞	Genetically varied daughter cells / 遗传上多样的子细胞

10. Observing Mitosis in Root Tips | 观察根尖有丝分裂实验

A core practical in the OCR specification involves preparing and observing a stained root tip squash under a light microscope to see cells in different stages of mitosis.

OCR大纲中一个核心实验是制备并观察染色的根尖压片，在光学显微镜下观察处于有丝分裂不同阶段的细胞。

The root tip is used because it contains a meristem – a region of rapidly dividing cells. You must first soften the root tip by heating it with hydrochloric acid, which breaks down the middle lamella between plant cells, allowing cells to separate.

之所以使用根尖，是因为它含有分生组织——一个快速分裂的细胞区域。你必须先用盐酸加热根尖使其软化，这会分解植物细胞之间的胞间层，使细胞彼此分离。

After rinsing the root tip in water, you stain it with a dye such as acetic orcein (or toluidine blue), which binds to DNA and makes chromosomes deeply coloured and visible.

用水冲洗根尖后，用醋酸地衣红（或甲苯胺蓝）等染料对其染色，染料与DNA结合，使染色体着色深且可见。

You then macerate the tip on a slide with a mounted needle, apply a coverslip, and gently squash by pressing down – but do not slide the coverslip sideways, or you will shear the chromosomes.

然后在载玻片上用解剖针将根尖捣碎，盖上盖玻片，轻轻按压来压片——但不要侧向移动盖玻片，否则会切断染色体。

Under the microscope, most cells will be in interphase (no visible chromosomes). You should be able to identify cells in prophase (condensed threads), metaphase (chromosomes at equator), anaphase (chromosomes moving apart), and telophase (two new nuclei forming).

在显微镜下，大多数细胞处于间期（没有可见染色体）。你应该能够识别出前期的细胞（浓缩的线状物）、中期（染色体在赤道板）、后期（染色体正分开移动）和末期（两个新核正在形成）。

Always calculate mitotic index if asked: (number of cells in mitosis ÷ total number of cells) × 100. It gives an indication of the rate of cell division in that tissue.

如果被问到，始终要计算有丝分裂指数：（处于分裂期的细胞数 ÷ 细胞总数）× 100。它表示该组织中细胞分裂的速率。

11. Common Misconceptions and Exam Tips | 常见误区与应试技巧

Many students confuse interphase with a resting stage – always emphasise it is a time of intense activity where DNA is replicated. Never say ‘resting’ in an exam answer.

许多学生将间期与静止期混淆——始终强调它是一个活跃活动、DNA复制的时期。考试作答时绝不要说“静止”。

Another frequent error is confusing chromatids with chromosomes. A chromosome made up of two chromatids is still one chromosome until the centromere splits in anaphase. So in metaphase, a human cell has 46 chromosomes (each with two chromatids), not 92.

另一个常见错误是混淆染色单体与染色体。一条由两个染色单体组成的染色体仍然是一条染色体，直到着丝粒在后期分裂。所以在中期，人类细胞有46条染色体（每条有两个染色单体），而不是92条。

Be precise with the terms ‘gene’, ‘DNA’, ‘chromosome’, and ‘chromatid’. Use them correctly: DNA is the molecule, a gene is a section of DNA, chromosomes are coiled structures made of DNA and proteins, and chromatids are identical copies of a chromosome.

准确使用“基因”“DNA”“染色体”和“染色单体”这些术语。正确使用它们：DNA是分子，基因是DNA的一段，染色体是由DNA和蛋白质组成的螺旋结构，而染色单体是染色体的相同拷贝。

In OCR exam questions, you may be asked to state that mitosis produces genetically identical cells, or to explain why this is important – link it to growth and repair, where cells must be exact copies. Contrast with meiosis, which introduces variation for survival advantages.

在OCR考题中，你可能需要陈述有丝分裂产生遗传上相同的细胞，或解释其重要性——要与生长和修复联系起来，此时细胞必须一模一样。与减数分裂进行对比，减数分裂引入变异以提供生存优势。

When describing stages, use the correct sequence and keywords: prophase (condense, nuclear envelope breaks), metaphase (line up equator, spindle attach), anaphase (centromere splits, pull apart), telophase (new nuclei form, cytokinesis). Diagrams in exams often require labelling these stages.

描述阶段时，使用正确的顺序和关键词：前期（凝缩，核膜破裂），中期（排列赤道，纺锤体附着），后期（着丝粒分裂，拉开），末期（新核形成，胞质分裂）。考试中的简图常常需要标注这些阶段。

Finally, always use scientific reasoning: if a question asks about cancer, relate it to uncontrolled mitosis and checkpoints in the cell cycle. Use phrases like ‘uncontrolled cell division’ and ‘tumour formation’.

最后，始终使用科学推理：如果一个问题问及癌症，要联系到不受控的有丝分裂和细胞周期检查点。使用“不受控制的细胞分裂”和“肿瘤形成”等表达。

With these concepts clear and precise language, you can tackle any IGCSE OCR mitosis question. Practice past papers and be ready to apply your knowledge to unfamiliar contexts, such as comparing growth in plants and animals.

有了这些清晰的概念和精准的语言，你就能应对任何IGCSE OCR有丝分裂考题。多做历年真题，并准备好将你的知识应用到不熟悉的情境中，比如比较植物和动物的生长。

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