📚 Mitosis in IB Biology: Key Concepts and Exam Focus | IB 生物：有丝分裂 考点精讲
Mitosis is the process by which a eukaryotic cell divides its nucleus into two genetically identical daughter nuclei, ensuring that each daughter cell receives a complete set of chromosomes. This carefully choreographed sequence is at the heart of growth, tissue repair, and asexual reproduction. In the IB Biology syllabus (Topic 1.6), understanding mitosis goes beyond naming the phases — it requires linking chromosome behaviour, DNA content changes, regulatory cyclins, and even experimental techniques such as mitotic index calculation. This article unpacks every essential aspect, pairing clear explanations with the precise terminology examiners expect.
有丝分裂是真核细胞将细胞核一分为二、产生两个遗传上完全相同的子细胞核的过程，保证了每个子细胞都能获得全套染色体。这一精密有序的过程是生物体生长、组织修复和无性繁殖的核心。在 IB 生物学大纲（主题 1.6）中，对有丝分裂的掌握远不止记忆各阶段名称——需要将染色体行为、DNA 含量变化、细胞周期蛋白调控，甚至如“有丝分裂指数”计算等实验技术联系起来。本文将逐层拆解每一个关键知识点，用清晰的语言配合阅卷官期待的专业术语，帮助你吃透考点。
1. The Cell Cycle Overview | 细胞周期全景
The cell cycle describes the series of events that lead to cell division. It consists of two broad stages: interphase — during which the cell grows and duplicates its DNA — and the mitotic phase (M phase), where the nucleus divides (mitosis) and the cytoplasm splits (cytokinesis). Interphase itself is subdivided into G1, S, and G2 phases. Cells that are not actively dividing may leave the cycle and enter a non-dividing state called G0. A typical mammalian cell spends about 90% of its cycle in interphase, underlining that preparation, not division, occupies most of a cell’s life.
细胞周期描述了细胞分裂的连续事件。它由两个大的阶段组成：间期——细胞生长并复制 DNA——以及有丝分裂期（M 期），此时核分裂（有丝分裂）和胞质分裂相继发生。间期又细分为 G1 期、S 期和 G2 期。不活跃分裂的细胞可能退出周期，进入称为 G0 的静止状态。一个典型的哺乳动物细胞约 90% 的时间处于间期，这说明细胞的一生中，准备工作远比分裂本身占据更长时间。
2. Interphase: G1, S, and G2 | 间期：G1、S 与 G2
During G1 (first gap), the cell increases in size, synthesises proteins and RNA, and carries out its normal metabolic functions. A critical checkpoint near the end of G1 assesses whether conditions are favourable for DNA replication. If the cell passes this restriction point, it commits to division. The S phase (synthesis) is when each DNA molecule is replicated, resulting in the doubling of genetic material. Importantly, the chromosome number does not change — a human cell still has 46 chromosomes — but each chromosome now consists of two identical sister chromatids held together at the centromere. In G2 (second gap), the cell continues to grow and synthesises proteins needed for mitosis, such as tubulin for spindle fibres. A second checkpoint ensures that all DNA has been accurately replicated and any damage is repaired before entering mitosis.
在 G1 期（第一间隙期），细胞体积增大，合成蛋白质和 RNA，并执行正常代谢功能。G1 末期的一个重要检查点会评估条件是否适合 DNA 复制。如果细胞通过了这一限制点，便决定进入分裂程序。S 期（合成期）是每个 DNA 分子复制的阶段，遗传物质倍增。关键在于，染色体数目没有改变——人类细胞仍然有 46 条染色体——但此时每条染色体由两条通过着丝粒连接的相同姐妹染色单体组成。在 G2 期（第二间隙期），细胞继续生长并合成分裂所需的蛋白质，例如组成纺锤丝的微管蛋白。第二个检查点确保所有 DNA 均已被准确复制，且任何损伤在进入有丝分裂前得到修复。
3. Chromosome Structure and Key Terminology | 染色体结构与关键术语
To follow mitosis accurately, you must be clear about chromosome anatomy. A chromosome is a highly organised structure of DNA and histone proteins. After S phase, each chromosome is composed of two sister chromatids, which are genetically identical copies. The centromere is the constricted region where sister chromatids are most tightly attached. During mitosis, a multi-protein complex called the kinetochore assembles on the centromere of each chromatid; this is where spindle microtubules attach. Telomeres cap the ends of chromosomes, protecting them from degradation. The terms 'chromosome', 'chromatid' and 'chromatin' are not interchangeable — chromatin refers to the uncoiled DNA-protein complex present during interphase, while the condensed, visible structures during mitosis are correctly called chromosomes.
要想准确理解有丝分裂，必须厘清染色体的结构术语。染色体是由 DNA 和组蛋白高度有序组装的结构。S 期之后，每条染色体由两条遗传上完全相同的姐妹染色单体组成。着丝粒是姐妹染色单体紧附的缢缩区域。有丝分裂期间，一个称为动粒的多蛋白复合体在每条染色单体的着丝粒上组装形成；这正是纺锤体微管附着的位点。端粒覆盖在染色体末端，保护其免于降解。“染色体”“染色单体”和“染色质”这三个术语不可互换——染色质指的是间期存在的松解状态的 DNA–蛋白质复合物，而在有丝分裂期间凝缩、可见的结构则正确地称为染色体。
4. Prophase: Condensation and Spindle Formation | 前期：凝缩与纺锤体形成
Prophase marks the beginning of mitosis. Chromatin fibres coil and fold — supercoiling — into compact, microscopically visible chromosomes, each consisting of two sister chromatids. The nucleolus disappears and the nuclear envelope begins to disintegrate. In the cytoplasm, the centrosomes (or microtubule-organising centres) migrate to opposite poles of the cell. From each centrosome, spindle fibres made of microtubules radiate outwards, forming the mitotic spindle. In animal cells, each centrosome contains a pair of centrioles, though centrioles are absent in higher plants, whose spindle assembly is still effective.
前期标志着有丝分裂的开始。染色质纤维通过螺旋化和折叠——即超螺旋——凝缩成显微镜下可见的紧密染色体，每条染色体包含两条姐妹染色单体。核仁消失，核膜开始解体。在细胞质中，中心体（或称微管组织中心）移向细胞两极。从每个中心体辐射出由微管构成的纺锤丝，形成有丝分裂纺锤体。在动物细胞中，每个中心体含有一对中心粒，而高等植物细胞没有中心粒，但其纺锤体组装依然能够高效完成。
5. Metaphase: Alignment at the Equator | 中期：赤道面列队
Metaphase is characterised by the alignment of chromosomes along the metaphase plate — an imaginary plane equidistant from the two spindle poles. Spindle microtubules attach to the kinetochores of sister chromatids from opposite poles, and the tug-of-war between them arranges all chromosomes at the cell’s equator. This alignment ensures that when sister chromatids separate, each new nucleus will receive one complete set of chromosomes. The spindle assembly checkpoint monitors attachment and tension; the cell will not proceed to anaphase until every chromosome is properly bioriented. Metaphase chromosomes are at their most condensed, making this the ideal stage for karyotyping.
中期最显著的特征是染色体排列在中期板——一个距离两极等距的假想平面。纺锤体微管从两极分别附着于姐妹染色单体的动粒上，两极之间的“拔河”将全部染色体排列在细胞的赤道面。这一排列保证了当姐妹染色单体分离时，每个新核都能获得一整套染色体。纺锤体组装检查点监测微管附着和张力；直到每条染色体都正确实现双极定向，细胞才会进入后期。中期染色体凝缩程度最高，是制作核型分析的理想阶段。
6. Anaphase: Separation of Sister Chromatids | 后期：姐妹染色单体分离
Anaphase begins abruptly when the cohesin complexes holding sister chromatids together are cleaved. This allows the kinetochore microtubules to shorten, pulling the now separated chromatids — now referred to as individual chromosomes — towards opposite poles. Simultaneously, the spindle poles themselves are pushed further apart by the elongation of polar microtubules. Anaphase delivers an equal set of chromosomes to each pole. At this moment, the chromosome number in the cell temporarily quadruples relative to the original diploid count (e.g., a human cell transiently contains 92 chromosomes), but because the cell is about to divide, each daughter will receive the diploid number (46).
后期在黏连姐妹染色单体的黏连蛋白复合体被切割的瞬间骤然启动。由此，动粒微管缩短，将已分开的染色单体——此时可称为独立的染色体——拉向细胞两极。与此同时，极微管的延长将两极进一步推开。后期将均等的染色体组运送至两极。在此瞬间，细胞内染色体数目短暂地变为原先二倍体数目的两倍（例如人类细胞暂时含有 92 条染色体），但因细胞即将分裂，每个子细胞最终将得到二倍体数目（46 条）。
7. Telophase and Cytokinesis | 末期与胞质分裂
Telophase essentially reverses the events of prophase. A nuclear envelope re-forms around each set of chromosomes, using fragments of the parent cell’s nuclear membrane. Chromosomes decondense back into chromatin, becoming less visible under the light microscope. The nucleolus reappears and spindle fibres disassemble. Telophase is often concurrent with cytokinesis, the division of the cytoplasm. In animal cells, a cleavage furrow created by a contractile ring of actin and myosin pinches the cell in two. In plant cells, because of the rigid cell wall, vesicles from the Golgi apparatus coalesce at the equator to form a cell plate, which eventually develops into a new cell wall separating the daughter cells.
末期大体上是前期事件的逆转。核膜围绕每一套染色体重新形成，利用的是母细胞核膜的膜片段。染色体解旋舒展成染色质，在光学显微镜下变得不易辨认。核仁重新出现，纺锤丝解体。末期常与胞质分裂——即细胞质的分割——同步进行。在动物细胞中，由肌动蛋白和肌球蛋白组成的收缩环形成分裂沟，缢缩细胞为两个。在植物细胞中，由于存在刚性细胞壁，来自高尔基体的囊泡在赤道面融合成细胞板，该板最终发育为分隔子细胞的新细胞壁。
8. Plant vs Animal Cytokinesis Compared | 植物与动物胞质分裂对比
Although the nuclear events of mitosis are essentially identical in plants and animals, cytokinesis differs markedly due to the presence of the plant cell wall. Animals use an inward furrow; plants build a partition from the inside out. The table below summarises these differences — a favourite comparison in IB exam questions.
尽管动植物细胞的有丝分裂核内事件基本相同，胞质分裂却因植物细胞壁的存在而差异显著。动物细胞使用内陷式的分裂沟；植物细胞则从内向外构建一道隔墙。下表总结了二者的差异——这是 IB 考试中经常出现的对比内容。
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow formed by contractile ring of actin and myosin | Cell plate formed by Golgi-derived vesicles |
| Direction of division | Outside → inside | Inside → outside |
| Structural constraint | No rigid cell wall; cell can constrict | Rigid cell wall prevents constriction; new wall built |
| Key molecules | Actin, myosin, Rho family GTPases | Pectin, hemicellulose, callose in early cell plate |
9. DNA and Chromosome Number Changes | DNA 与染色体数目变化
A common pitfall is confusing DNA content with chromosome number. In a typical diploid cell (2n), before S phase, DNA content can be described as 2C (where C is the haploid DNA content). After S phase, DNA content doubles to 4C, yet the chromosome number remains 2n — the difference is that each chromosome now contains two chromatids. At anaphase, when sister chromatids separate, chromosome number temporarily reaches 4n, but after cytokinesis each daughter cell returns to 2n with 2C DNA. Exam questions often ask you to interpret graphs of DNA mass or chromosome number against time; you must recognise the steep rise in DNA during S, the plateau through G2 and M until anaphase, and the halving at cytokinesis.
一个常见易错点是将 DNA 含量与染色体数目混为一谈。在一个典型的二倍体细胞（2n）中，S 期之前 DNA 含量可描述为 2C（C 为单倍体的 DNA 含量）。S 期后 DNA 含量加倍为 4C，但染色体数目仍为 2n——区别在于此时每条染色体含有两条染色单体。后期姐妹染色单体分离时，染色体数目暂时达到 4n，胞质分裂后每个子细胞又恢复为 2n，同时 DNA 含量为 2C。考试题常要求你解读 DNA 质量或染色体数目随时间变化的曲线图；你必须能识别 DNA 含量在 S 期的急剧上升，G2 和 M 期（直到后期）的平台，以及在胞质分裂时的减半。
10. Regulation by Cyclins and CDKs | 细胞周期蛋白与 CDK 的调控
The ordered progression through the cell cycle is driven by cyclin-dependent kinases (CDKs) that, as their name implies, are only active when bound to regulatory proteins called cyclins. Cyclin concentrations rise and fall periodically throughout the cycle. For example, cyclin D peaks in G1, cyclin E at the G1/S transition, cyclin A during S and G2, and cyclin B during mitosis. The cyclin–CDK complexes phosphorylate target proteins that trigger specific events — such as DNA replication initiation or nuclear envelope breakdown. The sharp drop in cyclin levels after mitosis is due to their targeted destruction via ubiquitin-mediated proteolysis, ensuring the cycle moves forward irreversibly.
细胞周期有序推进依赖于周期蛋白依赖性激酶（CDK），顾名思义，这类激酶只有与称为周期蛋白的调节蛋白结合后才具有活性。细胞周期蛋白的浓度在整个周期中周期性升降。例如，周期蛋白 D 在 G1 期达到峰值，周期蛋白 E 在 G1/S 转换点，周期蛋白 A 在 S 期和 G2 期，周期蛋白 B 在有丝分裂期。周期蛋白–CDK 复合体磷酸化靶蛋白，触发特定事件——如启动 DNA 复制或核膜解体。有丝分裂后，周期蛋白浓度的骤降归因于泛素介导的蛋白水解对其进行的定向销毁，保证了周期不可逆地向前推进。
11. Checkpoints: G1, G2, and Metaphase | 检查点：G1、G2 与中期
Three major checkpoints safeguard the fidelity of cell division. The G1 checkpoint (restriction point) verifies cell size, nutrients, growth factors, and DNA integrity. If conditions are unfavourable, the cell can enter G0. The G2 checkpoint ensures all DNA has been replicated without errors; if damage is detected, the cycle pauses until repair pathways fix it, or if irreparable, the cell may undergo apoptosis. The metaphase (spindle assembly) checkpoint prevents anaphase until all chromosomes are correctly attached to spindle fibres and aligned at the metaphase plate. Loss of checkpoint function is a hallmark of cancer cells, which continue to divide despite genomic instability.
三个主要检查点保障了细胞分裂的保真度。G1 检查点（限制点）检查细胞大小、营养、生长因子和 DNA 完整性。若条件不利，细胞可进入 G0 期。G2 检查点确保所有 DNA 已无差错地完成复制；若检测到损伤，周期会暂停，等待修复通路完成修复，若损伤不可修复，细胞可能启动凋亡。中期（纺锤体组装）检查点阻止后期启动，直到所有染色体都正确附着在纺锤丝上并排列在中期板。检查点功能丧失是癌细胞的标志特征，癌细胞即使在基因组不稳定的情况下仍持续分裂。
12. Mitotic Index: Calculation and Clinical Relevance | 有丝分裂指数：计算与临床意义
The mitotic index is the ratio of cells undergoing mitosis to the total number of cells in a population, often expressed as a percentage. It is calculated using the formula:
Mitotic Index = (number of cells in mitosis / total number of cells) × 100
A high mitotic index indicates that a tissue is rapidly proliferating — normal in meristems (plant root tips, shoot tips) and in early embryos, but also a red flag in tumour biopsies. Clinically, pathologists use the mitotic index to grade cancers; a higher mitotic index often correlates with more aggressive tumours. In the IB lab, you may be asked to calculate the mitotic index from a garlic or onion root tip squash, identifying cells in different stages under the microscope. This simple ratio provides powerful insight into the dynamic nature of cell division.
有丝分裂指数是正在进行有丝分裂的细胞数占细胞总数的比例，常以百分比表示。计算公式为：
有丝分裂指数 = （处于有丝分裂的细胞数 / 细胞总数）× 100
高有丝分裂指数表明组织在快速增殖——这常见于分生组织（植物根尖、茎尖）和早期胚胎，但在肿瘤活检中同样是一个警示信号。临床上，病理学家使用有丝分裂指数对癌症进行分级；较高的有丝分裂指数往往与侵袭性更强的肿瘤相关。在 IB 实验室中，你可能需要从大蒜或洋葱根尖压片中计算有丝分裂指数，并在显微镜下识别处于不同分裂阶段的细胞。这个简单的比值能有力地揭示细胞分裂的动态特征。
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