A-Level生物 细胞周期 有丝分裂

A-Level生物 细胞周期 有丝分裂

1. 引言：什么是细胞周期？ Introduction: What is the Cell Cycle?

The cell cycle is the ordered sequence of events by which a eukaryotic cell grows, duplicates its genetic material, and divides into two genetically identical daughter cells. Understanding the cell cycle is fundamental to biology because it underpins growth, tissue repair, asexual reproduction, and the development of multicellular organisms from a single fertilised egg. Disruption of the cell cycle lies at the heart of cancer biology, making this topic essential for both A-Level examinations and medical biology. 细胞周期是真核细胞生长、复制遗传物质并分裂为两个遗传上相同的子细胞的有序事件序列。理解细胞周期是生物学的基础，因为它支撑着生长、组织修复、无性繁殖以及多细胞生物从单个受精卵发育的过程。细胞周期的紊乱是癌症生物学的核心，使得这一主题对A-Level考试和医学生物学都至关重要。

2. 间期：G1期、S期和G2期 Interphase: G1, S, and G2 Phases

Interphase occupies approximately 90% of the total cell cycle duration and is divided into three distinct stages. During the G1 phase (Gap 1), the cell grows in size, synthesises proteins and organelles, and carries out its normal metabolic functions. The cell also monitors its internal and external environment at the G1 checkpoint to ensure conditions are favourable for DNA replication. Following G1, the S phase (Synthesis) involves the semi-conservative replication of all nuclear DNA, resulting in each chromosome now consisting of two identical sister chromatids held together at the centromere. The centrosome, which organises the mitotic spindle, also duplicates during S phase. The G2 phase (Gap 2) is a second period of growth during which the cell synthesises additional proteins, including tubulin for spindle fibre assembly, and undergoes a final checkpoint to verify that DNA replication has been completed accurately and without damage. 间期占据整个细胞周期大约90%的时间，分为三个不同的阶段。在G1期（第一个间隙期），细胞体积增大，合成蛋白质和细胞器，并执行正常的代谢功能。细胞还在G1检查点监测其内外部环境，以确保条件有利于DNA复制。G1期之后，S期（合成期）涉及所有核DNA的半保留复制，结果每条染色体现在由两条相同的姐妹染色单体组成，它们在着丝粒处连接在一起。组织有丝分裂纺锤体的中心体也在S期复制。G2期（第二个间隙期）是第二个生长期，在此期间细胞合成额外的蛋白质，包括用于纺锤丝组装的微管蛋白，并经历最终检查点以验证DNA复制是否已准确且无损伤地完成。

3. 有丝分裂：前期 Mitosis: Prophase

Prophase marks the beginning of mitosis, the phase during which the replicated chromosomes are physically segregated into two daughter nuclei. As prophase progresses, the chromatin fibres condense into visible chromosomes, each consisting of two sister chromatids joined at the centromere. The nucleolus disappears and the nuclear envelope begins to break down into small membrane vesicles. The two centrosomes, which duplicated during S phase, migrate to opposite poles of the cell driven by motor proteins walking along microtubules. From each centrosome, microtubules polymerise outward to form the mitotic spindle, a bipolar array of fibres that will orchestrate chromosome movement. In animal cells, each centrosome contains a pair of centrioles, although centrioles are not essential for spindle formation in all eukaryotes. 前期标志着有丝分裂的开始，在此阶段复制的染色体被物理分离到两个子细胞核中。随着前期的进行，染色质纤维凝缩成可见的染色体，每条由两条在着丝粒处连接的姐妹染色单体组成。核仁消失，核膜开始分解成小膜泡。在S期复制的两个中心体，在动力蛋白沿微管行走的驱动下迁移到细胞的两极。从每个中心体，微管向外聚合形成有丝分裂纺锤体，这是一个双极纤维阵列，将协调染色体的运动。在动物细胞中，每个中心体含有一对中心粒，尽管中心粒并非所有真核生物纺锤体形成所必需的。

4. 有丝分裂：中期 Mitosis: Metaphase

Metaphase is the stage at which chromosomes reach their maximum condensation and align along a plane equidistant from the two spindle poles. This plane is known as the metaphase plate or equatorial plane. Each chromosome is attached to spindle microtubules from both poles via its kinetochores, which are protein complexes assembled on the centromeric DNA. The kinetochore microtubules from opposite poles exert equal pulling forces on each sister chromatid pair, creating a dynamic equilibrium that holds the chromosomes at the centre of the cell. The metaphase checkpoint (also called the spindle assembly checkpoint) ensures that all chromosomes are correctly attached to the spindle before the cell proceeds to anaphase, preventing chromosome mis-segregation that would lead to aneuploidy. 中期是染色体达到最大凝缩程度并沿着与两极等距的平面排列的阶段。这个平面被称为中期板或赤道面。每条染色体通过其动粒（着丝粒DNA上组装成的蛋白质复合物）附着到来自两极的纺锤体微管上。来自相反极的动粒微管对每对姐妹染色单体施加相等的拉力，形成动态平衡，将染色体保持在细胞中央。中期检查点（也称为纺锤体组装检查点）确保所有染色体在细胞进入后期之前正确附着到纺锤体上，防止染色体错误分离导致非整倍体。

5. 有丝分裂：后期 Mitosis: Anaphase

Anaphase begins when the cohesin protein complexes that hold sister chromatids together are cleaved by the enzyme separase, which had been inhibited by securin until the metaphase checkpoint was satisfied. Once cleaved, the sister chromatids separate and are now considered individual chromosomes. Anaphase proceeds in two overlapping movements. In anaphase A, the kinetochore microtubules shorten as tubulin subunits are depolymerised at the kinetochore end, pulling each chromosome toward its respective spindle pole. In anaphase B, the spindle poles themselves move further apart as polar microtubules from opposite poles slide past each other and push the poles apart, elongating the cell. By the end of anaphase, two complete and identical sets of chromosomes have been segregated to opposite ends of the cell. 后期开始于将姐妹染色单体结合在一起的黏连蛋白复合物被分离酶切割之时，该酶一直被分离酶抑制蛋白抑制，直到中期检查点被满足。切割后，姐妹染色单体分开，现在被视为独立的染色体。后期以两个重叠的运动进行。在后期A中，动粒微管缩短，因为微管蛋白亚基在动粒末端解聚，将每条染色体拉向各自的纺锤体极。在后期B中，纺锤体极本身进一步分开，因为来自相反极的极性微管相互滑动并将两极推开，拉长细胞。到后期结束时，两套完整且相同的染色体已被分离到细胞的两端。

6. 有丝分裂：末期和胞质分裂 Telophase and Cytokinesis

Telophase is essentially the reverse of prophase. The separated chromosomes, now clustered at each pole, begin to decondense back into extended chromatin fibres. New nuclear envelopes reassemble around each chromosome set from fragments of the parent cell’s nuclear membrane and from components of the endoplasmic reticulum. Nucleoli reappear within each new nucleus, and the mitotic spindle microtubules depolymerise. Telophase marks the end of karyokinesis (nuclear division), but cell division is not complete until cytokinesis physically separates the cytoplasm. In animal cells, cytokinesis occurs through the formation of a cleavage furrow driven by a contractile ring of actin and myosin filaments that pinches the cell membrane inward until the cell is divided into two. In plant cells, which have a rigid cell wall, cytokinesis involves the formation of a cell plate at the former metaphase plate, built from Golgi-derived vesicles that fuse and eventually form a new cell wall between the daughter cells. 末期本质上是前期的逆过程。已分离的染色体，现在聚集在每个极，开始解凝缩回伸展的染色质纤维。新的核膜围绕每组染色体重新组装，源自母细胞核膜的片段和内质网的组分。核仁在每个新核内重新出现，有丝分裂纺锤体微管解聚。末期标志着核分裂的结束，但细胞分裂直到胞质分裂物理分离细胞质才完成。在动物细胞中，胞质分裂通过由肌动蛋白和肌球蛋白丝组成的收缩环驱动的分裂沟的形成进行，该环将细胞膜向内收紧，直到细胞被分成两个。在具有刚性细胞壁的植物细胞中，胞质分裂涉及在先前的中期板处形成细胞板，该细胞板由高尔基体衍生的囊泡构建，它们融合并最终在子细胞之间形成新的细胞壁。

7. 细胞周期的调控 Regulation of the Cell Cycle

The orderly progression through the cell cycle is controlled by a molecular regulatory system composed of cyclin-dependent kinases (CDKs) and their regulatory partners, the cyclins. CDK levels remain relatively constant throughout the cell cycle, but their activity oscillates because they require binding to specific cyclins whose concentrations rise and fall at defined stages. For example, the G1/S cyclin-CDK complex triggers progression past the G1 checkpoint into S phase, while the M-phase cyclin-CDK complex (also called MPF, maturation-promoting factor) drives entry into mitosis. The cell cycle features three major checkpoints at which the cycle can be halted if conditions are unfavourable: the G1 checkpoint (restriction point), the G2 checkpoint, and the M-phase checkpoint (spindle assembly checkpoint). At each checkpoint, sensor proteins assess the integrity of the cell’s DNA and the status of key events, and they can activate inhibitors such as p53, which arrests the cell cycle to allow DNA repair or triggers apoptosis if the damage is irreparable. 细胞周期的有序进行由一个分子调控系统控制，该系统由周期蛋白依赖性激酶（CDK）及其调控伙伴周期蛋白组成。CDK水平在整个细胞周期中保持相对恒定，但它们的活性波动，因为它们需要结合特定的周期蛋白，这些蛋白的浓度在特定阶段上升和下降。例如，G1/S周期蛋白-CDK复合物触发通过G1检查点进入S期，而M期周期蛋白-CDK复合物（也称为MPF，成熟促进因子）驱动进入有丝分裂。细胞周期具有三个主要检查点，在这些检查点，如果条件不利，周期可以被暂停：G1检查点（限制点）、G2检查点和M期检查点（纺锤体组装检查点）。在每个检查点，传感器蛋白评估细胞DNA的完整性和关键事件的状态，它们可以激活抑制剂如p53，它暂停细胞周期以允许DNA修复，或在损伤不可修复时触发凋亡。

8. 癌症：不受控制的细胞分裂 Cancer: Uncontrolled Cell Division

Cancer is fundamentally a disease of the cell cycle, arising when the regulatory mechanisms that normally restrict proliferation break down. The genes that code for proteins controlling the cell cycle fall into two broad categories. Proto-oncogenes encode proteins that promote cell division, such as growth factor receptors and components of signal transduction pathways. When mutated into oncogenes, these genes become hyperactive or overexpressed, producing a constant mitogenic signal equivalent to having a stuck accelerator. Tumour suppressor genes, by contrast, encode proteins that inhibit cell division or promote apoptosis. The p53 gene, one of the most frequently mutated genes in human cancers, is a critical tumour suppressor that activates DNA repair pathways and triggers programmed cell death when DNA damage is too severe. The development of cancer typically requires multiple mutations in both proto-oncogenes and tumour suppressor genes, following a multi-step model of carcinogenesis that explains why cancer incidence increases with age. 癌症从根本上说是一种细胞周期疾病，当通常限制增殖的调控机制崩溃时就会产生。编码控制细胞周期蛋白质的基因分为两大类。原癌基因编码促进细胞分裂的蛋白质，如生长因子受体和信号转导通路组分。当突变成癌基因时，这些基因变得过度活跃或过度表达，产生持续的促有丝分裂信号，相当于油门卡住。相比之下，肿瘤抑制基因编码抑制细胞分裂或促进凋亡的蛋白质。p53基因是人类癌症中最常突变的基因之一，是一个关键的肿瘤抑制因子，当DNA损伤过于严重时，它激活DNA修复通路并触发程序性细胞死亡。癌症的发展通常需要原癌基因和肿瘤抑制基因中的多重突变，遵循多步骤致癌模型，这解释了为什么癌症发病率随年龄增加。

9. 考试技巧 Exam Tips

In A-Level biology examinations, questions on mitosis frequently require you to identify and describe the key events of each stage, often using diagrams or photomicrographs of cells in various phases. A common pitfall is confusing the behaviour of chromosomes versus chromatids ： remember that before S phase, a chromosome consists of a single DNA molecule, while after S phase and before anaphase, each chromosome consists of two sister chromatids (two DNA molecules) held together at the centromere. Make sure you can distinguish between interphase (visible nucleus, no distinct chromosomes), prophase (condensed chromosomes, nuclear envelope breaking down), metaphase (chromosomes aligned at the equator), anaphase (chromatids separating to poles), and telophase (two nuclei forming, chromosomes decondensing). Pay careful attention to the difference between mitosis in animal cells (centrioles present, cleavage furrow for cytokinesis) and plant cells (no centrioles, cell plate formation for cytokinesis). When discussing cell cycle regulation, be prepared to explain the roles of cyclins, CDKs, and tumour suppressor proteins such as p53. 在A-Level生物学考试中，有关有丝分裂的题目经常要求你识别和描述每个阶段的关键事件，通常使用处于不同阶段细胞的图表或显微照片。一个常见的错误是混淆染色体和染色单体的行为：记住在S期之前，一条染色体由单个DNA分子组成，而在S期之后、后期之前，每条染色体由两条在着丝粒处连接在一起的姐妹染色单体（两个DNA分子）组成。确保你能区分间期（可见细胞核，无清晰染色体）、前期（凝缩染色体，核膜分解）、中期（染色体排列在赤道面）、后期（染色单体分离到两极）和末期（两个细胞核形成，染色体解凝缩）。特别注意动物细胞有丝分裂（存在中心粒，分裂沟胞质分裂）和植物细胞有丝分裂（无中心粒，细胞板形成胞质分裂）之间的差异。在讨论细胞周期调控时，准备好解释周期蛋白、CDK和肿瘤抑制蛋白如p53的作用。

10. 总结 Summary

The cell cycle is a tightly regulated process that ensures the faithful duplication and segregation of genetic material during every cell division. Interphase, comprising G1, S, and G2, provides the cell with time to grow, replicate its DNA, and prepare for division. Mitosis proceeds through four sequential phases ： prophase, metaphase, anaphase, and telophase ： followed by cytokinesis, yielding two genetically identical daughter cells. The molecular machinery of CDKs and cyclins, monitored by critical checkpoint mechanisms, maintains the fidelity of this process. When these controls fail, uncontrolled proliferation can lead to cancer, underscoring the profound importance of cell cycle regulation in human health. 细胞周期是一个严格调控的过程，确保在每次细胞分裂期间遗传物质的忠实复制和分离。间期包括G1期、S期和G2期，为细胞提供生长、复制DNA和为分裂做准备的时间。有丝分裂通过四个连续阶段：前期、中期、后期和末期：进行，然后是胞质分裂，产生两个遗传相同的子细胞。由CDK和周期蛋白组成的分子机制，在关键检查点机制的监控下，维持这一过程的保真度。当这些控制失败时，不受控制的增殖可导致癌症，凸显了细胞周期调控在人类健康中的深远重要性。