Meiosis | 减数分裂 - aleveler.com

📚 Meiosis | 减数分裂

Meiosis is a type of cell division that produces four genetically different haploid daughter cells from a single diploid parent cell. It is essential for sexual reproduction and occurs in the reproductive organs to form gametes (sperm and egg cells in animals, pollen and ovules in plants). Understanding meiosis helps explain how genetic variation arises and why offspring resemble but are not identical to their parents.

减数分裂是一种细胞分裂方式，从一个二倍体母细胞产生四个遗传组成不同的单倍体子细胞。它对有性生殖至关重要，发生在生殖器官中，形成配子（动物中的精子和卵细胞，植物中的花粉和胚珠）。理解减数分裂有助于解释遗传变异如何产生，以及为什么后代与父母相似但又不完全相同。

1. Definition and Significance | 定义与重要性

Meiosis halves the chromosome number, taking a diploid cell (2n) and producing haploid cells (n). In humans, diploid cells have 46 chromosomes (23 pairs), and after meiosis, gametes contain only 23 chromosomes. This reduction is vital so that when two gametes fuse during fertilisation, the normal diploid number is restored, preventing chromosome doubling in each generation.

减数分裂将染色体数目减半，二倍体细胞（2n）产生单倍体细胞（n）。在人类中，二倍体细胞有46条染色体（23对），减数分裂后配子只含有23条染色体。这种减半至关重要，因为在受精过程中两个配子融合时，正常的二倍体数目得以恢复，避免了每一代的染色体数目倍增。

2. Site of Meiosis | 减数分裂的发生部位

In humans, meiosis takes place in the testes (to produce sperm) and in the ovaries (to produce eggs). In flowering plants, meiosis occurs in the anthers (to produce pollen grains) and in the ovules (to produce egg cells). In all cases, the process occurs inside specialised reproductive structures.

在人类中，减数分裂发生在睾丸（产生精子）和卵巢（产生卵子）中。在开花植物中，减数分裂发生在花药（产生花粉粒）和胚珠（产生卵细胞）中。在所有情况下，这一过程都发生在特化的生殖结构内部。

3. Overview of Two Divisions | 两次分裂的概述

Meiosis consists of two successive divisions: meiosis I and meiosis II. Meiosis I is the reduction division, separating homologous chromosomes into two daughter cells, each with half the original chromosome number but still with two chromatids per chromosome. Meiosis II is similar to mitosis, separating sister chromatids, resulting in four haploid cells with single‑copy chromosomes.

减数分裂包括两个连续的分裂：减数第一次分裂（减I）和减数第二次分裂（减II）。减I是减数分裂，将同源染色体分开进入两个子细胞，每个子细胞的染色体数目减半，但每条染色体仍含有两个染色单体。减II类似于有丝分裂，将姐妹染色单体分开，最终形成四个单倍体细胞，每个细胞的染色体为单拷贝。

4. Meiosis I – Prophase I | 减数第一次分裂前期

Prophase I is the longest and most complex stage. Chromosomes condense and become visible. Homologous chromosomes pair up in a process called synapsis, forming bivalents. Crossing over occurs between non‑sister chromatids of homologous pairs, exchanging segments of DNA. The nuclear envelope breaks down, and spindle fibres begin to form.

前期I是最长、最复杂的阶段。染色体凝集并变得可见。同源染色体通过联会过程配对，形成二价体。在同源染色体的非姐妹染色单体之间发生交叉互换，交换DNA片段。核膜解体，纺锤丝开始形成。

5. Crossing Over and Genetic Variation | 交叉互换与遗传变异

Crossing over takes place at points called chiasmata. It creates new combinations of alleles on each chromosome, meaning that even the two sister chromatids in one chromosome may no longer be genetically identical. This recombination is a major source of genetic variation in offspring.

交叉互换发生在被称为交叉点的位置。它在每条染色体上创造了新的等位基因组合，意味着一条染色体中的两个姐妹染色单体可能不再遗传上完全相同。这种重组是后代遗传变异的一个主要来源。

6. Metaphase I and Independent Assortment | 中期I与独立分配

During metaphase I, bivalents line up at the equator of the cell, attached to spindle fibres by their centromeres. The orientation of each bivalent is random – the maternal and paternal chromosomes can face either pole. This random alignment leads to independent assortment of chromosomes, another key source of genetic variability.

在中期I，二价体排列在细胞的赤道板上，通过着丝粒附着在纺锤丝上。每个二价体的方向是随机的——母方和父方染色体可以朝向任意一极。这种随机排列导致了染色体的独立分配，这是遗传变异的另一个重要来源。

7. Anaphase I and Telophase I | 后期I与末期I

In anaphase I, homologous chromosomes are pulled apart to opposite poles of the cell. Unlike mitosis, the centromeres do not divide, so sister chromatids remain attached. In telophase I, the chromosomes may decondense slightly, and the nuclear envelope may reform. Cytokinesis then divides the cytoplasm, forming two haploid daughter cells, each still containing replicated chromosomes (two chromatids).

在后期I，同源染色体被拉向细胞的两极。与有丝分裂不同，着丝粒不分裂，因此姐妹染色单体仍然连在一起。在末期I，染色体可能略微解凝缩，核膜可能重新形成。然后胞质分裂将细胞质分开，形成两个单倍体子细胞，每个子细胞仍然含有复制后的染色体（每条染色体有两个染色单体）。

8. Meiosis II – Separating Sister Chromatids | 减数第二次分裂分离姐妹染色单体

Meiosis II begins immediately without DNA replication. It resembles a mitotic division. In prophase II, chromosomes condense, the nuclear envelope disappears, and spindle fibres form. In metaphase II, chromosomes line up singly on the equator. In anaphase II, centromeres divide, and sister chromatids are pulled to opposite poles. Telophase II and cytokinesis produce four genetically distinct haploid daughter cells.

减数第二次分裂紧接着开始，没有DNA复制。它类似一次有丝分裂。在前期II，染色体凝缩，核膜消失，纺锤丝形成。在中期II，染色体单独排列在赤道板上。在后期II，着丝粒分裂，姐妹染色单体被拉向两极。末期II和胞质分裂最终产生四个遗传上不同的单倍体子细胞。

9. Meiosis vs Mitosis | 减数分裂与有丝分裂的比较

Mitosis produces two genetically identical diploid daughter cells for growth and repair. Meiosis produces four genetically different haploid cells for sexual reproduction. Mitosis involves one division, while meiosis involves two. Crossing over and independent assortment occur only in meiosis. In mitosis, cells remain diploid; meiosis halves the chromosome number.

有丝分裂为生长和修复产生两个遗传上相同的二倍体子细胞。减数分裂为有性生殖产生四个遗传上不同的单倍体细胞。有丝分裂进行一次分裂，减数分裂则进行两次。交叉互换和独立分配只发生在减数分裂中。有丝分裂中细胞保持二倍体；减数分裂则将染色体数目减半。

10. Importance in Sexual Reproduction | 在有性生殖中的重要性

Meiosis ensures that the number of chromosomes remains constant from generation to generation. Without the reduction division, fertilisation would double the chromosome number each time, which would be lethal. The genetic variation produced by meiosis and random fertilisation is the raw material for natural selection and evolution.

减数分裂确保染色体数目代代保持恒定。如果没有减数作用，每次受精都会使染色体数目加倍，这将是致命的。减数分裂和随机受精产生的遗传变异是自然选择和进化的原材料。

11. Exam Focus: Key Diagrams and Terminology | 考试重点：关键图示与术语

You should be able to draw and label the stages of meiosis, especially prophase I (showing bivalents and chiasmata), metaphase I (showing independent assortment), and anaphase I (separating homologous chromosomes). Be familiar with terms: homologous chromosomes, bivalent, crossing over, chiasma, independent assortment, haploid, diploid, and gamete. Questions often ask you to state the number of chromosomes or DNA molecules at various stages.

你应当能够绘制并标注减数分裂的各个阶段，特别是前期I（显示二价体和交叉）、中期I（显示独立分配）和后期I（分离同源染色体）。熟悉以下术语：同源染色体、二价体、交叉互换、交叉、独立分配、单倍体、二倍体和配子。考题经常要求说明在不同阶段的染色体数目或DNA分子数目。

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Meiosis | 减数分裂