📚 Edexcel Biology 2.6 Cell Division, Cell Diversity and Cellular Organisation | Edexcel 生物 2.6 细胞分裂、细胞多样性与细胞组织
Cell division is the basis of growth, repair and reproduction in living organisms. In Edexcel Biology Topic 2.6, you will learn how the cell cycle is controlled, how mitosis produces genetically identical daughter cells, and how cells become specialised to form tissues, organs and systems. Understanding these ideas is essential for questions on microscopy, stem cells and human health.
细胞分裂是生物体生长、修复和繁殖的基础。在 Edexcel 生物主题 2.6 中，你将学习细胞周期如何受控、有丝分裂如何产生遗传上完全相同的子细胞，以及细胞如何特化形成组织、器官和系统。理解这些概念对于显微镜、干细胞和人类健康相关考题至关重要。
1. The Cell Cycle | 细胞周期
The cell cycle describes the sequence of events by which a cell grows, replicates its DNA and divides. It consists of interphase and the mitotic phase. Interphase is often mistakenly called the "resting stage", but it is actually a period of intense activity. It is divided into G₁, S and G₂ phases. During G₁ the cell grows and carries out normal metabolism; during S phase DNA is replicated, so each chromosome becomes two sister chromatids; during G₂ the cell continues to grow and checks for errors before division.
细胞周期描述细胞生长、复制 DNA 并分裂的一系列事件。它由分裂间期和分裂期组成。分裂间期常被误称为“静止期”，但实际上是一段高度活跃的时期。它分为 G₁ 期、S 期和 G₂ 期。在 G₁ 期细胞生长并进行正常代谢；在 S 期 DNA 复制，因此每条染色体变为两条姐妹染色单体；在 G₂ 期细胞继续生长并在分裂前检查错误。
The cell cycle is tightly controlled by checkpoints, especially at the G₁/S boundary and the G₂/M boundary. These checkpoints ensure that damaged DNA is not passed on to daughter cells. If errors are detected, the cycle may pause for repair or trigger programmed cell death (apoptosis). Loss of checkpoint control can lead to uncontrolled division and cancer.
细胞周期受检查点严格控制，特别是在 G₁/S 边界和 G₂/M 边界。这些检查点确保受损 DNA 不会传给子细胞。如果检测到错误，周期可能暂停进行修复，或触发程序性细胞死亡（细胞凋亡）。失去检查点控制可导致细胞不受控制地分裂并形成癌症。
2. Mitosis: The Stages | 有丝分裂：各阶段
Mitosis is nuclear division that produces two genetically identical nuclei. It is a continuous process but is described in four stages: prophase, metaphase, anaphase and telophase. In prophase, chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere. The nuclear envelope breaks down and spindle fibres begin to form from the centrosomes.
有丝分裂是产生两个遗传相同细胞核的核分裂过程。它是一个连续过程，但通常分为四个阶段：前期、中期、后期和末期。在前期，染色质凝缩为可见的染色体，每条染色体由两条姐妹染色单体在着丝粒处连接。核膜解体，纺锤丝从中心体开始形成。
In metaphase, chromosomes line up on the metaphase plate (equator) and attach to spindle fibres via their centromeres. In anaphase, the centromeres split and sister chromatids are pulled to opposite poles. In telophase, new nuclear envelopes form around each set of chromosomes, and the chromosomes decondense back into chromatin.
在中期，染色体排列在赤道板（中期板）上，并通过着丝粒附着于纺锤丝。在后期，着丝粒分裂，姐妹染色单体被拉向相反两极。在末期，每组染色体周围形成新的核膜，染色体解凝缩回染色质。
3. Cytokinesis in Animal and Plant Cells | 动物与植物细胞的胞质分裂
Cytokinesis follows mitosis and divides the cytoplasm to produce two separate daughter cells. In animal cells, a cleavage furrow forms and deepens until the cell membrane pinches off. In plant cells, because of the rigid cell wall, vesicles gather at the equator and fuse to form a cell plate, which develops into a new cell wall.
胞质分裂在有丝分裂之后进行，将细胞质分开，形成两个独立的子细胞。在动物细胞中，形成分裂沟并逐渐加深，直到细胞膜被缢断。在植物细胞中，由于存在坚硬的细胞壁，囊泡聚集在赤道处并融合形成细胞板，随后发育为新的细胞壁。
4. Observing Mitosis and Calculating the Mitotic Index | 观察有丝分裂与计算有丝分裂指数
You may be asked to describe how mitosis can be observed using a stained root tip squash. The root tip is stained with a dye such as toluidine blue or acetic orcein, then squashed gently to separate cells. Under a light microscope, you can identify cells in interphase and the four stages of mitosis. To quantify the rate of division, you calculate the mitotic index using the formula below.
你可能会被问到如何使用染色的根尖压片观察有丝分裂。根尖用甲苯胺蓝或醋酸地衣红等染料染色，然后轻轻压片使细胞分散。在光学显微镜下，你可以识别处于间期和有丝分裂四个阶段的细胞。为了量化分裂速率，你使用以下公式计算有丝分裂指数。
