📚 Mitosis | 有丝分裂
Mitosis is a type of cell division that produces two daughter cells, each having the same number and kind of chromosomes as the parent nucleus. It is fundamental for growth, repair, and asexual reproduction in living organisms. In Edexcel IGCSE Biology, understanding mitosis means grasping how a single diploid cell can divide to form two genetically identical diploid cells, and recognising the stages and significance of this process.
有丝分裂是一种细胞分裂方式，产生的两个子细胞含有与母核完全相同数量和种类的染色体。它对于生物体的生长、修复和无性繁殖至关重要。在Edexcel IGCSE生物学中，理解有丝分裂意味着掌握一个二倍体细胞如何分裂成两个遗传上完全相同的二倍体细胞，并认识该过程的各个阶段及其重要性。
1. What is Mitosis? | 什么是有丝分裂？
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane. This produces two daughter cells that are exact copies of the original parent cell.
有丝分裂是真核细胞将其细胞核内的染色体分离成两组相同染色体，并分配到两个子细胞核的过程。随后的胞质分裂将细胞质、细胞器和细胞膜分开。最终产生两个与原母细胞完全相同的子细胞。
In the Edexcel specification, you need to know that mitosis produces two daughter cells that are genetically identical to each other and to the parent cell. The cell that begins mitosis is a diploid cell (2ₙ), meaning it contains two sets of chromosomes, and each daughter cell remains diploid.
在Edexcel考纲中，你需要知道有丝分裂产生的两个子细胞彼此之间以及与母细胞在遗传上是完全相同的。起始有丝分裂的细胞是二倍体细胞 (2ₙ)，意味着它含有两套染色体，而每个子细胞仍保持二倍体状态。
2. Chromosomes, DNA, and Genes | 染色体、DNA 与基因
To understand mitosis, it is essential to recall that the nucleus of a cell contains chromosomes, which are long thread-like structures made of DNA. Each chromosome carries many genes, and a gene is a section of DNA that codes for a specific protein.
要理解有丝分裂，必须回想细胞核中含有染色体，这些染色体是由DNA组成的长线状结构。每条染色体携带许多基因，而基因是一段编码特定蛋白质的DNA片段。
In a diploid human cell, there are 46 chromosomes arranged as 23 pairs. Before mitosis, each chromosome is replicated to form two identical copies called chromatids, joined at a point called the centromere. These chromatids are exact DNA replicas, ensuring that after division each daughter cell receives an identical set of genetic information.
在人类二倍体细胞中，有46条染色体，排成23对。在有丝分裂之前，每条染色体都进行了复制，形成两条完全相同的拷贝，称为染色单体，通过称为着丝粒的结构相连。这些染色单体是完全相同的DNA复制品，确保分裂后每个子细胞获得相同的一套遗传信息。
3. The Cell Cycle and Interphase | 细胞周期与间期
Mitosis is part of a larger sequence known as the cell cycle. The period between one mitosis and the next is called interphase. During interphase, the cell carries out its normal functions, grows larger and, crucially, replicates its DNA. The chromosomes are not visible under a light microscope at this stage because the DNA is in a loosely coiled form called chromatin.
有丝分裂属于一个更大的进程，称为细胞周期。两次有丝分裂之间的时期称为间期。在间期，细胞进行正常功能活动、体积增大，并且关键的是复制其DNA。这一阶段在光学显微镜下看不到染色体，因为DNA处于松散缠绕的染色质状态。
It is this DNA replication during interphase that ensures each chromosome exists as two identical chromatids when mitosis begins. Without this duplication, the daughter cells would end up with half the required genetic material. In Edexcel IGCSE, you are not required to name the sub-phases of interphase (G₁, S, G₂), but understanding that DNA is copied before mitosis is essential.
正是间期的DNA复制保证了有丝分裂开始时每条染色体都以两条相同染色单体的形式存在。如果没有这种复制，子细胞最终只能获得所需遗传物质的一半。在Edexcel IGCSE中，不必说出间期的亚期名称 (G₁、S、G₂)，但必须理解DNA在有丝分裂前已被复制。
4. Prophase – Preparing for Division | 前期 – 分裂的准备
Prophase is the first stage of mitosis, during which the chromatin condenses and coils tightly to become visible as distinct chromosomes. Each chromosome appears as two sister chromatids held together by the centromere. The nuclear membrane begins to break down, and the nucleolus disappears. In animal cells, centrioles move to opposite poles of the cell and form spindle fibres made of protein microtubules.
前期是有丝分裂的第一个阶段，此时染色质凝集、螺旋缠绕，变得可见为清晰的染色体。每条染色体都呈现为由着丝粒连接的两条姐妹染色单体。核膜开始解体，核仁消失。在动物细胞中，中心粒移向细胞两极，并形成由蛋白质微管组成的纺锤丝。
Spindle fibres are essential for the movement of chromosomes in later stages. Plant cells do not have centrioles, but they still organise spindle fibres. By the end of prophase, the chromosomes are free in the cytoplasm and ready to attach to the spindle.
纺锤丝对于后期染色体的移动至关重要。植物细胞没有中心粒，但仍然会组织纺锤丝。前期结束时，染色体游离于细胞质中，准备附着到纺锤体上。
5. Metaphase – Chromosome Alignment | 中期 – 染色体排列
During metaphase, the chromosomes line up along the cell's equator, which is also called the metaphase plate. The spindle fibres attach to the centromere of each chromosome from opposite poles. This alignment ensures that when the chromatids are pulled apart, each new nucleus will receive one copy of each chromatid.
在中期，染色体在细胞的赤道面（也称为中期板）上排列。纺锤丝从两极分别附着在每条染色体的着丝粒上。这种排列确保了当染色单体被拉开时，每个新细胞核都将得到每条染色单体的一个拷贝。
Under a microscope, this is often the stage where chromosomes are most clearly seen, and a karyotype can be prepared. The arrangement is highly organised and under tight cellular control, ensuring the accuracy of chromosome segregation.
在显微镜下，中期往往是染色体看得最清楚的阶段，此时可以制备核型。这种排列高度有序，受到细胞精确调控，保证了染色体分离的准确性。
6. Anaphase – Chromatid Separation | 后期 – 染色单体分离
Anaphase begins when the centromeres divide, allowing each sister chromatid to become an independent daughter chromosome. The spindle fibres shorten, pulling the daughter chromosomes to opposite poles of the cell. Because each chromatid is an exact DNA copy, the genetic material is equally distributed.
后期从着丝粒分裂开始，使得每条姐妹染色单体成为独立的子染色体。纺锤丝缩短，将子染色体拉向细胞两极。由于每一条染色单体都是精确的DNA拷贝，遗传物质得以均等分配。
This separation is powered by motor proteins on the microtubules. By the end of anaphase, each pole contains a complete set of chromosomes. The cell is ready to complete division and form two nuclei.
这种分离是由微管上的马达蛋白驱动的。在后期结束时，每个细胞极都含有一套完整的染色体。细胞已准备好完成分裂并形成两个细胞核。
7. Telophase and Cytokinesis | 末期与胞质分裂
In telophase, the chromosomes arrive at the poles and begin to decondense back into chromatin, becoming less visible. A new nuclear membrane forms around each set of chromosomes, creating two distinct nuclei. The nucleolus reappears, and the spindle fibres disassemble.
在末期，染色体到达两极，开始解凝集回到染色质状态，变得不易看见。每组染色体周围形成新的核膜，从而产生两个独立的细胞核。核仁重新出现，纺锤丝解体。
Cytokinesis then partitions the cytoplasm and organelles between the two daughter cells. In animal cells, a cleavage furrow forms and pinches the cell membrane inward to divide the cell. In plant cells, vesicles from the Golgi apparatus coalesce at the equator to form a cell plate, which eventually becomes a new cell wall.
随后，胞质分裂将细胞质和细胞器分配至两个子细胞。在动物细胞中，形成分裂沟，向内缢缩细胞膜从而将细胞一分为二。在植物细胞中，来自高尔基体的囊泡在赤道面上聚集形成细胞板，最终成为新的细胞壁。
8. Outcome: Two Genetically Identical Diploid Cells | 结果：两个遗传相同的二倍体细胞
The end product of mitosis is two daughter cells, each with a diploid (2ₙ) number of chromosomes genetically identical to the original parent cell. There is no variation introduced by this process, because the chromosomes have been faithfully replicated and equally segregated.
有丝分裂的最终产物是两个子细胞，每个细胞都具有二倍体 (2ₙ) 染色体数目，且在遗传上与原始母细胞完全相同。该过程不会引入变异，因为染色体已经被忠实地复制并均等分开。
This contrasts with meiosis, which halves the chromosome number and creates genetic variation through independent assortment and crossing over. Mitosis, by contrast, is a mechanism of exact copying, making it ideal for producing cells for body growth and tissue repair.
这与减数分裂形成对比；减数分裂会减半染色体数目，并通过独立分配和交叉互换产生遗传变异。相比之下，有丝分裂是精确复制的机制，因此非常适合于产生可用于身体生长和组织修复的细胞。
9. Mitosis in Growth | 有丝分裂在生长中的作用
Mitosis is responsible for increasing the number of cells in an organism, leading to growth. In multicellular organisms, a fertilised egg (zygote) divides repeatedly by mitosis to produce all the somatic cells of the body. This allows organisms to increase in size from embryo to adult.
有丝分裂负责增加生物体中的细胞数量，从而实现生长。在多细胞生物中，受精卵通过多次有丝分裂产生身体的所有体细胞。这使得生物体能够从胚胎到成年增大体积。
In plants, mitosis occurs in meristems such as root tips and shoot tips, where it produces new cells for elongation and development. Understanding how mitosis drives growth explains why organisms need to continuously produce new cells that are exact copies of existing ones.
在植物中，有丝分裂发生在根尖和茎尖等分生组织中，产生用于伸长和发育的新细胞。理解有丝分裂如何驱动生长，可以解释为什么生物体需要不断产生与现有细胞完全相同的新细胞。
10. Mitosis in Repair and Cell Replacement | 有丝分裂在修复与细胞替换中的作用
When tissues are damaged, mitosis produces new cells to replace those that have been lost or destroyed. For example, skin cells are continuously shed and replaced by mitosis in the basal layer of the epidermis. Similarly, the lining of the gut is renewed constantly, and broken bones heal through cell division producing new bone cells.
当组织受损时，有丝分裂产生新细胞来替代那些脱落或被破坏的细胞。例如，皮肤细胞不断脱落，通过表皮基底层的有丝分裂得以更新。同样地，肠道内壁不断被更新，骨折愈合也是通过细胞分裂产生新的骨细胞。
Red blood cells also have a limited lifespan and must be replaced by mitosis of precursor cells in the bone marrow. This role of mitosis is vital for maintaining the integrity and function of tissues and organs throughout an organism's life.
红细胞寿命有限，必须通过骨髓中前体细胞的有丝分裂来替换。有丝分裂的这一作用对于维持生物体一生中组织器官的完整性和功能至关重要。
11. Asexual Reproduction and Cloning | 无性繁殖与克隆
Mitosis is the basis of asexual reproduction in many organisms. Unicellular eukaryotes such as amoeba and yeast reproduce mainly by mitosis, resulting in offspring that are genetically identical to the parent. In multicellular organisms, forms of asexual reproduction like budding in hydra or vegetative propagation in plants (e.g. runners in strawberry plants) rely on mitosis.
有丝分裂是许多生物无性繁殖的基础。单细胞真核生物如变形虫和酵母主要通过有丝分裂进行繁殖，产生的后代与亲本遗传上完全相同。在多细胞生物中，诸如水螅的出芽生殖或植物的营养繁殖（例如草莓藤蔓）等形式也依赖有丝分裂。
Cloning is a modern application of mitosis. By taking a nucleus from a somatic cell and inserting it into an enucleated egg cell, scientists can trigger mitosis to produce a clone. This process, known as somatic cell nuclear transfer (SCNT), relies on the fact that mitosis faithfully copies all the genetic information.
克隆是有丝分裂的现代应用。通过将体细胞的细胞核取出并植入去核的卵细胞，科学家可以触发有丝分裂来产生克隆个体。这个过程被称为体细胞核移植，它依赖的正是有丝分裂能够忠实地复制全部遗传信息这一事实。
12. Comparing Mitosis and Meiosis | 有丝分裂与减数分裂的比较
While mitosis produces two genetically identical diploid daughter cells, meiosis produces four genetically different haploid gametes (sex cells). Mitosis involves one round of division, whereas meiosis involves two. This distinction is critical for understanding sexual reproduction and genetic variation.
有丝分裂产生两个遗传上相同的二倍体子细胞，而减数分裂则产生四个遗传上不同的单倍体配子（性细胞）。有丝分裂只包含一轮分裂，而减数分裂包含两轮。这一区别对于理解有性生殖和遗传变异至关重要。
In the Edexcel IGCSE specification, you must understand that mitosis occurs during growth, repair, cloning and asexual reproduction, while meiosis is involved in the production of gametes. Mitosis maintains the full chromosome number; meiosis halves it. Remember that all body cells, except gametes, are produced by mitosis.
在Edexcel IGCSE考纲中，你必须理解有丝分裂发生在生长、修复、克隆和无性繁殖过程中，而减数分裂则参与配子的产生。有丝分裂维持全套染色体数目，减数分裂则使其减半。请记住，除了配子外，所有体细胞都是由有丝分裂产生的。
A table can help summarise the differences:
一个表格有助于总结这些区别：
| Feature | 特征 | Mitosis | 有丝分裂 | Meiosis | 减数分裂 |
|---|---|---|
| Number of daughter cells | 子细胞数量 | 2 | 4 |
| Chromosome number | 染色体数目 | Diploid (2ₙ) | 二倍体 | Haploid (ₙ) | 单倍体 |
| Genetic variation | 遗传变异 | None, identical to parent | 无，与亲本相同 | Yes, due to crossing over and independent assortment | 有，因交叉互换和独立分配 |
| Role | 作用 | Growth, repair, asexual reproduction, cloning | 生长、修复、无性繁殖、克隆 | Production of gametes for sexual reproduction | 产生用于有性生殖的配子 |
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