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During which phase of the cell cycle does DNA replication occur?细胞周期的哪个阶段发生 DNA 复制？
DNA replication occurs during the S (synthesis) phase of interphase. Each chromosome is duplicated to form two sister chromatids joined at the centromere, doubling the DNA content of the cell from 2C to 4C. [1] G1 and G2 are growth and preparation phases where proteins and organelles are synthesized. Mitosis (M phase) separates the already-replicated chromosomes; no new replication occurs during M. [2]DNA 复制发生在间期的 S（合成）期。每条染色体被复制成两条姐妹染色单体，通过着丝点相连，细胞 DNA 含量从 2C 倍增至 4C。[1] G1 和 G2 是生长和准备阶段，细胞合成蛋白质和细胞器。M 期（有丝分裂期）负责分离已复制的染色体，在 M 期不发生新的 DNA 复制。[2]
During which phase of mitosis do sister chromatids separate and move to opposite poles?有丝分裂的哪个时期，姐妹染色单体分离并移向两极？
In Anaphase of mitosis, the centromeres (kinetochores) split, separating each pair of sister chromatids into individual chromosomes. The shortening of spindle microtubules pulls each chromatid toward opposite poles. Because both sister chromatids are genetically identical, each pole receives a complete set (2n chromosomes). [2] Prophase: chromosomes condense; Metaphase: chromosomes align at the equatorial plate; Telophase: nuclear envelopes reform at each pole. None of these involve chromatid separation. [1]有丝分裂后期，着丝点（动粒）分裂，将每对姐妹染色单体分离成独立染色体。纺锤体微管缩短，将每条染色单体拉向两极。由于姐妹染色单体基因完全相同，每极获得完整的一套染色体（2n 条）。[2] 前期：染色体凝缩；中期：染色体排列在赤道板；末期：核膜在两极重新形成。这些时期均不涉及染色单体分离。[1]
Cytokinesis occurs differently in animal and plant cells.动物细胞与植物细胞的胞质分裂方式不同。
The cleavage furrow forms by the contraction of an actin-myosin contractile ring located just beneath the plasma membrane at the cell's equator. [1] The actin microfilaments and myosin motor proteins slide past each other, tightening the ring and pinching the plasma membrane inward until the cell is divided into two. [1]分裂沟由位于细胞质膜下方赤道区的肌动蛋白-肌球蛋白收缩环收缩形成。[1] 肌动蛋白微丝与肌球蛋白马达蛋白相互滑动，收紧收缩环，使质膜向内凹陷，直至将细胞一分为二。[1]
Plant cells are surrounded by a rigid cellulose cell wall that prevents the membrane from being pinched inward. [1] Instead, plant cells build a new cell plate by vesicles derived from the Golgi apparatus fusing at the cell's midline; the vesicles deposit cell-wall precursors that grow outward until a complete partition forms. [1]植物细胞被坚硬的纤维素细胞壁包围，阻止质膜向内收缩。[1] 因此，植物细胞由高尔基体来源的囊泡在细胞中线融合，形成新的细胞板；囊泡将细胞壁前体沉积到中线并向外扩展，直至形成完整隔膜。[1]
The Golgi apparatus (Golgi body) supplies the membrane vesicles that fuse to form the plant cell plate.高尔基体（高尔基装置）提供融合形成植物细胞板的膜囊泡。 [1]
Meiosis I is often called the reductive division.减数分裂 I 常被称为减数分裂。
During Prophase I, homologous chromosomes pair up in a process called synapsis. The two homologs align gene-for-gene to form a bivalent (tetrad) held together by the synaptonemal complex. [1] At points called chiasmata, non-sister chromatids from the two homologs overlap and break at matching positions, then rejoin with each other. [1] This exchange is called crossing over (recombination), and it produces chromatids with new combinations of alleles, generating genetic variation in the gametes. [1]前期 I 期间，同源染色体配对，称为联会。两条同源染色体按基因一一对应排列，形成由联会复合体固定的二价体（四分体）。[1] 在称为交叉结（chiasmata）的位置，来自两条同源染色体的非姐妹染色单体重叠，并在相同位置断裂后互换连接。[1] 这一交换称为交叉互换（重组），产生具有新等位基因组合的染色单体，在配子中产生遗传变异。[1]
In Anaphase I of meiosis, homologous chromosomes separate and move to opposite poles; the centromere holding the two sister chromatids together does NOT split. [1] In Anaphase of mitosis, the centromere does split, separating sister chromatids rather than homologs. [1]减数分裂后期 I 中，同源染色体分离，移向两极；连接两条姐妹染色单体的着丝点不分裂。[1] 有丝分裂后期中，着丝点确实分裂，分离的是姐妹染色单体而非同源染色体。[1]
Two cells are produced at the end of Meiosis I, each containing n = 5 chromosomes (haploid). Each chromosome still consists of two sister chromatids (10 chromatids per cell total).减数分裂 I 结束时产生两个细胞，各含 n = 5 条染色体（单倍体）。每条染色体仍由两条姐妹染色单体组成（每个细胞共 10 条染色单体）。 [1]
A human primary spermatocyte (2n = 46) undergoes complete meiosis.一个人类初级精母细胞（2n = 46）经历完整的减数分裂。
After Meiosis I: 2 secondary spermatocytes, each with n = 23 chromosomes (each chromosome still consisting of 2 sister chromatids). [1] After Meiosis II: 4 spermatids (which mature into spermatozoa), each with n = 23 chromosomes (single chromatids). [1] Chromosome number is 23 per cell (haploid) at both stages. [1]减数分裂 I 后：2 个次级精母细胞，各含 n = 23 条染色体（每条染色体仍由 2 条姐妹染色单体组成）。[1] 减数分裂 II 后：4 个精细胞（成熟为精子），各含 n = 23 条染色体（单条染色单体）。[1] 两个阶段染色体数目均为 23 条（单倍体）。[1]
In Anaphase II, the centromeres split and sister chromatids separate, moving to opposite poles (same as mitotic Anaphase). [1] As a result, each gamete receives individual chromosomes (single chromatids), maintaining the haploid number while halving the DNA content per cell from 2C to 1C. [1]后期 II 中，着丝点分裂，姐妹染色单体分离，移向两极（与有丝分裂后期相同）。[1] 因此，每个配子获得单条染色体（单染色单体），保持单倍体数目，但每个细胞 DNA 含量从 2C 降至 1C。[1]
Fertilization restores the diploid number by fusing two haploid gametes (sperm: n = 23; egg: n = 23) to form a zygote with 2n = 46. [1] The biological significance is that chromosome number remains constant across generations, preserving the species' genetic identity and ensuring proper gene dosage in every somatic cell of the offspring. [1]受精通过融合两个单倍体配子（精子：n = 23；卵子：n = 23）形成 2n = 46 的合子，从而恢复二倍体数目。[1] 其生物学意义在于染色体数目在世代间保持稳定，维护物种基因组的完整性，并确保后代每个体细胞基因的正常剂量。[1]
If DNA replicated between Meiosis I and II, the gametes would end up with twice the intended amount of DNA (2n instead of n), defeating the purpose of meiosis. Suppressing this replication step ensures each gamete is truly haploid.若在减数分裂 I 与 II 之间发生 DNA 复制，配子将含有两倍预期数量的 DNA（2n 而非 n），违背了减数分裂的目的。抑制这一复制步骤正是确保每个配子真正为单倍体的关键。 [1]
A cell with 2n = 8 chromosomes is observed at various stages of its cell cycle.一个具有 2n = 8 条染色体的细胞在细胞周期不同阶段被观察到。
The G1 checkpoint checks that the cell is large enough, nutrients are sufficient, and no DNA damage is present before committing to DNA replication. If conditions are unfavorable, the cell can exit to G0. [1] The G2 checkpoint checks that DNA replication is complete and accurate, and that the cell is large enough to divide. It prevents cells with damaged or incompletely replicated DNA from entering mitosis. [1]G1 检验点在决定进行 DNA 复制前检查：细胞是否足够大、营养是否充足、是否存在 DNA 损伤。若条件不满足，细胞可退出细胞周期进入 G0 期。[1] G2 检验点检查 DNA 复制是否完整且准确，以及细胞是否足够大。它阻止 DNA 受损或复制不完全的细胞进入有丝分裂。[1]
G1: rapid synthesis of proteins, organelles, and RNA; cell grows in size. S: all DNA is replicated, requiring large energy input. G2: organelle duplication (including centrosomes), further protein synthesis, and quality-control of replicated DNA. [1] Together, these activities mean the cell is metabolically the most active during interphase; the cell is doing the opposite of resting. [1]G1：快速合成蛋白质、细胞器和 RNA；细胞体积增大。S 期：所有 DNA 被复制，需要大量能量输入。G2：细胞器复制（包括中心体）、进一步蛋白质合成和复制 DNA 的质量控制。[1] 这些活动共同表明细胞在间期的代谢最为活跃；细胞所做的恰恰与静止相反。[1]
A student examines cells under a microscope. Identify the mitotic phase and justify using two observable features.学生在显微镜下观察细胞。辨认有丝分裂时期，并用两个可观察特征说明理由。
Phase: Early Prophase. Feature 1: chromosomes are visibly condensed (DNA coiling around histones, making them shorter and thicker). [1] Feature 2: nuclear envelope is still intact; it breaks down later in Prophase (Prometaphase), not yet at this stage. Absence of spindle fibers is consistent with the spindle not yet formed. [1]时期：前期（早期）。特征 1：染色体明显凝缩（DNA 绕组蛋白卷缩，变得更短更粗）。[1] 特征 2：核膜仍然完整；核膜在前期稍后（前中期）才破裂，此时尚未破裂。无纺锤丝与纺锤体尚未形成一致。[1]
Phase: Metaphase. Feature 1: chromosomes are aligned at the equatorial (metaphase) plate midway between the two poles; this arrangement is characteristic of Metaphase only. [1] Feature 2: nuclear envelope is absent (broken down during Prometaphase); spindle fibers attached to kinetochores confirm the cell is ready for chromosome separation. [1]时期：中期。特征 1：染色体排列在两极之间中点的赤道板（中期板）上，这是中期所独有的特征。[1] 特征 2：核膜缺失（在前中期已解体）；纺锤丝附着于动粒，证实细胞已准备好进行染色体分离。[1]
Phase: Telophase. Feature 1: nuclear envelopes begin to reform around each chromosome cluster at the poles (ER fragments reorganize around decondensing chromosomes). [1] Feature 2: cell is elongated and (in animal cells) a cleavage furrow is beginning to form along the equator. Chromosomes are also decondensing. [1]时期：末期。特征 1：核膜开始在两极各染色体群周围重新形成（内质网碎片在去凝缩的染色体周围重组）。[1] 特征 2：细胞伸长，（在动物细胞中）分裂沟开始沿赤道形成。染色体也在去凝缩。[1]
The spindle assembly checkpoint (SAC) monitors whether every kinetochore on every chromosome is properly attached to spindle microtubules from opposite poles (biorientation). If any kinetochore is unattached, the SAC generates a "wait" signal that prevents the cell from entering Anaphase. [1] If this checkpoint fails, the cell enters Anaphase before all chromosomes are bioriented; some chromosomes may move to the wrong pole or fail to move entirely, resulting in daughter cells with too many or too few chromosomes (aneuploidy). [1]纺锤体组装检验点（SAC）监测每条染色体上的每个动粒是否正确地从两极双向连接到纺锤体微管（双向定向）。若有任何动粒未附着，SAC 发出"等待"信号，阻止细胞进入后期。[1] 若该检验点失效，细胞在所有染色体完成双向定向前进入后期；某些染色体可能移向错误的极或根本不移动，导致子细胞染色体数目过多或过少（非整倍体）。[1]
Compare mitosis and meiosis, and explain the sources of genetic variation that meiosis produces but mitosis does not.比较有丝分裂与减数分裂，并解释减数分裂产生而有丝分裂不产生的遗传变异来源。
During Prophase I, non-sister chromatids from homologous chromosomes physically overlap at chiasmata. [1] Double-strand breaks occur at the same loci on both chromatids; the broken ends are exchanged and rejoined, producing recombinant chromatids that carry alleles from both original homologs. Because allele combinations differ from either parent chromosome, genetic variation increases. [1]前期 I 中，来自同源染色体的非姐妹染色单体在交叉结处物理重叠。[1] 两条染色单体的相同位点发生双链断裂；断裂末端互换并重新连接，产生携带两条原始同源染色体等位基因的重组染色单体。由于等位基因组合与任一亲本染色体不同，遗传变异增加。[1]
$n = 5$ homologous pairs. Each pair can orient independently at Metaphase I with two possible orientations per pair. Number of chromosomally distinct gametes $= 2^5 = 32.$ [1] Calculation: $2^5 = 2 \times 2 \times 2 \times 2 \times 2 = 32.$ Therefore independent assortment alone can generate 32 chromosomally distinct gamete types. [1]$n = 5$ 对同源染色体。每对在中期 I 可独立定向，每对有两种可能方向。染色体组合不同的配子数 $= 2^5 = 32.$ [1] 计算过程：$2^5 = 2 \times 2 \times 2 \times 2 \times 2 = 32.$ 因此，仅靠独立分配可产生 32 种染色体组合不同的配子类型。[1]
Cancer cells are characterized by uncontrolled cell division that ignores normal cell-cycle checkpoints.癌细胞的特征是无控制地分裂，无视正常的细胞周期检验点。
Proto-oncogenes encode proteins that promote cell division (growth factors, receptors, signaling proteins, transcription factors); they are normal, essential genes. [0.5] Tumour-suppressor genes encode proteins that inhibit cell division, promote apoptosis, or repair DNA. [0.5] (i) When a proto-oncogene mutates into an oncogene, the growth-promoting protein becomes overactive; the cell receives constant signals to divide even without external growth factors, bypassing the G1 checkpoint. [1] (ii) When a tumour-suppressor gene (e.g., p53, Rb) is inactivated by mutation, the normal braking mechanism on cell division is lost; damaged cells bypass checkpoints and divide uncontrollably. [1]原癌基因编码促进细胞分裂的蛋白质（生长因子、受体、信号蛋白、转录因子）；它们是正常的、必不可少的基因。[0.5] 肿瘤抑制基因编码抑制细胞分裂、促进细胞凋亡或修复 DNA 的蛋白质。[0.5] (i) 当原癌基因突变为癌基因时，促生长蛋白变得过度活跃；细胞即使没有外部生长因子也接收持续分裂信号，绕过 G1 检验点。[1] (ii) 当肿瘤抑制基因（如 p53、Rb）因突变失活时，细胞分裂的正常制动机制丧失；受损细胞绕过检验点并无控制地分裂。[1]
Cancer requires multiple mutations because cells have multiple independent checkpoint layers, and tumour-suppressor genes require both alleles to be inactivated (loss of heterozygosity) before control is lost. A single oncogene mutation is typically insufficient if tumour-suppressor pathways remain intact. [1] Over a lifetime, somatic cells accumulate mutations with each DNA replication cycle; older individuals have had more replications and more opportunity for multiple checkpoint gene mutations to co-occur, explaining the age-related rise in cancer incidence. [1]癌症需要多个突变，因为细胞具有多个独立的检验点控制层，肿瘤抑制基因的两个等位基因都需要失活（杂合性丧失）才会失去控制。如果肿瘤抑制通路仍然完整，单个癌基因突变通常不足以引发癌症。[1] 在一生中，体细胞每次 DNA 复制都会积累突变；年龄较大的个体经历了更多复制次数，多个检验点基因突变共同发生的机会更多，这解释了癌症发病率随年龄上升的规律。[1]
A cell with 2n = 6 chromosomes (3 homologous pairs: long, medium, short) undergoes meiosis.一个 2n = 6 的细胞（3 对同源染色体：长、中、短）进行减数分裂。
Award marks for a diagram showing: (1) all three bivalents (long, medium, short pairs) aligned at the cell's equatorial plate [1]; (2) each bivalent labeled showing maternal and paternal homologs [1]; (3) two possible orientations drawn for at least one pair (e.g., long pair: maternal-left/paternal-right in orientation A, vs maternal-right/paternal-left in orientation B), labeled as independent assortment possibilities. [1]示意图需包含：(1) 三个二价体（长、中、短对）全部排列在细胞赤道板上 [1]；(2) 每个二价体标注母方和父方同源染色体 [1]；(3) 至少对一对画出两种可能方向（例如，长对：方向 A 为母方靠左/父方靠右，方向 B 为母方靠右/父方靠左），并标注这是独立分配的两种可能性。[1]
Total gametes produced: 4. [1] Chromosome number per gamete: n = 3. [1] The four gametes are NOT genetically identical; they differ due to (1) independent assortment of homologs at Metaphase I, and (2) crossing over during Prophase I producing recombinant chromatids. [1]产生的配子总数：4 个。[1] 每个配子的染色体数目：n = 3。[1] 四个配子基因不完全相同；它们因以下原因不同：(1) 中期 I 同源染色体的独立分配；(2) 前期 I 的交叉互换产生重组染色单体。[1]
$n = 3$ chromosome pairs. $$2^n = 2^3 = 8$$ [1] Therefore, independent assortment alone can produce 8 chromosomally distinct gamete types from this cell. Crossing over would increase this number beyond 8. [1]$n = 3$ 对染色体。$$2^n = 2^3 = 8$$ [1] 因此，仅靠独立分配，该细胞可产生 8 种染色体组合不同的配子类型。交叉互换会使这一数字超过 8 种。[1]
Spermatogenesis and oogenesis both use meiosis to produce gametes, but differ in cell number produced and cytoplasm distribution.精子发生与卵子发生均通过减数分裂产生配子，但产生的细胞数目和细胞质分配方式不同。
Starting cell: primary spermatocyte (2n). [1] After Meiosis I: 2 secondary spermatocytes, each with n chromosomes (each chromosome still 2 chromatids). [1] After Meiosis II: 4 spermatids (each n, single chromatids), all of which mature into functional spermatozoa; total = 4 spermatozoa per primary spermatocyte. [1]起始细胞：初级精母细胞（2n）。[1] 减数分裂 I 后：2 个次级精母细胞，各含 n 条染色体（每条染色体仍有 2 条染色单体）。[1] 减数分裂 II 后：4 个精细胞（各含 n 条单条染色单体），均成熟为功能性精子；每个初级精母细胞共产生 4 个精子。[1]
Oogenesis uses unequal (asymmetric) cytokinesis at both Meiosis I and Meiosis II. [1] After Meiosis I, the primary oocyte divides into one large secondary oocyte (retaining most cytoplasm, organelles, and nutrients) and one small first polar body with little cytoplasm. [1] After Meiosis II, the secondary oocyte divides unequally again into one large egg (ootid, which matures into the ovum) and one small second polar body. The first polar body may also divide. All polar bodies eventually degenerate. Total: 1 functional egg + 3 polar bodies. [1]卵子发生在减数分裂 I 和 II 均使用不均等（不对称）的胞质分裂。[1] 减数分裂 I 后，初级卵母细胞分裂为一个大的次级卵母细胞（保留大部分细胞质、细胞器和营养物质）和一个小的第一极体（细胞质极少）。[1] 减数分裂 II 后，次级卵母细胞再次不均等分裂为一个大的卵细胞（卵母细胞，成熟为卵子）和一个小的第二极体。第一极体也可能分裂。所有极体最终退化。总计：1 个功能性卵子 + 3 个极体。[1]
By concentrating all cytoplasm, organelles (mitochondria), ribosomes, mRNA stores, and yolk into one large cell, oogenesis maximizes the nutrients and developmental machinery available to the egg after fertilization. [1] This large cytoplasmic reserve supports the early embryo through the first cell divisions (cleavage) before the embryo's own genome is activated. In contrast, equal cytokinesis in spermatogenesis is acceptable because sperm contribute only DNA to the zygote, not cytoplasm. [1]通过将所有细胞质、细胞器（线粒体）、核糖体、mRNA 储备和卵黄集中到一个大细胞中，卵子发生最大化了受精后卵子可获得的营养和发育机制。[1] 这一大量细胞质储备在胚胎自身基因组被激活前，支持早期胚胎完成最初几次细胞分裂（卵裂）。相比之下，精子发生中均等的胞质分裂是可行的，因为精子只向合子贡献 DNA 而非细胞质。[1]
A polar body is haploid (n), containing one complete set of chromosomes.极体为单倍体（n），含有一套完整的染色体。 [1]
A research team compares Population A (normal mitosis in tissue) and Population B (cancer cells from a tumour).研究团队比较种群 A（组织中正常有丝分裂的细胞）与种群 B（肿瘤中的癌细胞）。
The 90:10 ratio is expected because interphase encompasses three sub-stages (G1, S, G2) that collectively require far more time than M phase; DNA synthesis alone (S phase) can take 6-8 hours in a typical mammalian cell, whereas the entire M phase may take only 1 hour. [1] If the G1 checkpoint is bypassed, cells with insufficient size, nutrient reserves, or DNA damage proceed to S phase, replicating damaged DNA and producing mutations. [1] If the G2 checkpoint is bypassed, cells with incompletely replicated or damaged DNA enter mitosis, leading to chromosome breaks, deletions, and daughter cells with incorrect chromosome complements. [1]90:10 的比例是预期结果，因为间期包含三个子阶段（G1、S、G2），合计所需时间远多于 M 期；仅 DNA 合成（S 期）在典型哺乳动物细胞中就需要 6-8 小时，而整个 M 期可能只需 1 小时。[1] 若 G1 检验点被绕过，大小不足、营养储备不足或携带 DNA 损伤的细胞进入 S 期，复制受损 DNA 并产生突变。[1] 若 G2 检验点被绕过，DNA 复制不完整或受损的细胞进入有丝分裂，导致染色体断裂、缺失，以及子细胞含有错误的染色体组成。[1]
Crossing over during meiosis generates novel allele combinations in gametes, increasing the genetic diversity of offspring. In a changing environment, diverse genotypes improve the probability that at least some individuals will survive new stresses (natural selection acts on this variation). [1] Crossing over does not occur in mitosis because in mitosis homologous chromosomes do not pair (no synapsis, no bivalent formation); mitosis must produce genetically identical daughter cells for tissue homeostasis. Introducing recombination in somatic mitosis could disrupt gene dosage and produce mosaic mutations. [1]减数分裂中的交叉互换在配子中产生新的等位基因组合，增加后代的遗传多样性。在环境变化中，多样的基因型增加了至少部分个体能够在新压力下存活的概率（自然选择作用于这种变异）。[1] 有丝分裂中不发生交叉互换，因为有丝分裂中同源染色体不配对（无联会，无二价体形成）；有丝分裂必须产生基因相同的子细胞以维持组织稳态。在体细胞有丝分裂中引入重组可能破坏基因剂量并产生嵌合体突变。[1]
Accept any one of: UV radiation (causes thymine dimers, can mutate p53); ionizing radiation (X-rays, gamma rays cause double-strand breaks); chemical carcinogens (e.g., tobacco smoke compounds such as benzopyrene that form DNA adducts); chronic viral infection (HPV E6/E7 proteins inactivate Rb and p53).以下任意一个均可接受：紫外线辐射（导致胸腺嘧啶二聚体，可突变 p53）；电离辐射（X 射线、伽马射线导致双链断裂）；化学致癌物（如烟草烟雾中的苯并芘与 DNA 形成加合物）；慢性病毒感染（HPV E6/E7 蛋白使 Rb 和 p53 失活）。 [1]