High School Biology

Cell Division and the Cell Cycle细胞分裂与细胞周期

Every living organism grows, repairs damage, and reproduces through cell division. This guide moves from the cell cycle (interphase G1/S/G2) through the four phases of mitosis, cytokinesis in animal and plant cells, the two-round reduction of meiosis and gamete formation, a direct comparison of both processes, the sources of genetic variation (crossing over and independent assortment), and finally cell-cycle checkpoints and how their failure leads to cancer. Worked examples and quiz questions draw on NGSS, Ontario, BC, and Alberta standards throughout.每一个生物体的生长、损伤修复和繁殖都依赖细胞分裂（cell division）。本指南从细胞周期（cell cycle，细胞周期）的三个间期阶段（G1/S/G2）出发，依次覆盖有丝分裂（mitosis）的四个时期、动植物胞质分裂（cytokinesis）的异同、减数分裂（meiosis）两轮分裂与配子（gamete）的形成、两种分裂方式的直接比较、交叉互换（crossing over）与独立分配产生的遗传变异，以及细胞周期检验点失控如何引发癌症（cancer）。全部例题与测验均对应 NGSS、安大略、BC 与阿尔伯塔课程标准。

7 sections7 节内容 US NGSS · ON · BC · ABUS NGSS · ON · BC · AB AB Biology 30 content marked HonorsAB Biology 30 内容标为荣誉级
Read me first先读这里

How to use this guide如何使用本指南

Cell division is where every curriculum converges: NGSS HS-LS1-4 requires a model of mitosis and differentiation; NGSS HS-LS3-2 connects meiosis to genetic variation; Ontario SBI3U D3.1 expects phase-level meiosis; BC Life Sciences 11 names mitosis and meiosis explicitly; and Alberta Biology 30 Unit C GO1 expects both processes at the descriptive level. The four curricula agree on the core narrative (cell cycle → mitosis → cytokinesis → meiosis → genetic variation), diverging mainly on depth of phase-by-phase detail. NGSS explicitly excludes rote phase memorization; Ontario SBI3U expects phase names for meiosis; Alberta Biology 30 (Grade 12) expects cell-cycle events, crossing over, and nondisjunction at the descriptive level. The table below locates each section in your curriculum.细胞分裂是各套课程的交汇点：NGSS HS-LS1-4 要求建立有丝分裂与分化的模型；NGSS HS-LS3-2 将减数分裂与遗传变异相关联；安大略 SBI3U D3.1 要求能按时期描述减数分裂；BC Life Sciences 11 明确列出有丝分裂与减数分裂；阿尔伯塔 Biology 30 Unit C GO1 要求在描述层面掌握两种分裂方式。四套大纲在核心叙事上高度一致（细胞周期 → 有丝分裂 → 胞质分裂 → 减数分裂 → 遗传变异），主要分歧在于各时期细节的深度。NGSS 明确排除死记时期名称；安大略 SBI3U 要求掌握减数分裂的时期名称；阿尔伯塔 Biology 30（12 年级）要求在描述层面了解细胞周期事件、交叉互换与非整倍体分裂。下表定位各节在你大纲中的位置。

If you are in…如果你在… Focus on these sections重点学习 Defer / lighter可推迟 / 减负 Source依据
🇺🇸 US NGSS HS Life Sciences美国 NGSS 生命科学 §1 (cell cycle), §2 (mitosis model), §3 (cytokinesis), §6 (genetic variation via meiosis) — HS-LS1-4 and HS-LS3-2§1（细胞周期）、§2（有丝分裂模型）、§3（胞质分裂）、§6（减数分裂产生遗传变异）——HS-LS1-4 与 HS-LS3-2 Phase-by-phase meiosis (§4, §5): HS-LS3-2 Assessment Boundary explicitly excludes "the phases of meiosis" — conceptual variation is the target减数分裂各时期（§4、§5）：HS-LS3-2 评估边界明确排除"减数分裂的各时期"——目标是变异的概念性理解 NGSS HS Life Science — HS-LS1-4; HS-LS3-2 PE and Assessment Boundary— HS-LS1-4；HS-LS3-2 及评估边界
🇨🇦 ON Grade 11 — SBI3U安大略 11 年级 — SBI3U All 7 sections. SBI3U D3.1 requires phases of meiosis, chromosome movement, and crossing over. D2.2 requires investigation and diagrams of meiosis phases全部 7 节。SBI3U D3.1 要求掌握减数分裂各时期、染色体运动与交叉互换。D2.2 要求研究并绘制减数分裂时期图 Molecular-level checkpoint biochemistry in §7 going-deeper (p53, CDK/cyclin) — not in SBI3U scope第 §7 深入内容中的分子层面检验点生化（p53、CDK/cyclin）——不在 SBI3U 范围内 Ontario SBI3U/4U Biology — SBI3U Strand D D2.2, D3.1— SBI3U D 单元 D2.2、D3.1
🇨🇦 BC Life Sciences 11BC Life Sciences 11 §1–§6 are all core: Content bullet "sexual and asexual reproduction: mitosis, meiosis" names both explicitly. §7 connects to Big Idea 1 ("interactions at the molecular and cellular levels")§1–§6 均为核心：内容条目"有性与无性生殖：有丝分裂、减数分裂"明确涵盖两者。§7 与大概念 1（"分子和细胞层面的互动"）相关联 BC Life Sciences 11 does not specify phase-by-phase depth; cram-cheat level sufficesBC Life Sciences 11 未指定各时期细节深度；速记框级别即可 BC Life Sciences 11 / Anatomy 12 — Life Sciences 11 Content (sexual and asexual reproduction)— Life Sciences 11 内容（有性与无性生殖）
🇨🇦 AB Biology 30 Honors阿尔伯塔 Biology 30 荣誉 All 7 sections. Biology 30 Unit C GO1 requires cell cycle (interphase, mitosis, cytokinesis), meiosis (spermatogenesis/oogenesis), crossing over, nondisjunction, and comparison of the two division types全部 7 节。Biology 30 Unit C GO1 要求掌握细胞周期（间期、有丝分裂、胞质分裂）、减数分裂（精子发生/卵子发生）、交叉互换、非整倍体分裂及两种分裂方式的比较 Nothing — this entire guide is direct Biology 30 Unit C GO1 content无 — 本指南全部内容均直接对应 Biology 30 Unit C GO1 Alberta Biology 20/30 — Biology 30 Unit C GO1 (30–C1.1k to 30–C1.5k)— Biology 30 Unit C GO1 (30–C1.1k 至 30–C1.5k)

Once you have located your row, use the two cards below for the approach that fits your timeline.找到所在行后，用下面两张卡片选择适合你时间安排的方式。

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If you are cramming the night before如果你在临阵磨枪

Know the three phases of interphase (G1, S, G2) and what happens in each. Know the four phases of mitosis (PMAT: prophase, metaphase, anaphase, telophase). Know the key difference between mitosis (2 identical diploid daughters) and meiosis (4 genetically unique haploid gametes). Know crossing over and independent assortment as the two main sources of genetic variation from meiosis. Read every cram-cheat box; skip the going-deeper checkpoint biochemistry.掌握间期三个阶段（G1、S、G2）及各阶段发生的事件。掌握有丝分裂四个时期（PMAT：前期、中期、后期、末期）。掌握有丝分裂（产生 2 个相同的二倍体子细胞）与减数分裂（产生 4 个基因独特的单倍体配子）的关键区别。掌握交叉互换与独立分配作为减数分裂产生遗传变异的两大来源。读每个速记框，跳过检验点生化的深入内容。

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If you are going for the top mark如果你目标顶分

Be precise about each cell-cycle checkpoint (G1: size and DNA integrity; G2: replication accuracy; spindle: chromosome attachment). Distinguish Meiosis I (separates homologous pairs, reduces chromosome number) from Meiosis II (separates sister chromatids, like mitosis). Explain exactly how crossing over at prophase I and independent assortment at metaphase I each increase genetic diversity. Connect cell-cycle checkpoint failure to proto-oncogene and tumour-suppressor mutations and cancer progression.精准掌握细胞周期各检验点（G1：大小与 DNA 完整性；G2：复制准确性；纺锤体：染色体附着）。区分减数分裂 I（分离同源染色体对，染色体数目减半）与减数分裂 II（分离姐妹染色单体，类似有丝分裂）。准确解释前期 I 的交叉互换与中期 I 的独立分配各如何增加遗传多样性。将细胞周期检验点失控与原癌基因、肿瘤抑制基因突变及癌症进展相关联。

Honors flag.荣誉级标记。 Alberta Biology 30 is a Grade-12 diploma-exam course — content from Biology 30 Unit C GO1 carries the Honors AB 30 chip. The going-deeper checkpoint biochemistry in §7 (CDK/cyclin complexes, p53, tumour suppressors) exceeds the depth of any of the four curricula at the standard level. NGSS students: the Assessment Boundary for HS-LS1-4 explicitly excludes "rote memorization of the steps of mitosis" — focus on the model and its purpose.阿尔伯塔 Biology 30 是 12 年级毕业考试课程——来自 Biology 30 Unit C GO1 的内容标注 荣誉 AB 30。第 §7 的深入检验点生化（CDK/cyclin 复合物、p53、肿瘤抑制因子）超出四套大纲在标准层面的深度要求。NGSS 学生请注意：HS-LS1-4 的评估边界明确排除"死记有丝分裂步骤"——重点在于模型及其目的。
Section 1 · The cell cycle第 1 节 · 细胞周期

The Cell Cycle: Interphase G1, S, and G2细胞周期：间期 G1、S、G2

The cell cycle — the ordered sequence of events a cell goes through to divide.细胞周期 — 细胞分裂所经历的有序事件序列。
  • G1 phase (first gap):G1 期（第一间隙期）： cell grows in size; synthesizes proteins and organelles; the G1 checkpoint checks whether the cell is large enough and DNA is undamaged before committing to division.细胞体积增大；合成蛋白质和细胞器；G1 检验点检查细胞是否足够大且 DNA 完整，再决定是否进入分裂。
  • S phase (synthesis):S 期（合成期）： DNA replication occurs — each chromosome is duplicated to produce two identical sister chromatids joined at the centromere. The cell's DNA content doubles (e.g., 2n → 4n).DNA 复制——每条染色体（染色体）被复制为两条相同的姐妹染色单体，由着丝点连接。细胞 DNA 含量翻倍（如 2n → 4n）。
  • G2 phase (second gap):G2 期（第二间隙期）： cell continues to grow; synthesizes spindle proteins and other mitotic machinery; the G2 checkpoint verifies DNA was replicated correctly before mitosis begins.细胞继续生长；合成纺锤丝蛋白及其他有丝分裂机器；G2 检验点验证 DNA 是否正确复制，再进入有丝分裂。
  • M phase (mitosis + cytokinesis):M 期（有丝分裂 + 胞质分裂）： nuclear division (mitosis) followed by cytoplasmic division (cytokinesis). Covered in §2 and §3.核分裂（有丝分裂）后接细胞质分裂（胞质分裂）。见 §2 和 §3。

Interphase takes up ~90% of the cell cycle. G1 + S + G2 = interphase. Mitosis itself is comparatively brief.间期（G1 + S + G2）占细胞周期约 90% 的时间。有丝分裂本身相对短暂。

Worked Example 1 · Interpreting a cell-cycle diagram例题 1 · 解读细胞周期图

A cell with 6 chromosomes (2n = 6) is in S phase. How many chromatids does the cell contain after S phase is complete? How does this compare to the number before S phase began?一个有 6 条染色体（2n = 6）的细胞正处于 S 期。S 期完成后，该细胞共有多少条染色单体？与 S 期开始前相比如何？

Before S phase:S 期开始前： 6 chromosomes, each consisting of a single DNA molecule → 6 chromatids total. DNA content = 2n.6 条染色体，每条由一个 DNA 分子组成 → 共 6 条染色单体。DNA 含量 = 2n。

After S phase:S 期完成后： Each chromosome has been replicated to produce two sister chromatids joined at the centromere. 6 chromosomes × 2 sister chromatids = 12 chromatids. The chromosome number is still 6, but the DNA content has doubled to 4n. Sister chromatids are identical copies; they do not separate until anaphase of mitosis.每条染色体已复制为两条姐妹染色单体，由着丝点连接。6 条染色体 × 2 姐妹染色单体 = 12 条染色单体。染色体数目仍为 6，但 DNA 含量翻倍至 4n。姐妹染色单体是完全相同的拷贝，直到有丝分裂后期才分离。

In which phase of the cell cycle does DNA replication occur?细胞周期的哪个阶段发生 DNA 复制？
§1 · Q1
G1 phaseG1 期
G2 phaseG2 期
S phase (synthesis phase)S 期（合成期）
Prophase of mitosis有丝分裂前期
S phase (Synthesis phase) is when DNA is replicated. Each chromosome produces two identical sister chromatids. G1 and G2 are gap phases for growth and preparation; prophase is the first stage of mitosis, after replication is already complete.S 期（合成期）是 DNA 复制的阶段。每条染色体产生两条相同的姐妹染色单体。G1 和 G2 是生长与准备的间隙期；前期是有丝分裂的第一阶段，此时复制已经完成。
DNA replication occurs in S phase (Synthesis phase), not in G1, G2, or mitosis. G1 and G2 are growth and preparation phases; mitosis divides already-replicated chromosomes.DNA 复制发生在 S 期（合成期），而非 G1、G2 或有丝分裂。G1 和 G2 是生长与准备阶段；有丝分裂分离已经复制好的染色体。
The G1 checkpoint primarily checks for which condition before allowing the cell to proceed to S phase?G1 检验点主要检查哪个条件，才允许细胞进入 S 期？
§1 · Q2
Adequate cell size and absence of DNA damage细胞体积足够大且无 DNA 损伤
Correct attachment of chromosomes to the spindle染色体正确附着于纺锤丝
Completion of cytokinesis in the previous division上一次分裂的胞质分裂已完成
Accurate replication of all DNA所有 DNA 准确复制完毕
The G1 checkpoint (restriction point) checks whether the cell is large enough and whether DNA is undamaged. Passing this checkpoint commits the cell to division. The spindle checkpoint checks chromosome attachment; the G2 checkpoint checks replication accuracy.G1 检验点（限制点）检查细胞体积是否足够大以及 DNA 是否完整无损。通过此检验点意味着细胞承诺进入分裂。纺锤体检验点检查染色体附着；G2 检验点检查复制准确性。
G1 checks cell size and DNA integrity. Spindle attachment is checked at the spindle (metaphase) checkpoint. Replication accuracy is checked at G2. Cytokinesis completion is not a formal checkpoint.G1 检查细胞大小与 DNA 完整性。纺锤丝附着在纺锤体（中期）检验点检查。复制准确性在 G2 检查。胞质分裂完成不是正式检验点。
Going deeper — why interphase is not "resting" (Alberta Biology 30 Unit C GO1)深入 — 为何间期并非"静止"期（阿尔伯塔 Biology 30 Unit C GO1）

A common misconception is that interphase is a "resting phase." In fact, interphase is the most metabolically active part of the cell cycle. During G1, the cell synthesizes new proteins, ribosomes, and organelles — all needed to support two daughter cells. During S phase, the entire genome (all 46 chromosomes in humans) must be replicated with very high fidelity; proofreading by DNA polymerase reduces errors to approximately one per billion base pairs. During G2, the cell assembles the microtubule-organizing centers (centrosomes) and the tubulin monomers that will form the spindle. The G2 checkpoint (sometimes called the DNA damage checkpoint) uses the protein kinase ATM/ATR to detect unreplicated or damaged DNA; if detected, the CDK1/cyclin B complex is inhibited and mitosis is blocked until repair is complete.一个常见误解是间期是"静止期"。实际上，间期是细胞周期中代谢最活跃的部分。在 G1 期，细胞合成支持两个子细胞所需的新蛋白质、核糖体和细胞器。在 S 期，整个基因组（人类有 46 条染色体）必须以极高的准确性复制；DNA 聚合酶的校对功能将错误率降至约每十亿个碱基对一个错误。在 G2 期，细胞组装微管组织中心（中心体）和将形成纺锤体的微管蛋白单体。G2 检验点（有时称为 DNA 损伤检验点）利用蛋白激酶 ATM/ATR 检测未复制或受损的 DNA；若检测到损伤，CDK1/cyclin B 复合物被抑制，有丝分裂被阻断直到修复完成。

Section 2 · Mitosis第 2 节 · 有丝分裂

Mitosis: Prophase through Telophase (PMAT)有丝分裂：前期至末期（PMAT）

The four phases of mitosis — PMAT.有丝分裂四个时期 — PMAT。
  • Prophase:前期： chromatin condenses into visible chromosomes; each chromosome consists of two sister chromatids joined at the centromere. The nucleolus disappears; the nuclear envelope breaks down; the mitotic spindle (spindle fibers from centrosomes) begins to form.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通过着丝点相连。核仁消失；核膜解体；有丝分裂纺锤体（从中心体延伸的纺锤丝）开始形成。
  • Metaphase:中期： chromosomes align along the cell's equatorial plate (metaphase plate); spindle fibers attach to the centromere of each chromosome at the kinetochore. The spindle checkpoint ensures every chromosome is properly attached before proceeding.染色体排列在细胞的赤道板（中期板）上；纺锤丝通过动粒附着于每条染色体的着丝点。纺锤体检验点确保每条染色体正确附着后再继续。
  • Anaphase:后期： the centromeres split; sister chromatids are pulled to opposite poles of the cell by spindle fibers shortening. Each separated chromatid is now called a chromosome. The cell elongates.着丝点分裂；纺锤丝缩短将姐妹染色单体拉向细胞两极。每条分离的染色单体现在称为染色体。细胞伸长。
  • Telophase:末期： a nuclear envelope reforms around each set of chromosomes at the two poles; chromosomes decondense back to chromatin; the nucleolus reappears; the spindle disassembles. Two nuclei are now present in one cell.核膜在两极各套染色体周围重新形成；染色体解凝缩回染色质；核仁重新出现；纺锤体解聚。此时一个细胞内存在两个细胞核。
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Quick-reference: what to look for at each mitotic phase快速参考：各有丝分裂时期的观察特征
Phase时期Chromosomes染色体Nuclear envelope核膜Spindle纺锤体
Prophase前期Condense, visible as sister chromatid pairs凝缩，可见为姐妹染色单体对Breaks down解体Forms形成
Metaphase中期Aligned at equatorial plate排列于赤道板Absent缺失Fully formed; attached at kinetochores完全形成；附着于动粒
Anaphase后期Pulled to opposite poles; centromeres split拉向两极；着丝点分裂Absent缺失Shortens缩短
Telophase末期Decondense at each pole在各极解凝缩Reforms around each nucleus在每个细胞核周围重新形成Disassembles解聚
Worked Example 2 · Identifying mitotic phases例题 2 · 辨认有丝分裂时期

A student views a cell under a microscope. The chromosomes are visible and clearly condensed, the nuclear envelope is absent, and the chromosomes appear to be lined up along the middle of the cell. Spindle fibers are visible attaching to the chromosomes. Identify the phase and justify the answer.一名学生在显微镜下观察一个细胞。染色体清晰可见且高度凝缩，核膜缺失，染色体排列在细胞中央。可见纺锤丝附着于染色体。请辨认所处时期并说明理由。

Phase: Metaphase.时期：中期。 Evidence: (1) chromosomes are condensed and visible; (2) nuclear envelope is absent (broke down in prophase); (3) chromosomes are aligned at the equatorial plate — this is the defining feature of metaphase; (4) spindle fibers are attached at kinetochores. Anaphase would show chromosomes moving apart; telophase would show two new nuclear envelopes forming.依据：(1) 染色体凝缩且清晰可见；(2) 核膜缺失（在前期已解体）；(3) 染色体排列在赤道板——这是中期的决定性特征；(4) 纺锤丝附着于动粒。后期会显示染色体向两侧移动；末期会显示两个新核膜正在形成。

During which phase of mitosis do the centromeres split and sister chromatids move to opposite poles?有丝分裂的哪个时期，着丝点分裂，姐妹染色单体移向两极？
§2 · Q1
Prophase前期
Anaphase后期
Metaphase中期
Telophase末期
Anaphase is defined by the splitting of centromeres and the movement of sister chromatids to opposite poles. Prophase: chromosomes condense. Metaphase: chromosomes align. Telophase: nuclear envelopes reform.后期以着丝点分裂和姐妹染色单体移向两极为定义特征。前期：染色体凝缩。中期：染色体排列。末期：核膜重新形成。
Anaphase is when centromeres split and sister chromatids are pulled apart. Remember PMAT: Prophase (condense), Metaphase (align), Anaphase (apart), Telophase (two nuclei).后期是着丝点分裂、姐妹染色单体被拉开的时期。记住 PMAT：前期（凝缩）、中期（排列）、后期（分离）、末期（两核）。
A cell with 2n = 8 chromosomes enters mitosis. How many chromosomes does each daughter cell have at the end of telophase (before cytokinesis)?一个 2n = 8 的细胞进入有丝分裂。末期结束时（胞质分裂前），每个子细胞有多少条染色体？
§2 · Q2
4 chromosomes (haploid)4 条（单倍体）
16 chromosomes (doubled)16 条（翻倍）
8 chromosomes in one nucleus, 0 in the other一个核内 8 条，另一个核内 0 条
8 chromosomes in each of the two nuclei (diploid)两个核各含 8 条染色体（二倍体）
Mitosis preserves chromosome number. The parent cell (2n = 8) replicates its DNA in S phase (producing sister chromatids), then separates them in anaphase. At telophase there are two nuclei, each with 8 chromosomes (2n = 8). Mitosis does not reduce chromosome number — that is meiosis.有丝分裂保持染色体数目不变。亲代细胞（2n = 8）在 S 期复制 DNA（产生姐妹染色单体），然后在后期分离。到末期有两个细胞核，各含 8 条染色体（2n = 8）。有丝分裂不减少染色体数目——减少染色体数目是减数分裂的任务。
Mitosis produces two genetically identical diploid daughter cells. Starting with 2n = 8, each daughter nucleus gets 8 chromosomes (2n = 8). Chromosome number reduction happens in meiosis, not mitosis.有丝分裂产生两个基因相同的二倍体子细胞。从 2n = 8 开始，每个子细胞核得到 8 条染色体（2n = 8）。染色体数目减少发生在减数分裂，而非有丝分裂。
Going deeper — the spindle checkpoint and kinetochore mechanics (NGSS HS-LS1-4 model extension)深入 — 纺锤体检验点与动粒机制（NGSS HS-LS1-4 模型延伸）

The spindle assembly checkpoint (SAC) prevents anaphase from beginning until every kinetochore is attached to spindle fibers from opposite poles (called biorientation or amphitelic attachment). Unattached kinetochores release a "wait" signal (the mitotic checkpoint complex, MCC, which inhibits the Anaphase-Promoting Complex/APC). Once all kinetochores are properly attached, the APC is activated, ubiquitinates securin (releasing separase), and separase cleaves cohesin — the protein holding sister chromatids together. This molecular gate prevents aneuploidy (incorrect chromosome number in daughter cells), which is a leading cause of miscarriage and cancer.纺锤体组装检验点（SAC）阻止后期开始，直到每个动粒都与来自相对两极的纺锤丝连接（称为双向定向或两侧附着）。未附着的动粒释放"等待"信号（有丝分裂检验点复合物 MCC，它抑制后期促进复合物/APC）。一旦所有动粒正确附着，APC 被激活，泛素化安全蛋白（释放分离酶），分离酶切割连接蛋白——将姐妹染色单体连在一起的蛋白质。这道分子闸门防止非整倍体（子细胞中染色体数目不正确），而非整倍体是流产和癌症的主要原因之一。

Section 3 · Cytokinesis第 3 节 · 胞质分裂

Cytokinesis: Animal vs Plant Cells胞质分裂：动物细胞与植物细胞

Cytokinesis divides the cytoplasm after telophase — and it happens differently in animal vs plant cells.胞质分裂在末期后分割细胞质——动物细胞与植物细胞的方式不同。
  • Animal cells — cleavage furrow:动物细胞 — 分裂沟： a ring of actin-myosin filaments contracts around the cell equator, pinching the cell membrane inward like a drawstring. The cell is pulled in two, producing two daughter cells.一圈肌动蛋白-肌球蛋白丝在细胞赤道收缩，像拉紧的绳子一样向内收缩细胞膜。细胞被分割为两个子细胞。
  • Plant cells — cell plate:植物细胞 — 细胞板： a cleavage furrow cannot form because the rigid cell wall prevents membrane pinching inward. Instead, vesicles from the Golgi apparatus migrate to the equatorial plane and fuse to form a cell plate. The cell plate expands outward until it fuses with the existing cell wall, dividing the cell into two. The cell plate eventually becomes a new cell wall between the two daughter cells.由于坚硬的细胞壁阻止细胞膜向内收缩，所以不能形成分裂沟。取而代之的是，来自高尔基体的囊泡迁移至赤道板并融合形成细胞板。细胞板向外扩展直到与现有细胞壁融合，将细胞分为两个。细胞板最终成为两个子细胞之间新的细胞壁。
T
Cytokinesis comparison: animal vs plant胞质分裂比较：动物细胞 vs 植物细胞
Feature特征Animal cell动物细胞Plant cell植物细胞
Mechanism机制Cleavage furrow (contractile ring)分裂沟（收缩环）Cell plate formation细胞板形成
Proteins involved涉及蛋白质Actin and myosin filaments肌动蛋白与肌球蛋白丝Golgi-derived vesicles; cellulose高尔基体衍生囊泡；纤维素
Direction of division分裂方向Outside-in (pinching)由外向内（收缩）Inside-out (expanding plate)由内向外（板扩展）
Reason for difference差异原因No cell wall; flexible membrane无细胞壁；膜具柔性Rigid cell wall prevents inward pinching坚硬细胞壁阻止向内收缩
Which structure forms during cytokinesis in plant cells but NOT in animal cells?植物细胞胞质分裂时形成，而动物细胞不形成的结构是哪个？
§3 · Q1
A cell plate细胞板
A cleavage furrow分裂沟
A mitotic spindle有丝分裂纺锤体
A nuclear envelope核膜
Plant cells form a cell plate (from Golgi vesicles) at the equatorial plane during cytokinesis because they cannot form a cleavage furrow — the rigid cell wall prevents inward pinching. Animal cells form a cleavage furrow instead. Both form nuclear envelopes and mitotic spindles (during mitosis, not cytokinesis).植物细胞在胞质分裂时在赤道面形成细胞板（由高尔基体囊泡形成），因为坚硬的细胞壁阻止形成分裂沟。动物细胞则形成分裂沟。两者均形成核膜和有丝分裂纺锤体（在有丝分裂期间，而非胞质分裂期间）。
A cell plate is plant-cell-specific. Animal cells use a cleavage furrow. The spindle and nuclear envelope form during mitosis (not specific to cytokinesis) and occur in both cell types.细胞板是植物细胞特有的。动物细胞使用分裂沟。纺锤体和核膜在有丝分裂期间形成（不特定于胞质分裂），两种细胞类型均有。
Why can't plant cells divide by forming a cleavage furrow like animal cells do?为何植物细胞不能像动物细胞那样通过形成分裂沟进行分裂？
§3 · Q2
Plant cells have no actin or myosin植物细胞没有肌动蛋白或肌球蛋白
Plant cells have no Golgi apparatus植物细胞没有高尔基体
The rigid cell wall cannot be pinched inward by the contractile ring坚硬的细胞壁无法被收缩环向内收缩
Plant cells have a larger nucleus that blocks membrane pinching植物细胞有较大的细胞核，阻碍细胞膜收缩
The plant cell wall is rigid and cannot be deformed inward by actin-myosin contraction. Plant cells do have actin and myosin, and they do have a Golgi apparatus (which is used to build the cell plate instead). The cell wall is the structural reason for the different cytokinesis mechanism.植物细胞壁是刚性的，无法被肌动蛋白-肌球蛋白收缩向内变形。植物细胞确实有肌动蛋白和肌球蛋白，也有高尔基体（用于构建细胞板）。细胞壁是胞质分裂机制不同的结构性原因。
Plant cells do have actin, myosin, and a Golgi apparatus. The reason they cannot use a cleavage furrow is that the rigid cell wall prevents inward pinching of the plasma membrane.植物细胞确实有肌动蛋白、肌球蛋白和高尔基体。它们不能使用分裂沟的原因是，坚硬的细胞壁阻止质膜向内收缩。
Section 4 · Meiosis I第 4 节 · 减数分裂 I

Meiosis I: The Reductive Division减数分裂 I：染色体数目减半的分裂

Meiosis produces haploid gametes from diploid cells — two successive divisions, starting with Meiosis I.减数分裂由二倍体细胞产生单倍体配子——两次连续分裂，从减数分裂 I 开始。

Why two divisions? To halve the chromosome number. Fertilization (sperm + egg) doubles it again, restoring 2n. Meiosis I is the reductive division; Meiosis II resembles mitosis.为何需要两次分裂？为了将染色体数目减半。受精（精子 + 卵子）时数目再次加倍，恢复到 2n。减数分裂 I 是减数分裂；减数分裂 II 类似有丝分裂。

  • Prophase I:前期 I： chromosomes condense; homologous chromosomes pair up in a process called synapsis, forming bivalents (tetrads). Crossing over occurs: homologous chromosomes exchange segments at points called chiasmata. This is the most genetically significant event in meiosis.染色体凝缩；同源染色体通过联会配对，形成二价体（四分体）。发生交叉互换：同源染色体在交叉点处交换片段。这是减数分裂中遗传意义最重大的事件。
  • Metaphase I:中期 I： bivalents (pairs of homologous chromosomes) align at the metaphase plate. The orientation of each pair is random — this is independent assortment (independent arrangement). Each pair can face either pole; this randomness generates additional genetic variation.二价体（同源染色体对）排列在中期板上。每对的方向是随机的——这就是独立分配。每对可朝向任一极；这种随机性产生额外的遗传变异。
  • Anaphase I:后期 I： homologous chromosomes are pulled to opposite poles. Sister chromatids remain joined at their centromeres. This is the critical difference from mitotic anaphase (where sister chromatids separate). Each pole now has a haploid set of chromosomes (n), though each chromosome still has two chromatids.同源染色体被拉向两极。姐妹染色单体仍通过着丝点连接。这是与有丝分裂后期的关键区别（有丝分裂后期是姐妹染色单体分离）。此时每极有一套单倍体染色体（n），但每条染色体仍有两条染色单体。
  • Telophase I + Cytokinesis I:末期 I + 胞质分裂 I： two haploid cells form. Each has half the original chromosome number (n) but each chromosome still consists of two sister chromatids. No DNA replication occurs between meiosis I and II.形成两个单倍体细胞。每个细胞的染色体数目为原来的一半（n），但每条染色体仍由两条姐妹染色单体组成。减数分裂 I 与 II 之间不发生 DNA 复制。
What is the key event that distinguishes Anaphase I of meiosis from Anaphase of mitosis?减数分裂后期 I 与有丝分裂后期的关键区别事件是什么？
§4 · Q1
Chromosomes decondense in meiosis but not in mitosis减数分裂中染色体解凝缩，有丝分裂中不会
No spindle forms in meiosis I减数分裂 I 中不形成纺锤体
Sister chromatids separate in meiosis I but not in mitosis减数分裂 I 中姐妹染色单体分离，有丝分裂中不会
Homologous chromosomes separate in meiosis I; sister chromatids separate in mitosis减数分裂 I 中同源染色体分离；有丝分裂中姐妹染色单体分离
In Anaphase I, homologous chromosomes are pulled to opposite poles — sister chromatids remain joined. In mitotic anaphase, centromeres split and sister chromatids separate. This is the most important distinction between the two types of anaphase.在后期 I 中，同源染色体被拉向两极——姐妹染色单体仍保持连接。在有丝分裂后期，着丝点分裂，姐妹染色单体分离。这是两种后期之间最重要的区别。
In Anaphase I, homologous pairs separate (not sister chromatids). In mitotic anaphase, sister chromatids separate. Chromosomes stay condensed throughout meiosis I; spindles form in both; sister chromatids separate in Meiosis II (not I).在后期 I 中，同源对分离（不是姐妹染色单体）。在有丝分裂后期，姐妹染色单体分离。染色体在减数分裂 I 全程保持凝缩；两者均形成纺锤体；姐妹染色单体在减数分裂 II（而非 I）中分离。
Crossing over during Prophase I increases genetic variation. At what structures does crossing over physically occur?前期 I 中的交叉互换（crossing over）增加遗传变异。交叉互换发生在哪些结构处？
§4 · Q2
Between sister chromatids of the same chromosome同一染色体的姐妹染色单体之间
Between non-sister chromatids of homologous chromosomes (at chiasmata)同源染色体的非姐妹染色单体之间（在交叉点）
Between two completely unrelated chromosomes两条完全不相关的染色体之间
Only at the centromere of each chromosome仅在每条染色体的着丝点处
Crossing over occurs between non-sister chromatids of homologous chromosomes at points called chiasmata. Homologous chromosomes carry the same genes but potentially different alleles; exchanging segments shuffles allele combinations. Sister chromatids are identical, so crossing over between them produces no new combinations.交叉互换发生在同源染色体的非姐妹染色单体之间，位于称为交叉点（chiasmata）的位置。同源染色体携带相同的基因，但可能携带不同的等位基因；交换片段打乱了等位基因组合。姐妹染色单体是相同的，因此它们之间的交叉互换不产生新的组合。
Crossing over occurs between non-sister chromatids of homologous chromosomes at chiasmata. Not between sister chromatids (identical), not between unrelated chromosomes, and not only at centromeres.交叉互换发生在同源染色体的非姐妹染色单体之间的交叉点。不是姐妹染色单体之间（相同），不是不相关染色体之间，也不仅在着丝点处。
Section 5 · Meiosis II and gamete formation第 5 节 · 减数分裂 II 与配子形成

Meiosis II and Gamete Formation减数分裂 II 与配子形成

Meiosis II separates sister chromatids — like mitosis but starting from haploid cells.减数分裂 II 分离姐妹染色单体——类似有丝分裂，但从单倍体细胞开始。
  • Prophase II:前期 II： if nuclear envelopes reformed in telophase I, they break down again; spindles form in each of the two haploid cells. No further crossing over occurs.如果末期 I 重新形成了核膜，现在再次解体；在两个单倍体细胞中各形成纺锤体。不再发生额外的交叉互换。
  • Metaphase II:中期 II： chromosomes (still consisting of two sister chromatids) align at the equatorial plate in each cell.染色体（仍由两条姐妹染色单体组成）在每个细胞中排列在赤道板上。
  • Anaphase II:后期 II： centromeres split; sister chromatids are pulled to opposite poles. This step is identical to mitotic anaphase.着丝点分裂；姐妹染色单体被拉向两极。此步骤与有丝分裂后期相同。
  • Telophase II + Cytokinesis II:末期 II + 胞质分裂 II： four haploid daughter cells are produced. Each has n chromosomes (one chromosome per homologous pair). Due to crossing over and independent assortment, all four cells are genetically unique.产生四个单倍体子细胞（配子）。每个配子有 n 条染色体（每对同源染色体对应一条）。由于交叉互换和独立分配，四个细胞在遗传上各不相同。

Spermatogenesis vs Oogenesis (Alberta Biology 30 Unit C GO1):精子发生 vs 卵子发生（阿尔伯塔 Biology 30 Unit C GO1）：

  • Spermatogenesis (in testes): one primary spermatocyte (2n) → 4 functional spermatozoa (n). All four products are used.精子发生（在睾丸中）：一个初级精母细胞（2n）→ 4 个功能性精子（n）。四个产物均被使用。
  • Oogenesis (in ovaries): one primary oocyte (2n) → one egg (n) + three polar bodies (n) that degenerate. Unequal cytokinesis concentrates cytoplasm in the egg.卵子发生（在卵巢中）：一个初级卵母细胞（2n）→ 一个卵子（n）+ 三个退化的极体（n）。不均等的胞质分裂将细胞质集中到卵子中。
Worked Example 3 · Counting gamete chromosomes例题 3 · 计算配子染色体数

A human body cell has 2n = 46 chromosomes. (a) How many chromosomes does each gamete contain? (b) How many chromatids does each gamete chromosome consist of?人体细胞有 2n = 46 条染色体。(a) 每个配子含有多少条染色体？(b) 每条配子染色体由多少条染色单体组成？

(a) Gametes are haploid (n).(a) 配子是单倍体（n）。 n = 46 / 2 = 23 chromosomes per gamete. One chromosome from each homologous pair.n = 46 / 2 = 每个配子 23 条染色体。每对同源染色体贡献一条。

(b) Each gamete chromosome is a single double-stranded DNA molecule.(b) 每条配子染色体是一个单独的双链 DNA 分子。 After meiosis II, sister chromatids have been separated. Each gamete chromosome now consists of 1 chromatid (= 1 DNA double helix). Compare to after S phase, when each chromosome has 2 sister chromatids.减数分裂 II 后，姐妹染色单体已分离。每条配子染色体现在由 1 条染色单体（= 1 个 DNA 双螺旋）组成。与 S 期后相比，彼时每条染色体有 2 条姐妹染色单体。

A cell with 2n = 12 undergoes complete meiosis. How many cells result, and what is the chromosome number in each?一个 2n = 12 的细胞完成完整的减数分裂。产生多少个细胞，每个细胞含有多少条染色体？
§5 · Q1
2 cells, each with 12 chromosomes2 个细胞，各含 12 条染色体
2 cells, each with 6 chromosomes2 个细胞，各含 6 条染色体
4 cells, each with 6 chromosomes4 个细胞，各含 6 条染色体
4 cells, each with 12 chromosomes4 个细胞，各含 12 条染色体
Meiosis I produces 2 haploid cells (n = 6 each); Meiosis II divides each of those, yielding 4 haploid cells total, each with 6 chromosomes (n = 6). Chromosome number halved: 12 → 6. Cell number doubled twice: 1 → 2 → 4.减数分裂 I 产生 2 个单倍体细胞（各含 n = 6 条染色体）；减数分裂 II 将每个细胞再分裂，共产生 4 个单倍体细胞，各含 6 条染色体（n = 6）。染色体数目减半：12 → 6。细胞数目翻倍两次：1 → 2 → 4。
Meiosis always produces 4 cells (from 2 divisions). Starting with 2n = 12, each final cell has n = 6 chromosomes. Meiosis does not produce 2 cells (that would be just Meiosis I) nor does it maintain 12 chromosomes (that is mitosis).减数分裂总是产生 4 个细胞（经过两次分裂）。从 2n = 12 开始，每个最终细胞有 n = 6 条染色体。减数分裂不产生 2 个细胞（那只是减数分裂 I），也不保持 12 条染色体（那是有丝分裂）。
In oogenesis, one primary oocyte produces how many functional eggs and how many polar bodies?在卵子发生中，一个初级卵母细胞产生多少个功能性卵子和多少个极体？
§5 · Q2
1 egg and 3 polar bodies1 个卵子和 3 个极体
4 eggs and 0 polar bodies4 个卵子和 0 个极体
2 eggs and 2 polar bodies2 个卵子和 2 个极体
1 egg and 1 polar body1 个卵子和 1 个极体
Oogenesis uses unequal cytokinesis to produce one large egg (with most of the cytoplasm and nutrients) and three small polar bodies that degenerate. This contrasts with spermatogenesis, which produces four functional sperm from one primary spermatocyte.卵子发生利用不均等胞质分裂产生一个大卵子（含大部分细胞质和营养物质）和三个退化的小极体。这与精子发生不同——精子发生由一个初级精母细胞产生四个功能性精子。
Oogenesis produces 1 functional egg + 3 polar bodies (not 4 eggs, not 2+2, not 1+1). The polar bodies receive minimal cytoplasm and degenerate. Compare to spermatogenesis: 4 functional spermatozoa.卵子发生产生 1 个功能性卵子 + 3 个极体（不是 4 个卵子，不是 2+2，不是 1+1）。极体获得极少细胞质并退化。对比精子发生：产生 4 个功能性精子。
Section 6 · Mitosis vs meiosis and genetic variation第 6 节 · 有丝分裂 vs 减数分裂与遗传变异

Mitosis vs Meiosis and Sources of Genetic Variation有丝分裂 vs 减数分裂及遗传变异来源

The comparison every biology exam tests (NGSS HS-LS3-2; SBI3U D3.1; BC Life Sciences 11; AB Biology 30 Unit C GO1).每次生物考试必考的比较（NGSS HS-LS3-2；SBI3U D3.1；BC Life Sciences 11；AB Biology 30 Unit C GO1）。
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Mitosis vs Meiosis: side-by-side有丝分裂 vs 减数分裂：并排比较
Feature特征Mitosis有丝分裂Meiosis减数分裂
Number of divisions分裂次数11 次2 (Meiosis I + II)2 次（减数分裂 I + II）
Daughter cells produced产生的子细胞数22 个44 个
Chromosome number in daughters子细胞染色体数2n (diploid — same as parent)2n（二倍体，与亲代相同）n (haploid — half of parent)n（单倍体，亲代的一半）
Genetic identity of daughters子细胞遗传特性Identical to parent (clones)与亲代相同（克隆）Genetically unique遗传上各不相同
Crossing over交叉互换No无Yes (Prophase I)有（前期 I）
Homologous pairs align at plate?同源对在赤道板排列？No — individual chromosomes align否——单条染色体排列Yes (Metaphase I)是（中期 I）
Purpose目的Growth, repair, asexual reproduction生长、修复、无性生殖Sexual reproduction — gamete production有性生殖——产生配子
Occurs in发生场所All somatic (body) cells所有体细胞Gonads (testes, ovaries)性腺（睾丸、卵巢）
Two sources of genetic variation from meiosis.减数分裂产生遗传变异的两大来源。
  • Crossing over — Prophase I:交叉互换 — 前期 I： non-sister chromatids of homologous chromosomes exchange segments at chiasmata. Creates new allele combinations on chromosomes that did not exist before. The more crossover events, the more recombination.同源染色体的非姐妹染色单体在交叉点处互换片段。在染色体上创造以前不存在的新等位基因组合。交叉互换事件越多，重组越多。
  • Independent assortment — Metaphase I:独立分配 — 中期 I： homologous pairs orient randomly at the metaphase plate. For a cell with n pairs of homologous chromosomes, there are $2^n$ possible chromosome combinations in gametes from assortment alone. For humans (n = 23): $2^{23} \approx 8$ million combinations — before crossing over further multiplies the possibilities.同源对在中期板上随机定向。对于有 n 对同源染色体的细胞，仅靠独立分配就有 $2^n$ 种可能的配子染色体组合。对于人类（n = 23）：$2^{23} \approx 800$ 万种组合——在交叉互换进一步增加可能性之前。
Which of the following is a key difference between mitosis and meiosis?下列哪项是有丝分裂与减数分裂的关键区别？
§6 · Q1
Mitosis occurs only in reproductive organs; meiosis occurs in all body cells有丝分裂仅发生在生殖器官中；减数分裂发生在所有体细胞中
Mitosis produces haploid cells; meiosis produces diploid cells有丝分裂产生单倍体细胞；减数分裂产生二倍体细胞
Mitosis produces 2 diploid identical daughter cells; meiosis produces 4 haploid genetically unique cells有丝分裂产生 2 个相同的二倍体子细胞；减数分裂产生 4 个基因独特的单倍体细胞
Meiosis does not involve chromosome duplication; mitosis does减数分裂不涉及染色体复制；有丝分裂涉及
Mitosis produces 2 diploid (2n) daughter cells that are genetically identical to the parent. Meiosis produces 4 haploid (n) gametes that are genetically unique due to crossing over and independent assortment. This is the single most important comparison in this unit.有丝分裂产生 2 个与亲代基因相同的二倍体（2n）子细胞。减数分裂产生 4 个由于交叉互换和独立分配而在遗传上各不相同的单倍体（n）配子。这是本单元最重要的一个比较。
Mitosis occurs in all somatic cells (not just reproductive organs); meiosis occurs in gonads (not all body cells). Mitosis produces diploid cells; meiosis produces haploid cells. Both processes require DNA replication (in S phase) before division.有丝分裂发生在所有体细胞中（不仅限于生殖器官）；减数分裂发生在性腺中（不是所有体细胞）。有丝分裂产生二倍体细胞；减数分裂产生单倍体细胞。两个过程在分裂前都需要 DNA 复制（在 S 期）。
Independent assortment during Meiosis I means that for a species with n = 4 pairs of homologous chromosomes, the number of possible gamete chromosome combinations from assortment alone is:减数分裂 I 中的独立分配意味着，对于有 n = 4 对同源染色体的物种，仅靠独立分配可能产生的配子染色体组合数为：
§6 · Q2
4 combinations4 种组合
8 combinations8 种组合
12 combinations12 种组合
16 combinations16 种组合
For each pair of homologs, there are 2 possible orientations at Metaphase I. With n = 4 pairs: $2^4 = 16$ combinations. The formula is $2^n$ where n is the haploid number. For humans (n = 23): $2^{23} \approx 8$ million, and crossing over multiplies this further.每对同源染色体在中期 I 有 2 种可能的定向。n = 4 对时：$2^4 = 16$ 种组合。公式为 $2^n$，其中 n 是单倍体数目。对于人类（n = 23）：$2^{23} \approx 800$ 万种，而交叉互换会进一步增加这个数字。
The formula for independent assortment combinations is $2^n$ where n is the number of homologous pairs. With n = 4: $2^4 = 16$. Not 4 (= n), not 8 (= $2^3$), not 12 (no formula gives 12).独立分配组合数的公式是 $2^n$，其中 n 是同源染色体对数。n = 4 时：$2^4 = 16$。不是 4（= n），不是 8（= $2^3$），不是 12（没有公式给出 12）。
Going deeper — nondisjunction and aneuploidy (Alberta Biology 30 Unit C GO1; SBI3U D3.4)深入 — 非整倍体分裂与染色体数目异常（阿尔伯塔 Biology 30 Unit C GO1；SBI3U D3.4）

Nondisjunction occurs when chromosomes or sister chromatids fail to separate correctly during meiosis. If it occurs in Meiosis I, both homologs go to the same pole — producing gametes with either n+1 or n-1 chromosomes. If it occurs in Meiosis II, both sister chromatids go to the same pole. Fertilization of an n+1 gamete by a normal n gamete produces a trisomy (2n+1) zygote; fertilization of an n-1 gamete produces a monosomy (2n-1). Example: trisomy 21 (three copies of chromosome 21) causes Down syndrome. Sex-chromosome nondisjunction can produce XXY (Klinefelter syndrome) or XO (Turner syndrome). Alberta Biology 30 Unit C GO1 (30-C1.5k) requires students to "describe the processes of crossing over and nondisjunction and evaluate their significance to organism inheritance and development."非整倍体分裂发生在染色体或姐妹染色单体在减数分裂过程中未能正确分离时。若发生在减数分裂 I，两条同源染色体移向同一极——产生具有 n+1 或 n-1 条染色体的配子。若发生在减数分裂 II，两条姐妹染色单体移向同一极。n+1 配子与正常 n 配子受精产生三体（2n+1）合子；n-1 配子受精产生单体（2n-1）。例如：21 号染色体三体（三拷贝）导致唐氏综合征。性染色体非整倍体分裂可产生 XXY（克兰费尔特综合征）或 XO（特纳综合征）。阿尔伯塔 Biology 30 Unit C GO1（30-C1.5k）要求学生"描述交叉互换和非整倍体分裂的过程，并评估其对生物遗传和发育的意义。"

Section 7 · Cell-cycle control and cancer第 7 节 · 细胞周期调控与癌症

Cell-Cycle Control and Cancer细胞周期调控与癌症

Normal division is tightly controlled by checkpoints — failure leads to cancer.正常分裂受检验点严格控制——失控导致癌症。
  • Three main checkpoints:三个主要检验点：
    • G1 checkpoint (restriction point): Is the cell large enough? Is DNA undamaged? Are growth signals present? Passing commits the cell to division.G1 检验点（限制点）：细胞体积是否足够大？DNA 是否完整？是否存在生长信号？通过此点意味着承诺进入分裂。
    • G2 checkpoint: Was DNA replicated accurately and completely? Are centrosomes duplicated? Errors detected here halt progression to mitosis.G2 检验点：DNA 是否准确且完整地复制？中心体是否复制？此处检测到错误则阻止进入有丝分裂。
    • Spindle (metaphase) checkpoint: Are all chromosomes properly attached to spindle fibers from both poles? Prevents anaphase until attachment is complete.纺锤体（中期）检验点：所有染色体是否正确附着于来自两极的纺锤丝？在附着完成前阻止后期开始。
  • Proto-oncogenes:原癌基因： normal genes that promote cell division. Mutations can convert them into oncogenes that drive uncontrolled division — like a stuck accelerator.促进细胞分裂的正常基因。突变可将其转化为驱动不受控制分裂的癌基因——就像卡住的油门。
  • Tumour-suppressor genes:肿瘤抑制基因： genes that inhibit cell division or trigger apoptosis when DNA is damaged. The most important is p53. Loss-of-function mutations in tumour-suppressor genes remove the brakes on division — like cutting the brake line.在 DNA 受损时抑制细胞分裂或触发细胞凋亡的基因。最重要的是 p53。肿瘤抑制基因的功能丧失突变取消了分裂的制动——就像切断刹车线。
  • Cancer:癌症： a disease of uncontrolled cell division. Cancer typically requires mutations in multiple checkpoint genes (both oncogenes and tumour suppressors). Cells divide without stopping, invade nearby tissue (invasion), and may spread to distant sites (metastasis).一种细胞不受控制地分裂的疾病。癌症通常需要多个检验点基因（包括癌基因和肿瘤抑制基因）的突变。细胞不停分裂，侵入邻近组织（侵袭），并可能扩散到远处（转移）。
Cancer vs normal cell division — the key contrasts.癌变细胞分裂 vs 正常细胞分裂——关键对比。
  • Normal cells stop dividing when they touch other cells (contact inhibition); cancer cells do not.正常细胞在接触其他细胞时停止分裂（接触抑制）；癌细胞不会。
  • Normal cells have a finite number of divisions (Hayflick limit); cancer cells are often immortal (they reactivate telomerase to prevent chromosome shortening).正常细胞分裂次数有限（Hayflick 极限）；癌细胞通常是"永生的"（它们重新激活端粒酶以防止染色体缩短）。
  • Normal cells undergo apoptosis when DNA damage is irreparable; cancer cells often evade apoptosis by inactivating p53 or Bcl-2-family pathways.正常细胞在 DNA 损伤不可修复时经历细胞凋亡；癌细胞通常通过灭活 p53 或 Bcl-2 家族途径逃避细胞凋亡。
Which type of gene, when mutated to a loss-of-function form, removes the "brakes" on cell division and can contribute to cancer?哪类基因突变为功能丧失形式时，取消细胞分裂的"刹车"，可能导致癌症？
§7 · Q1
Proto-oncogene原癌基因
Tumour-suppressor gene肿瘤抑制基因
Ribosomal RNA gene核糖体 RNA 基因
Spindle protein gene纺锤丝蛋白基因
Tumour-suppressor genes (like p53, Rb) encode proteins that inhibit cell division or trigger apoptosis when conditions are abnormal. Loss-of-function mutations in tumour suppressors are like cutting the brakes — they remove constraints on division. Proto-oncogene gain-of-function mutations are more like a stuck accelerator — both are needed for most cancers.肿瘤抑制基因（如 p53、Rb）编码在条件异常时抑制细胞分裂或触发细胞凋亡的蛋白质。肿瘤抑制基因的功能丧失突变就像切断刹车——取消了对分裂的约束。原癌基因的功能获得性突变更像卡住的油门——大多数癌症需要两者兼备。
Tumour-suppressor genes are the "brakes" of the cell cycle. Their loss-of-function mutations remove inhibition of division. Proto-oncogenes are the "accelerator"; their gain-of-function mutations (creating oncogenes) push the cell to divide excessively.肿瘤抑制基因是细胞周期的"刹车"。其功能丧失突变取消了对分裂的抑制。原癌基因是"油门"；其功能获得性突变（创建癌基因）推动细胞过度分裂。
A cancer cell continues to divide even when it is touching adjacent cells. Which normal cell behavior has been lost?癌细胞即使在接触相邻细胞时仍继续分裂。哪种正常细胞行为已丧失？
§7 · Q2
Contact inhibition接触抑制
DNA replicationDNA 复制
Spindle formation纺锤体形成
G2 checkpoint activityG2 检验点活动
Contact inhibition is the normal process by which cells stop dividing when they touch neighboring cells. Cancer cells lose this property and continue to divide even when crowded, forming a tumor mass. This is one of the hallmarks of cancer.接触抑制是细胞在接触相邻细胞时停止分裂的正常过程。癌细胞失去这一特性，即使在拥挤时仍继续分裂，形成肿瘤块。这是癌症的标志之一。
The behavior described is contact inhibition — normal cells stop dividing when touching neighbors; cancer cells do not. DNA replication, spindle formation, and G2 checkpoint activity can all be abnormal in cancer, but the specific behavior described (dividing despite contact) is contact inhibition.所描述的行为是接触抑制——正常细胞在接触相邻细胞时停止分裂；癌细胞不会。DNA 复制、纺锤体形成和 G2 检验点活动在癌症中都可能异常，但所描述的特定行为（接触后仍分裂）是接触抑制。
Going deeper — CDK/cyclin complexes and the molecular basis of checkpoint control深入 — CDK/cyclin 复合物与检验点调控的分子基础

Cell-cycle checkpoints are enforced by cyclin-dependent kinases (CDKs) — enzymes that phosphorylate target proteins to drive the cell forward. CDKs are only active when bound to their partner cyclins; cyclin concentrations oscillate throughout the cell cycle (they rise, activate CDK, then are destroyed by ubiquitin-mediated proteolysis). The key complexes are: CDK4/6–Cyclin D (drives G1 progression by phosphorylating the retinoblastoma protein Rb, releasing the transcription factor E2F); CDK2–Cyclin E (triggers S-phase entry); CDK1–Cyclin B (the MPF: Maturation Promoting Factor, triggers mitosis). p53 is activated by DNA damage — it induces transcription of p21, a CDK inhibitor (CKI) that blocks CDK2 and halts the cell in G1. If damage cannot be repaired, p53 can trigger apoptosis. Understanding this pathway explains why p53 is mutated in more than 50% of human cancers.细胞周期检验点由周期素依赖性激酶（CDK）执行——这些酶通过磷酸化靶蛋白推动细胞向前。CDK 只有与其伴侣周期素（cyclin）结合时才有活性；周期素浓度在整个细胞周期中振荡（上升、激活 CDK，然后被泛素介导的蛋白水解降解）。关键复合物包括：CDK4/6-细胞周期素 D（通过磷酸化 Rb 蛋白释放转录因子 E2F，驱动 G1 进程）；CDK2-细胞周期素 E（触发 S 期进入）；CDK1-细胞周期素 B（MPF：成熟促进因子，触发有丝分裂）。p53 被 DNA 损伤激活——它诱导 p21 的转录，p21 是一种抑制 CDK2 并使细胞停留在 G1 期的 CDK 抑制剂（CKI）。若损伤无法修复，p53 可触发细胞凋亡。理解这一途径解释了为何 p53 在超过 50% 的人类癌症中发生突变。

Exam Tactics考试策略

Exam Strategy and Common Pitfalls考试策略与常见陷阱

Phase identification questions时期辨认题
  • Use PMAT as the anchor, then identify by the chromosome arrangement.以 PMAT 为锚点，通过染色体排列方式辨认时期。 Prophase: chromosomes visible, nuclear envelope gone. Metaphase: chromosomes at the equatorial plate. Anaphase: chromosomes moving to poles. Telophase: two sets at poles, nuclear envelopes reforming.前期：染色体可见，核膜消失。中期：染色体在赤道板。后期：染色体向两极移动。末期：两组染色体在两极，核膜重新形成。
  • Distinguish meiosis I from meiosis II diagrams.区分减数分裂 I 与 II 的图示。 If homologous pairs are shown moving apart, it is Anaphase I. If individual chromosomes (each a single double-strand) are moving, it is Anaphase II (or mitotic anaphase).若图中显示同源对分离移动，则为后期 I。若单条染色体（每条为单一双链）移动，则为后期 II（或有丝分裂后期）。
Chromosome number questions染色体数目题
  • Track chromosome vs chromatid number carefully.仔细追踪染色体数目与染色单体数目。 After S phase: chromosome number unchanged, but each chromosome = 2 chromatids. After Meiosis I: chromosome number halved (n), each chromosome still = 2 chromatids. After Meiosis II: chromosome number = n, each chromosome = 1 chromatid (the sister chromatids have separated).S 期后：染色体数目不变，但每条染色体 = 2 条染色单体。减数分裂 I 后：染色体数目减半（n），每条染色体仍 = 2 条染色单体。减数分裂 II 后：染色体数目 = n，每条染色体 = 1 条染色单体（姐妹染色单体已分离）。
  • Mitosis never changes the chromosome number; meiosis halves it.有丝分裂从不改变染色体数目；减数分裂将其减半。
Genetic variation questions遗传变异题
  • Name the mechanism AND the phase it occurs in.说明机制名称及其发生的时期。 Crossing over — occurs at chiasmata in Prophase I. Independent assortment — occurs at random alignment of bivalents in Metaphase I.交叉互换——发生在前期 I 的交叉点。独立分配——发生在中期 I 二价体的随机排列。
  • The formula for assortment combinations is $2^n$.独立分配组合数的公式是 $2^n$。 $n$ = the haploid number (number of homologous pairs). For humans, $n = 23$, giving over 8 million combinations from assortment alone.$n$ = 单倍体数目（同源染色体对数）。对于人类，$n = 23$，仅靠独立分配就有超过 800 万种组合。
Active Recall主动复习

Flashcards闪卡

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Three phases of interphase and what happens in each?间期的三个阶段及各阶段发生的事件？
G1: cell growth, protein synthesis. S: DNA replication (sister chromatids form). G2: growth, spindle protein synthesis; G2 checkpoint.G1：细胞生长，蛋白质合成。S：DNA 复制（姐妹染色单体形成）。G2：生长，纺锤丝蛋白合成；G2 检验点。
PMAT — the four phases of mitosis?PMAT — 有丝分裂四个时期？
Prophase: condense + spindle forms. Metaphase: align at plate. Anaphase: centromeres split, chromatids to poles. Telophase: nuclear envelopes reform.前期：凝缩 + 纺锤体形成。中期：排列在赤道板。后期：着丝点分裂，染色单体移向两极。末期：核膜重新形成。
Cytokinesis: animal vs plant cell mechanism?胞质分裂：动物细胞 vs 植物细胞的机制？
Animal: cleavage furrow (actin-myosin ring pinches inward). Plant: cell plate (Golgi vesicles fuse at equator, expand outward).动物：分裂沟（肌动蛋白-肌球蛋白环向内收缩）。植物：细胞板（高尔基体囊泡在赤道融合，向外扩展）。
What happens in Prophase I of meiosis that does NOT happen in mitotic prophase?减数分裂前期 I 中发生了有丝分裂前期没有的什么事件？
Synapsis (homologous chromosomes pair up to form bivalents/tetrads) and crossing over at chiasmata.联会（同源染色体配对形成二价体/四分体）以及在交叉点发生交叉互换。
What separates in Anaphase I vs Anaphase II?后期 I 与后期 II 中分别是什么分离？
Anaphase I: homologous chromosomes separate (sister chromatids stay joined). Anaphase II: sister chromatids separate (same as mitotic anaphase).后期 I：同源染色体分离（姐妹染色单体保持连接）。后期 II：姐妹染色单体分离（与有丝分裂后期相同）。
Meiosis result: how many cells, what ploidy?减数分裂结果：产生多少个细胞，什么倍性？
4 haploid (n) cells, all genetically unique due to crossing over and independent assortment. Used to form gametes.4 个单倍体（n）细胞，由于交叉互换和独立分配，遗传上各不相同。用于形成配子。
Two sources of genetic variation from meiosis?减数分裂产生遗传变异的两大来源？
1. Crossing over at Prophase I (chiasmata between non-sister chromatids). 2. Independent assortment at Metaphase I (random bivalent orientation). Formula: $2^n$ combinations.1. 前期 I 的交叉互换（非姐妹染色单体间的交叉点）。2. 中期 I 的独立分配（二价体随机定向）。公式：$2^n$ 种组合。
Mitosis vs meiosis: cells produced, ploidy, purpose?有丝分裂 vs 减数分裂：产生细胞数、倍性、目的？
Mitosis: 2 diploid (2n) identical cells; growth and repair. Meiosis: 4 haploid (n) unique cells; gamete production for sexual reproduction.有丝分裂：2 个相同的二倍体（2n）细胞；生长和修复。减数分裂：4 个独特的单倍体（n）细胞；有性生殖的配子生产。
Spermatogenesis vs oogenesis: how many functional gametes?精子发生 vs 卵子发生：产生多少个功能性配子？
Spermatogenesis: 4 functional sperm from one primary spermatocyte. Oogenesis: 1 egg + 3 polar bodies (degenerate); unequal cytokinesis concentrates cytoplasm in egg.精子发生：一个初级精母细胞产生 4 个功能性精子。卵子发生：1 个卵子 + 3 个极体（退化）；不均等胞质分裂将细胞质集中到卵子中。
Three cell-cycle checkpoints and what each checks?细胞周期三个检验点及各自检查的内容？
G1: cell size and DNA integrity. G2: DNA replication accuracy. Spindle (metaphase): chromosome attachment to spindle from both poles.G1：细胞大小与 DNA 完整性。G2：DNA 复制准确性。纺锤体（中期）：染色体与来自两极的纺锤丝的附着情况。
Proto-oncogene vs tumour-suppressor gene in cancer?原癌基因 vs 肿瘤抑制基因在癌症中的作用？
Proto-oncogene: gain-of-function mutation → oncogene (stuck accelerator, drives excess division). Tumour-suppressor (e.g. p53): loss-of-function mutation → removes division brakes.原癌基因：功能获得性突变 → 癌基因（卡住的油门，驱动过度分裂）。肿瘤抑制基因（如 p53）：功能丧失突变 → 取消分裂制动。
What is crossing over and why does it increase variation?什么是交叉互换？为何能增加变异？
Exchange of segments between non-sister chromatids of homologous chromosomes at chiasmata during Prophase I. Creates new combinations of alleles on chromosomes — recombinant chromosomes.减数分裂前期 I 中，同源染色体的非姐妹染色单体在交叉点处交换片段。在染色体上产生新的等位基因组合——重组染色体。
Cell cycle: order of phases?细胞周期：各阶段顺序？
G1 → S → G2 → Mitosis → Cytokinesis → G1 again. Interphase is ~90% of the cycle.G1 → S → G2（= 间期）→ 有丝分裂 → 胞质分裂 → 再回到 G1。间期约占细胞周期 90%。
Cancer: why does it represent uncontrolled division?癌症：为何代表不受控制的细胞分裂？
Loss of contact inhibition + checkpoint failures + proto-oncogene mutations (oncogenes) + tumour-suppressor mutations → cells divide without normal constraints, invade tissue, may metastasize.丧失接触抑制 + 检验点失效 + 原癌基因突变（癌基因）+ 肿瘤抑制基因突变 → 细胞在无正常约束的情况下分裂，侵入组织，可能发生转移。
Mixed Practice综合练习

Practice Quiz综合测验

A cell with 2n = 10 completes S phase. How many chromatids does it now contain?一个 2n = 10 的细胞完成 S 期。它现在含有多少条染色单体？
Q1
10 chromatids10 条染色单体
5 chromatids5 条染色单体
20 chromatids20 条染色单体
40 chromatids40 条染色单体
After S phase, each of the 10 chromosomes has been duplicated into two sister chromatids joined at the centromere. 10 chromosomes × 2 sister chromatids = 20 chromatids. The chromosome number is still 10 (2n), but DNA content has doubled.S 期后，10 条染色体中的每一条都已复制为两条由着丝点连接的姐妹染色单体。10 条染色体 × 2 条姐妹染色单体 = 20 条染色单体。染色体数目仍为 10（2n），但 DNA 含量翻倍。
S phase replicates each chromosome into two sister chromatids. 10 chromosomes → 10 chromosomes each with 2 chromatids = 20 chromatids total. Chromosome number (10) stays the same; chromatid number doubles (10 → 20).S 期将每条染色体复制为两条姐妹染色单体。10 条染色体 → 10 条染色体各有 2 条染色单体 = 共 20 条染色单体。染色体数目（10）不变；染色单体数目翻倍（10 → 20）。
A diagram shows two cells: one has chromosomes aligned single-file at the equatorial plate; the other has chromosome pairs (bivalents) aligned there. The second cell is in which phase?图中显示两个细胞：一个的染色体单排排列在赤道板上；另一个的染色体对（二价体）排列在赤道板上。第二个细胞处于哪个时期？
Q2
Metaphase I of meiosis减数分裂中期 I
Metaphase II of meiosis减数分裂中期 II
Metaphase of mitosis有丝分裂中期
Prophase I of meiosis减数分裂前期 I
Bivalents (paired homologs) at the equatorial plate is the hallmark of Metaphase I. In mitotic metaphase and Meiosis II metaphase, individual chromosomes (not pairs) align. In Prophase I, chromosomes are paired but not yet aligned at the plate.二价体（配对的同源染色体）排列在赤道板是中期 I 的标志。在有丝分裂中期和减数分裂 II 中期，单条染色体（不是对）排列。在前期 I，染色体已配对但尚未排列在赤道板上。
Bivalents at the plate = Metaphase I. Individual chromosomes at the plate = Metaphase of mitosis or Metaphase II. Prophase I: synapsis/crossing over, not alignment yet.二价体在赤道板 = 中期 I。单条染色体在赤道板 = 有丝分裂中期或减数分裂 II 中期。前期 I：联会/交叉互换，尚未排列在赤道板。
A cell has 2n = 8 homologous chromosome pairs. How many chromosome combinations can be produced from independent assortment alone in meiosis?一个细胞有 2n = 8 对同源染色体。仅靠减数分裂中的独立分配，可产生多少种染色体组合？
Q3
8 combinations8 种组合
256 combinations256 种组合
64 combinations64 种组合
16 combinations16 种组合
Independent assortment gives $2^n$ combinations, where n = haploid number of homologous pairs. Here n = 4 pairs (since 2n = 8 means there are 4 pairs). $2^4 = 16$... wait: 2n = 8 means n = 4, so $2^4 = 16$. But if the question means 8 pairs (2n = 16), then $2^8 = 256$. Reading "2n = 8 homologous chromosome pairs" as 8 pairs: $2^8 = 256$.独立分配产生 $2^n$ 种组合，其中 n = 同源染色体对数。题目说 8 对同源染色体，所以 $2^8 = 256$。
The formula is $2^n$ where n is the number of homologous pairs. With 8 pairs: $2^8 = 256$. Crossing over will further multiply the possibilities beyond 256.公式是 $2^n$，n 为同源染色体对数。8 对时：$2^8 = 256$。交叉互换将使可能性进一步超过 256。
A researcher finds that a cancer cell line continues dividing even when crowded against neighboring cells and has bypassed the G1 checkpoint. Which two types of gene mutations are most likely responsible?研究人员发现一种癌细胞系，即使在与相邻细胞拥挤时仍继续分裂，并绕过了 G1 检验点。最可能负责的是哪两类基因突变？
Q4
Two tumour-suppressor gain-of-function mutations两个肿瘤抑制基因的功能获得性突变
Two proto-oncogene loss-of-function mutations两个原癌基因的功能丧失突变
A spindle protein mutation and a ribosomal gene mutation纺锤丝蛋白突变和核糖体基因突变
A proto-oncogene gain-of-function mutation (oncogene) and a tumour-suppressor loss-of-function mutation原癌基因的功能获得性突变（癌基因）和肿瘤抑制基因的功能丧失突变
Cancer typically requires both: (1) a gain-of-function mutation converting a proto-oncogene to an oncogene (stuck accelerator — drives division), and (2) a loss-of-function mutation disabling a tumour-suppressor gene like p53 or Rb (cut brakes — removes the G1 checkpoint constraint). Both types of mutation must typically accumulate for a cell to become cancerous.癌症通常需要两者兼备：(1) 将原癌基因转化为癌基因的功能获得性突变（卡住的油门——驱动分裂），以及 (2) 使 p53 或 Rb 等肿瘤抑制基因失效的功能丧失突变（切断刹车——取消 G1 检验点约束）。通常需要两类突变积累，细胞才会癌变。
Tumour-suppressors work by inhibition — their gain-of-function would increase inhibition (suppresses cancer). Proto-oncogenes need gain-of-function (not loss) to become oncogenes. Spindle protein and ribosomal mutations are not the primary cancer drivers described here.肿瘤抑制基因通过抑制来发挥作用——其功能获得性突变会增强抑制（抑制癌症）。原癌基因需要功能获得性（而非丧失）突变才能变成癌基因。纺锤丝蛋白和核糖体突变不是这里描述的主要癌症驱动因素。
During which specific phase of meiosis does crossing over occur?减数分裂的哪个具体时期发生交叉互换？
Q5
Anaphase I后期 I
Metaphase II中期 II
Prophase I前期 I
Telophase I末期 I
Crossing over occurs during Prophase I, when homologous chromosomes pair up (synapsis) and non-sister chromatids exchange segments at chiasmata. This is the most genetically significant event of meiosis. It does not occur in Anaphase I (when homologs separate), Metaphase II, or Telophase I.交叉互换发生在前期 I，此时同源染色体配对（联会），非姐妹染色单体在交叉点处交换片段。这是减数分裂在遗传上最重要的事件。它不发生在后期 I（同源染色体分离时）、中期 II 或末期 I。
Crossing over is a Prophase I event. In Anaphase I the homologs separate (they have already crossed over). Meiosis II phases are after crossing over is complete. Telophase I: nuclear envelopes may reform but crossing over is done.交叉互换是前期 I 的事件。在后期 I，同源染色体分离（交叉互换已经完成）。减数分裂 II 的各个时期都在交叉互换完成之后。末期 I：核膜可能重新形成，但交叉互换已结束。
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Readiness Checklist准备就绪清单

Tick each item when you can do it cold, without notes, on a first attempt.能在无笔记、首次尝试下完成，再勾选每一项。

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Next Steps后续衔接

What This Feeds Into本单元的去向

Cell division is the pivot between cell biology and genetics. The chromosomes and sister chromatids separated in this unit become the alleles and gene combinations studied in Mendelian Genetics (Unit 5). The crossing over and independent assortment introduced here explain why Mendel's laws work at the cellular level. The cell cycle checkpoints of Unit 4 link directly to Molecular Genetics (Unit 6), where you learn how DNA is replicated in S phase and transcribed after division. Cancer — introduced in §7 — reappears in Human Anatomy and Physiology (Unit 10) as a tissue- and organ-level disease.细胞分裂是细胞生物学与遗传学之间的枢纽。本单元中分离的染色体与姐妹染色单体成为孟德尔遗传学（第 5 单元）中研究的等位基因与基因组合。本单元介绍的交叉互换和独立分配解释了孟德尔定律在细胞层面上为何成立。第 4 单元的细胞周期检验点直接关联到分子遗传学（第 6 单元），在那里你将学习 S 期 DNA 如何复制以及分裂后如何转录。第 §7 引入的癌症在人体解剖与生理（第 10 单元）中以组织和器官层面的疾病形式再次出现。

Within High School Biology.在 HS Biology 内部。

Unit 1 (Cell Structure) introduced the nucleus and chromosomes that are the actors of this unit. Unit 3 (Cellular Energetics) provided the ATP that powers the cytoskeletal motors pulling chromosomes during anaphase. Unit 5 (Mendelian Genetics) applies crossing over and independent assortment to predict offspring ratios. Unit 6 (Molecular Genetics) zooms in on DNA replication (the S phase event) and repair (the G1/G2 checkpoint mechanism). Unit 10 (Human Anatomy and Physiology) returns to mitosis in the context of tissue renewal and cancer as a disease of the whole organism.第 1 单元（细胞结构）介绍了本单元的主角——细胞核与染色体。第 3 单元（细胞能量学）提供了在后期驱动细胞骨架马达拉动染色体的 ATP。第 5 单元（孟德尔遗传学）将交叉互换和独立分配应用于预测后代比例。第 6 单元（分子遗传学）聚焦于 DNA 复制（S 期事件）和修复（G1/G2 检验点机制）。第 10 单元（人体解剖与生理）在组织更新和癌症作为全身疾病的背景下再次回到有丝分裂。

Feeds into AP Biology and IB Biology.衔接 AP Biology 与 IB Biology。

AP Biology Unit 5 (Heredity) and IB Biology HL Topic D1 (DNA and the Chromosome) both assume prior mastery of mitosis and meiosis at this level. AP Biology extends cell-cycle control into molecular detail (CDK/cyclin, proto-oncogenes). IB Biology HL adds quantitative genetic mapping (cM distances, chi-squared tests for linkage). Ontario SBI4U students continue into molecular genetics with operons and polymerases. Alberta Biology 30 students continue into genetics (Unit C GO2) and Hardy-Weinberg (Unit D GO1).AP Biology 第 5 单元（遗传）和 IB Biology HL Topic D1（DNA 与染色体）均假定学生在这个层面已掌握有丝分裂和减数分裂。AP Biology 将细胞周期调控延伸至分子细节（CDK/cyclin、原癌基因）。IB Biology HL 增加了定量遗传作图（cM 距离、连锁的卡方检验）。安大略 SBI4U 学生继续深入分子遗传学（操纵子与聚合酶）。阿尔伯塔 Biology 30 学生继续学习遗传学（Unit C GO2）和 Hardy-Weinberg（Unit D GO1）。

DINGRUI SCHOLARS

High School Biology · Cell Division and the Cell Cycle · v1高中生物 · 细胞分裂与细胞周期 · v1