📚 Two Types of Nuclear Division: Mitosis and Meiosis | 两种核分裂：有丝分裂与减数分裂
In A-Level Biology, nuclear division is the process by which a nucleus divides to distribute genetic material into daughter nuclei. The two main types are mitosis, which produces genetically identical nuclei for growth and repair, and meiosis, which produces genetically varied haploid nuclei for sexual reproduction.
在 A-Level 生物中，核分裂是细胞核分裂并将遗传物质分配到子核的过程。两种主要类型是有丝分裂和减数分裂：有丝分裂产生遗传上完全相同的子核，用于生长和修复；减数分裂产生遗传上多样的单倍体子核，用于有性生殖。
1. The Role of Nuclear Division | 核分裂的作用
Nuclear division ensures that each daughter cell receives a complete set of genetic instructions. Without accurate nuclear division, cells would inherit too many or too few chromosomes, leading to malfunction or disease.
核分裂确保每个子细胞获得一套完整的遗传指令。如果没有准确的核分裂，细胞会继承过多或过少的染色体，从而导致功能异常或疾病。
Mitosis is used for growth, replacement of damaged cells, and asexual reproduction in many organisms. Meiosis is used only to produce gametes in sexually reproducing organisms.
有丝分裂用于生长、替换受损细胞以及许多生物的无性生殖。减数分裂仅用于在有性生殖生物中产生配子。
2. Key Terminology: Chromosomes, Chromatids and Ploidy | 关键术语：染色体、染色单体和倍性
Before comparing the two types of nuclear division, it is essential to define chromosome structure. A chromosome consists of two identical sister chromatids joined at a centromere after DNA replication.
在比较两种核分裂之前，必须先定义染色体结构。DNA 复制后，一条染色体由两条相同的姐妹染色单体组成，它们在着丝粒处相连。
es, shown as 2n. Haploid cells contain one set, shown as n. In humans, 2n = 46 and n = 23.
二倍体细胞含有两套完整的染色体，记作 2n。单倍体细胞含有一套染色体，记作 n。在人类中，2n=46，n=23。
Homologous chromosomes are pairs of chromosomes that carry the same genes at the same loci, but may have different alleles. One homologue comes from the mother and one from the father.
同源染色体是携带相同基因座位的成对染色体，但可能具有不同的等位基因。一条同源染色体来自母方，另一条来自父方。
3. Overview of Mitosis | 有丝分裂概览
Mitosis is a single nuclear division that produces two daughter nuclei. Each daughter nucleus has the same chromosome number as the parent nucleus, so the division is described as equational.
有丝分裂是一次单一的核分裂，产生两个子核。每个子核的染色体数目与亲代核相同，因此这种分裂被称为均等分裂。
Because the two daughter nuclei are genetically identical to each other and to the parent nucleus, mitosis preserves genetic stability in somatic cells.
由于两个子核之间以及与亲代核之间在遗传上完全相同，有丝分裂在体细胞中保持了遗传稳定性。
In animal cells, mitosis is accompanied by cytokinesis, where the cytoplasm divides by a cleavage furrow. In plant cells, a cell plate forms across the equator.
在动物细胞中，有丝分裂伴随胞质分裂，细胞质通过分裂沟分开。在植物细胞中，赤道板处形成细胞板。
4. Stages of Mitosis | 有丝分裂的阶段
The mitotic phase is divided into prophase, metaphase, anaphase and telophase. Although the process is continuous, these stages help describe the movement of chromosomes.
有丝分裂期分为前期、中期、后期和末期。虽然这一过程是连续的，但这些阶段有助于描述染色体的运动。
In prophase, chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and spindle fibres begin to form from the centrosomes.
在前期，染色质凝缩成可见的染色体，核膜解体，纺锤丝开始从中心体形成。
In metaphase, chromosomes line up along the metaphase plate. Spindle fibres attach to the centromeres of each chromosome.
在中期，染色体排列在赤道板上。纺锤丝附着在每条染色体的着丝粒上。
In anaphase, the centromeres split, and sister chromatids are pulled to opposite poles by shortening spindle fibres.
在后期，着丝粒分裂，姐妹染色单体被缩短的纺锤丝拉向相反的两极。
In telophase, nuclear envelopes reform around each group of chromosomes, and the chromosomes decondense back to chromatin.
在末期，核膜围绕每组染色体重新形成，染色体解凝缩回染色质。
5. Importance of Mitosis | 有丝分裂的重要性
Mitosis is responsible for increasing cell number during growth and development. It also replaces worn-out or damaged cells, such as skin and blood cells.
有丝分裂负责在生长和发育过程中增加细胞数量。它还可以替换磨损或受损的细胞，例如皮肤细胞和血细胞。
In plants, mitosis occurs in meristems at root tips and shoot tips, allowing continuous growth. In animals, stem cells divide by mitosis to produce specialised cells.
在植物中，有丝分裂发生在根尖和茎尖的分生组织中，从而允许持续生长。在动物中，干细胞通过有丝分裂产生特化细胞。
Asexual reproduction relies entirely on mitosis. For example, budding in yeast and vegetative propagation in plants produce clones of the parent.
无性生殖完全依赖有丝分裂。例如，酵母的出芽生殖和植物的营养繁殖会产生与亲本相同的克隆。
6. Overview of Meiosis | 减数分裂概览
Meiosis is a reduction division consisting of two consecutive nuclear divisions: meiosis I and meiosis II. One diploid parent cell produces four haploid daughter cells.
减数分裂是一种减数分裂，由两个连续的核分裂组成：减数第一次分裂和减数第二次分裂。一个二倍体亲代细胞产生四个单倍体子细胞。
Meiosis is essential for sexual reproduction because it halves the chromosome number in gametes. Fertilisation then restores the diploid number in the zygote.
减数分裂对有性生殖至关重要，因为它使配子中的染色体数目减半。受精后，合子中的二倍体数目得以恢复。
In humans, meiosis occurs in the ovaries and testes to produce egg cells and sperm cells respectively.
在人类中，减数分裂分别发生在卵巢和睾丸中，产生卵细胞和精子细胞。
7. Stages of Meiosis | 减数分裂的阶段
Meiosis I is the reduction division. In prophase I, homologous chromosomes pair up to form bivalents, and crossing over occurs at chiasmata.
减数第一次分裂是减数分裂。在前期 I，同源染色体配对形成二价体，并在交叉处发生交叉互换。
In metaphase I, bivalents line up at the equator with their spindle attachments facing opposite poles. This arrangement allows independent assortment.
在中期 I，二价体排列在赤道板上，其纺锤丝附着朝向相反的两极。这种排列方式允许独立分配。
In anaphase I, homologous chromosomes are separated to opposite poles. Sister chromatids remain attached at their centromeres.
在后期 I，同源染色体被分离到相反的两极。姐妹染色单体仍在其着丝粒处保持连接。
In telophase I, two haploid nuclei form, each containing one chromosome from each homologous pair. Meiosis II then separates sister chromatids, similar to mitosis.
在末期 I，形成两个单倍体核，每个核含有每对同源染色体中的一条。随后，减数第二次分裂分离姐妹染色单体，其过程类似于有丝分裂。
Meiosis II produces four genetically different haploid nuclei from the two nuclei formed in meiosis I.
减数第二次分裂由减数第一次分裂形成的两个核产生四个遗传上不同的单倍体核。
8. Genetic Variation in Meiosis | 减数分裂中的遗传变异
Meiosis generates genetic variation through two main mechanisms: crossing over and independent assortment of homologous chromosomes.
减数分裂通过两种主要机制产生遗传变异：交叉互换和同源染色体的独立分配。
Crossing over exchanges segments of DNA between non-sister chromatids of homologous chromosomes. This creates new combinations of alleles on a chromosome.
交叉互换在同源染色体的非姐妹染色单体之间交换 DNA 片段。这会在一条染色体上产生新的等位基因组合。
Independent assortment occurs in metaphase I when homologous pairs line up randomly. The number of possible combinations is 2ⁿ, where n is the haploid number.
独立分配发生在中期 I，此时同源染色体对随机排列。可能的组合数为 2ⁿ，其中 n 是单倍体数目。
Random fertilisation further increases variation because any sperm can fuse with any egg.
随机受精进一步增加了变异，因为任何一个精子都可以与任何一个卵细胞融合。
9. Comparing Mitosis and Meiosis | 比较有丝分裂与减数分裂
The table below summarises the main differences between mitosis and meiosis for A-Level exam answers.
下表总结了有丝分裂与减数分裂之间的主要区别，适用于 A-Level 考试作答。
| Feature | 特征 | Mitosis | 有丝分裂 | Meiosis | 减数分裂 |
|---|---|---|
| Number of divisions | 分裂次数 | 1 | 2 |
| Daughter cells | 子细胞 | 2 diploid | 2 个二倍体 | 4 haploid | 4 个单倍体 |
| Genetic identity | 遗传同一性 | Identical | 相同 | Varied | 多样 |
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