📚 Cell Division IGCSE CIE Biology | IGCSE CIE 生物：细胞分裂 考点精讲
Cell division is a fundamental process by which a parent cell divides into two or more daughter cells. In the IGCSE CIE Biology syllabus, you need to understand the details of the cell cycle, mitosis for growth and repair, and meiosis for sexual reproduction. Mastering the steps, key terms and differences between these processes is essential for your examination success.
细胞分裂是母细胞分裂成两个或多个子细胞的基本过程。在 IGCSE CIE 生物课程中，你需要理解细胞周期的细节、用于生长和修复的有丝分裂以及用于有性生殖的减数分裂。掌握这些过程的步骤、关键术语以及它们之间的差异是考试成功的关键。
1. Introduction to Cell Division | 细胞分裂简介
Cell division enables organisms to grow, replace damaged cells and reproduce. In unicellular organisms such as bacteria, binary fission allows the whole organism to reproduce asexually. In multicellular eukaryotes, mitosis produces genetically identical cells for body growth and tissue repair, while meiosis generates gametes with half the chromosome number for sexual reproduction.
细胞分裂使生物体能够生长、替换受损细胞和繁殖。在细菌等单细胞生物中，二分裂使整个生物体能够进行无性繁殖。在多细胞真核生物中，有丝分裂产生遗传上完全相同的细胞用于身体生长和组织修复，而减数分裂产生染色体数目减半的配子用于有性生殖。
The two main types of nuclear division in eukaryotes are mitosis (equational division) and meiosis (reduction division). You must be able to describe the stages, the behaviour of chromosomes, and the biological significance of each process.
真核生物中核分裂的两种主要类型是有丝分裂（均等分裂）和减数分裂（减数分裂）。你必须能够描述每个过程的阶段、染色体的行为以及生物学意义。
2. The Cell Cycle | 细胞周期
The cell cycle is the series of events that a cell goes through from one division to the next. It consists of interphase (G₁, S, G₂) and the mitotic phase (mitosis and cytokinesis). Most of the cell’s life is spent in interphase, during which it grows, carries out its normal functions and replicates its DNA.
细胞周期是细胞从一次分裂到下一次分裂所经历的一系列事件。它包括间期（G₁ 期、S 期、G₂ 期）和有丝分裂期（有丝分裂和胞质分裂）。细胞一生中大部分时间处于间期，在此期间它生长、执行正常功能并复制 DNA。
In the S phase (synthesis phase), each chromosome duplicates, forming two identical sister chromatids held together at the centromere. The centrosome also duplicates. During G₁ and G₂ phases, the cell synthesises proteins and organelles, and checks for errors before committing to division.
在 S 期（合成期），每条染色体复制，形成两条由着丝粒连接在一起的相同姐妹染色单体。中心体也在此期复制。在 G₁ 和 G₂ 期，细胞合成蛋白质和细胞器，并在进入分裂前检查错误。
- G₁ phase: cell grows and carries out normal metabolism
- S phase: DNA replication (chromosomes become double-stranded)
- G₂ phase: final preparations for mitosis, checking for DNA damage
- G₁ 期：细胞生长并进行正常代谢
- S 期：DNA 复制（染色体变为双链）
- G₂ 期：有丝分裂的最后准备，检查 DNA 损伤
3. Interphase – Not a Resting Phase | 间期——并非休止期
Interphase is often misleadingly called a ‘resting phase’, but cells are metabolically very active. This is when the DNA is in the form of chromatin – long, thin strands that are not visible as individual chromosomes under a light microscope. The nucleolus is prominent, and the nuclear envelope is intact.
间期常被误称为“休止期”，但实际上细胞代谢非常活跃。此时 DNA 以染色质形式存在——长而细的丝状结构，在光学显微镜下看不到单独的染色体。核仁显著，核膜完整。
By the end of interphase, the cell has doubled its DNA content (from 2n to 4n in diploid organisms) and is ready to enter mitosis. The centrosomes, if present, have also doubled and will later help organise the spindle fibres.
到间期结束时，细胞的 DNA 含量加倍（二倍体生物从 2n 变为 4n），准备进入有丝分裂。中心体（如果存在）也已加倍，随后将帮助组织纺锤丝。
4. Mitosis: An Overview | 有丝分裂概述
Mitosis is the division of the nucleus that produces two daughter nuclei genetically identical to the parent nucleus. It ensures that each daughter cell receives an exact copy of the parent cell’s chromosomes. Mitosis is divided into four main stages: prophase, metaphase, anaphase and telophase (PMAT).
有丝分裂是细胞核的分裂，产生两个与母细胞核遗传上相同的子细胞核。它确保每个子细胞获得母细胞染色体的精确副本。有丝分裂分为四个主要阶段：前期、中期、后期和末期（PMAT）。
You must be able to recognise diagrams or photomicrographs of cells in different stages of mitosis in root tip squashes or other suitable material. In animal cells, the centrosomes and spindle fibres are involved; in plant cells, the spindle forms without centrioles.
你必须能够识别根尖压片或其他合适材料中处于有丝分裂不同阶段的细胞图或显微照片。在动物细胞中，中心体和纺锤丝参与该过程；在植物细胞中，纺锤体在没有中心粒的情况下形成。
5. Stages of Mitosis: Prophase and Metaphase | 有丝分裂阶段：前期和中期
Prophase: Chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere. The nucleolus disappears, and the nuclear envelope begins to break down. In animal cells, centrosomes move to opposite poles of the cell and spindle fibres begin to form.
前期：染色质凝缩成可见的染色体，每条染色体由两条在着丝粒处连接的姐妹染色单体组成。核仁消失，核膜开始解体。在动物细胞中，中心体向细胞两极移动，纺锤丝开始形成。
Metaphase: Chromosomes align along the metaphase plate (equator) of the cell. The spindle fibres attach to the centromeres via kinetochores. This arrangement ensures that when the chromatids separate, each new nucleus will receive one copy of each chromosome.
中期：染色体排列在细胞的赤道板（中期板）上。纺锤丝通过动粒附着在着丝粒上。这种排列确保了当染色单体分离时，每个新细胞核将获得每条染色体的一个副本。
6. Stages of Mitosis: Anaphase and Telophase | 有丝分裂阶段：后期和末期
Anaphase: The centromeres divide, and the spindle fibres shorten, pulling the sister chromatids apart to opposite poles of the cell. Once separated, each chromatid is considered an independent chromosome. The cell becomes temporarily 4n in DNA content as the chromatids move apart.
后期：着丝粒分裂，纺锤丝缩短，将姐妹染色单体拉向细胞的两极。一旦分开，每条染色单体被视为一条独立的染色体。随着染色单体分开，细胞的 DNA 含量暂时变为 4n。
Telophase: Chromosomes decondense back into chromatin, nuclear envelopes re-form around each set of chromosomes, and the nucleoli reappear. The spindle fibres disassemble. At this point, two genetically identical nuclei are present in one cell.
末期：染色体解旋回染色质状态，每组染色体周围重新形成核膜，核仁重新出现。纺锤丝解体。此时，一个细胞内有两个遗传上相同的细胞核。
7. Cytokinesis | 胞质分裂
Cytokinesis is the division of the cytoplasm, which usually begins during telophase and overlaps with the end of mitosis. In animal cells, a cleavage furrow forms, pinching the cell membrane inward until two separate daughter cells are formed.
胞质分裂是细胞质的分裂，通常在末期开始，与有丝分裂的结束重叠。在动物细胞中，形成分裂沟，将细胞膜向内紧缩，直到形成两个独立的子细胞。
In plant cells, because of the rigid cell wall, cytokinesis occurs by the formation of a cell plate. Vesicles derived from the Golgi apparatus coalesce at the equator, delivering materials to build a new cell wall and middle lamella between the daughter cells.
在植物细胞中，由于有坚硬的细胞壁，胞质分裂通过形成细胞板进行。来自高尔基体的囊泡在赤道处聚集，提供材料在子细胞之间构建新的细胞壁和胞间层。
8. Importance of Mitosis in Living Organisms | 有丝分裂在生物体中的重要性
Mitosis is essential for growth – an organism increases in size by increasing the number of cells. It also replaces worn-out or damaged cells, for example in skin, blood and the lining of the gut. Asexual reproduction in many plants and some animals (e.g. budding in Hydra) relies entirely on mitosis, producing offspring that are genetically identical clones of the parent.
有丝分裂对于生长至关重要——生物体通过增加细胞数量来增大体积。它还替换磨损或受损的细胞，例如在皮肤、血液和肠道内壁。许多植物和一些动物的无性繁殖（如水螅的出芽生殖）完全依赖有丝分裂，产生与亲本遗传上完全相同的克隆后代。
Mitosis also maintains the diploid chromosome number in somatic cells. By producing genetically identical cells, it ensures that the genetic information remains consistent throughout the body, which is critical for proper functioning.
有丝分裂还维持体细胞中的二倍体染色体数。通过产生遗传相同的细胞，它确保遗传信息在全身保持一致，这对于正常功能至关重要。
9. Meiosis: An Overview | 减数分裂概述
Meiosis is a specialised type of cell division that halves the chromosome number (from diploid 2n to haploid n). This is crucial for sexual reproduction, as it produces gametes (sperm and egg cells in animals, pollen and ovules in plants). Fertilisation then restores the diploid number.
减数分裂是一种特殊类型的细胞分裂，能将染色体数目减半（从二倍体 2n 到单倍体 n）。这对有性生殖至关重要，因为它产生配子（动物的精子和卵细胞，植物的花粉和胚珠）。受精然后恢复二倍体数目。
Meiosis consists of two consecutive divisions: Meiosis I (reductional division) and Meiosis II (equational division). By the end of meiosis, one diploid parent cell has given rise to four haploid daughter cells that are genetically non-identical.
减数分裂由两个连续的分裂组成：减数分裂 I（减数分裂）和减数分裂 II（均等分裂）。到减数分裂结束时，一个二倍体母细胞产生四个遗传上不相同时的单倍体子细胞。
10. Stages of Meiosis I | 减数分裂 I 的阶段
Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis) forming bivalents, and crossing over may occur between non-sister chromatids. The nuclear envelope breaks down, and spindle fibres form. This is the longest phase and introduces genetic variation.
前期 I：染色体凝缩，同源染色体配对（联会）形成二价体，非姐妹染色单体之间可能发生交叉。核膜解体，纺锤丝形成。这是最长的阶段，并引入遗传变异。
Metaphase I: Bivalents align at the metaphase plate with independent assortment – the orientation of each pair is random. Spindle fibres attach to the centromeres of each homologous chromosome.
中期 I：二价体排列在赤道板上，伴有独立分配——每对的方向是随机的。纺锤丝附着在每个同源染色体的着丝粒上。
Anaphase I: Homologous chromosomes are pulled to opposite poles, reducing the chromosome number from diploid to haploid. Sister chromatids remain attached at their centromeres.
后期 I：同源染色体被拉向两极，染色体数目从二倍体减为单倍体。姐妹染色单体在着丝粒处仍相连。
Telophase I: Chromosomes may decondense slightly, nuclear envelopes re-form, and cytokinesis often produces two haploid cells. These cells proceed directly to Meiosis II without DNA replication.
末期 I：染色体稍有解螺旋，核膜重新形成，胞质分裂通常产生两个单倍体细胞。这些细胞直接进入减数分裂 II，而不进行 DNA 复制。
11. Meiosis II and Final Outcome | 减数分裂 II 及最终结果
Meiosis II resembles a mitotic division but starts with haploid cells. In prophase II, chromosomes condense again, the nuclear envelope breaks down, and spindle fibres form. In metaphase II, chromosomes line up individually along the equator.
减数分裂 II 类似于有丝分裂，但起始细胞是单倍体。在前期 II 中，染色体再次凝缩，核膜解体，纺锤丝形成。在中期 II 中，染色体单独排列在赤道板上。
In anaphase II, the centromeres split and sister chromatids are pulled to opposite poles. Telophase II and cytokinesis follow, resulting in four haploid daughter cells, each genetically unique due to crossing over and independent assortment.
在后期 II 中，着丝粒分裂，姐妹染色单体被拉向两极。随后是末期 II 和胞质分裂，最终产生四个单倍体子细胞，由于交叉和独立分配，每个细胞在遗传上都是独特的。
The overall equation for meiosis can be summarised as: one 2n cell → four n cells. Genetic variation is enhanced by crossing over (exchange of alleles between homologous chromosomes) and random segregation of chromosomes.
减数分裂的总方程可总结为：一个 2n 细胞 → 四个 n 细胞。交叉（同源染色体之间交换等位基因）和染色体的随机分离增强了遗传变异。
12. Comparing Mitosis and Meiosis | 有丝分裂与减数分裂比较
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cell chromosome number | Diploid (2n), same as parent | Haploid (n), half of parent |
| Genetic variation | None (clones) | High, due to crossing over and independent assortment |
| Pairing of homologous chromosomes | No | Yes, in Prophase I |
| Role in multicellular organisms | Growth, repair, asexual reproduction | Production of gametes for sexual reproduction |
| 特征 | 有丝分裂 | 减数分裂 |
|---|---|---|
| 分裂次数 | 一次 | 两次 |
| 子细胞染色体数目 | 二倍体 (2n)，与亲本相同 | 单倍体 (n)，亲本的一半 |
| 遗传变异 | 无（克隆） | 高，由于交叉和独立分配 |
| 同源染色体配对 | 不发生 | 发生，在前期 I |
| 在多细胞生物中的作用 | 生长、修复、无性繁殖 | 产生用于有性生殖的配子 |
Remember that in plants, meiosis produces spores that grow into haploid gametophytes by mitosis, not gametes directly. In animals, meiosis directly produces gametes.
记住，在植物中，减数分裂产生孢子，孢子通过有丝分裂长成单倍体配子体，而不是直接产生配子。在动物中，减数分裂直接产生配子。
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