The Cell Cycle and Interphase
细胞周期与间期
A cell cannot simply split in two and hope for the best: it first spends a long, busy interphase — growing in G1, replicating its DNA in S phase, and checking and preparing in G2 — because mitosis can only separate copies that already exist, so the copying must be finished before division begins.
细胞不能说分就分、碰运气分成两半：它必须先经历一个漫长而忙碌的（分裂）间期 (interphase)——在 G1 期（第一间隙期）长大、在 S 期（合成期）完成 DNA 复制、在 G2 期（第二间隙期）检查并做准备。原因很简单：有丝分裂 (mitosis) 只能把「已经存在的两份拷贝」分开，它本身不会「造」出新的遗传物质，所以复制必须在分裂开始之前就全部完成。
- Explain why cells divide, referring to growth, repair and replacement, and the limit that the surface area to volume ratio places on cell size.解释 (Explain) 细胞为什么要分裂，要讲到生长、修复与更新，以及表面积与体积比 (surface area to volume ratio) 对细胞体积设下的上限。
- Outline the cell cycle as a repeating sequence of interphase (G1, S and G2) followed by M phase (mitosis and cytokinesis).概述 (Outline) 细胞周期 (cell cycle)：（分裂）间期（G1 期 → S 期 → G2 期）之后进入 M 期（分裂期），M 期 = 有丝分裂 (mitosis) + 胞质分裂 (cytokinesis)，并且不断循环。
- Describe the events of G1, S and G2, and state in which sub-phase DNA is replicated.描述 (Describe) G1、S、G2 三个亚期各自发生了什么，并说出 DNA 复制 (DNA replication) 究竟发生在哪一期（S 期）。
- Justify why DNA replication must be completed during interphase, before mitosis begins.论证 (Justify) 为什么 DNA 复制必须在（分裂）间期内、在有丝分裂开始之前就全部完成。
- Interpret a cell cycle diagram or a set of phase durations, and calculate the percentage of the cycle spent in interphase.解读 (Interpret) 细胞周期图或给定的各时期时长数据，并计算 (Calculate) 间期占整个细胞周期的百分比。
- Distinguish between the cell cycle and mitosis, and between a chromosome before and after S phase.区分 (Distinguish)「细胞周期」与「有丝分裂」这两个概念，以及一条染色体在 S 期前后的不同形态：S 期前是一条 DNA 细丝，S 期后由两条姐妹染色单体组成。
- Why cells divide at all · 细胞为什么非分裂不可
- The cell cycle: one loop with two very unequal parts · 细胞周期：一个循环，两个长短悬殊的部分
- Inside interphase: G1, S and G2 · 走进间期：G1、S、G2 到底在干什么
- The key question: why must the DNA be copied BEFORE mitosis? · 核心问题：为什么 DNA 必须在有丝分裂之前复制好？
- Writing about the cell cycle: verbs, numbers, diagrams and quality control · 把细胞周期写成分数：动词、数字、作图与质量控制（检查点）
Why cells divide at all5
Before you learn the steps of the cell cycle, you need a reason for the cycle to exist at all. Cells divide for three reasons, and you should be able to name all three without hesitating. The first is growth: you began as a single fertilised egg and you are now built from trillions of cells, and that happened because cells divided again and again. An organism grows mainly by making MORE cells, not by inflating the cells it already has. The second is repair and replacement: damaged tissue is rebuilt, and worn-out cells are replaced. The third is asexual reproduction, in which a whole new organism is produced from one parent by cell division alone — budding in yeast and Hydra, or a strawberry plant sending out a runner. Growth and repair are the two you will be asked about most often, and we come back to all three when we look at the roles of mitosis later in the topic.
在学细胞周期的具体步骤之前，先要知道「为什么要有这个周期」。细胞分裂有三个理由，你要能脱口而出：第一是生长——你最初只是一个受精卵，现在由数以万亿计的细胞构成，靠的就是细胞一次又一次地分裂。请注意：生物体长大主要靠「细胞数量变多」，而不是把原有的细胞吹得越来越大。第二是修复与更新（组织修复 tissue repair）：受损的组织会重新长好，用旧了的细胞会由新细胞替换掉。第三是无性生殖 (asexual reproduction)：仅靠一个亲本的细胞分裂就产生一个全新的个体——例如酵母和水螅的出芽生殖，或草莓长出匍匐茎繁殖新植株。考试里问得最多的是前两个；这三点在后面讲有丝分裂的作用时还会展开细讲。
Repair and replacement is the one that catches students out, because it is easy to assume division stops once you have finished growing. It does not. The outer layer of your skin is shed constantly and replaced by cells dividing deeper in the epidermis. The lining of your gut is replaced every few days, because digestion is chemically brutal on it. Mature red blood cells have no nucleus and cannot divide at all, so replacements are produced by the division of precursor cells in your bone marrow. Wound healing is the same process concentrated in one place: cells at the edges of a cut divide until the gap is closed. Right now, as you read this sentence, millions of your cells are somewhere in the middle of a cell cycle.
最容易被忽视的是「修复与更新」，因为大家总以为长大以后细胞就不分裂了。并非如此。你皮肤最外层的细胞每天都在脱落，由表皮深层不断分裂的细胞补上；肠道内壁的细胞每隔几天就整批更新一次，因为消化过程对它们的化学侵蚀太厉害；成熟的红细胞根本没有细胞核 (nucleus)、完全不能分裂，所以新的红细胞是由骨髓里的前体细胞分裂产生的。伤口愈合其实就是同一个过程集中发生在一个地方：切口边缘的细胞不断分裂，直到把缺口填满。就在你读这句话的此刻，你体内有数以百万计的细胞正处在细胞周期的某一步。
That leaves an obvious question: why divide at all — why not simply let one cell grow bigger? The limit is geometric. Everything a cell needs (oxygen, glucose) and everything it must get rid of (carbon dioxide, wastes) has to cross the cell surface membrane, so surface area is the cell's supply line, while the volume of cytoplasm inside is the demand. As a cell enlarges, its volume grows faster than its surface area, so the surface area to volume ratio falls and each unit of cytoplasm is served by less membrane. The distance from the membrane to the centre also increases, and diffusion over longer distances is slow. Past a certain size the cell simply cannot supply its own interior. So instead of continuing to grow, it divides — and the two smaller daughter cells each have a healthier surface area to volume ratio again.
于是有个很自然的问题：既然要长大，为什么不让一个细胞一直变大，非要分裂？限制来自几何学。细胞需要的一切（氧气、葡萄糖）和必须排掉的一切（二氧化碳、代谢废物）都要穿过细胞膜，所以「表面积」就是这个细胞的供给线，而内部细胞质的「体积」就是需求量。细胞一变大，体积增长的速度比表面积快，于是表面积与体积比 (surface area to volume ratio) 下降，平均每单位细胞质能分到的膜面积越来越少；同时从细胞膜到细胞中心的距离也变远，而距离越长，扩散就越慢。超过一定大小，细胞就真的供不起自己的内部了。所以它不再继续变大，而是分裂——分成两个较小的子细胞 (daughter cell)，各自又重新拥有比较有利的表面积与体积比。
Model a cell as a cube. A cube with 1 cm sides has a surface area of 6 cm2 (six faces, each 1 cm2) and a volume of 1 cm3, giving a surface area to volume ratio of 6:1. Double the sides to 2 cm and the surface area becomes 24 cm2 while the volume becomes 8 cm3 — a ratio of only 3:1. At 3 cm sides it is 54 cm2 to 27 cm3, or 2:1. The supply line has not kept up with the demand. Real cells are not cubes, and some cells get around the problem with a flattened or branched shape, but the arithmetic is exactly why most cells stay microscopic and divide instead of swelling.
把细胞想成一个立方体来算。边长 1 cm：表面积 = 6 个面 × 1 cm2 = 6 cm2，体积 = 1 cm3，比值 6:1。边长翻倍到 2 cm：表面积 = 24 cm2，体积 = 8 cm3，比值只剩 3:1。边长 3 cm：54 cm2 对 27 cm3，比值 2:1。可见供给（表面积）完全跟不上需求（体积）。真实细胞当然不是立方体，有些细胞靠把自己压扁、或长出许多分支来增大表面积，从而绕开这个问题，但这套算术正好解释了为什么绝大多数细胞都保持在显微尺度、宁可分裂也不一味长大。
Students often write that cell division happens 'while an organism is growing', as though an adult's cells have stopped. Adults are not finished: skin, gut lining and blood are replaced continuously throughout life, and wound healing can start at any age. If a question asks for the roles of cell division, growth alone is a partial answer — name replacement and repair as well.
很多同学写「细胞分裂发生在生物体生长的时候」，好像成年人的细胞就不分裂了。其实成年人体内的皮肤、肠道内壁、血细胞终生都在持续更新，伤口愈合任何年龄都会发生。题目问细胞分裂的作用时，只写「生长」只答对了一部分，必须同时写出「更新与修复」。
Put a thin chip and a thick potato wedge in the same oven for the same time. The thin one cooks through. The wedge browns on the outside while the middle is still raw, because heat only gets in across the surface and then has to travel inwards. A cell has the same problem, in two parts. Everything it needs crosses the cell surface membrane, so surface area is the supply line and the cytoplasm inside is the demand — and as the wedge gets thicker, its volume climbs faster than its surface does, so the surface area to volume ratio falls. The second part is distance: the centre of a wedge is further from any surface, and diffusion over longer distances is slow. Where it breaks down: leave the wedge in the oven longer and the middle does eventually cook, but a cell cannot solve its problem by waiting, because its demand for oxygen and glucose never stops and its wastes keep building — the supply across the membrane has to keep up continuously. And a wedge cannot turn itself into two thin chips, which is precisely what a cell does instead of growing.
用薯条打比方讲表面积与体积比。细薯条能熟透，粗薯角外面焦了里面还是生的：因为热量只能从表面进去，再往里传。细胞是同一个道理，而且有两层：一，细胞需要的一切都要穿过细胞膜 (cell surface membrane)，所以表面积＝供给线，内部细胞质的体积＝需求量；薯角越粗，体积增长比表面积快，表面积与体积比 (SA:V) 就下降。二，距离问题：越粗，中心离表面越远，而扩散距离越长越慢。类比的边界（必须点明）：薯角在烤箱里多烤一会儿，中心总归会熟；但细胞不能靠「多等一会儿」解决问题——它对氧气和葡萄糖的需求一刻不停，废物也在不断产生，所以穿过细胞膜的供给必须持续跟上，而不是「迟早补上」。另外，薯角没法把自己变成两根细薯条，而细胞恰恰就是这么做的——不再长大，而是分裂。
The cell cycle: one loop with two very unequal parts34
The cell cycle is the ordered, repeating sequence of events that runs from the moment a cell is produced to the moment it divides to produce two cells of its own. It has two main parts, and getting the naming right here will earn you marks all year. The first part is interphase, which is divided into three sub-phases in this order: G1, S and G2. The second part is M phase, which is made up of mitosis — the division of the nucleus — followed by cytokinesis, the division of the cytoplasm into two separate cells. Notice what that sentence does not say. Interphase is not a stage of mitosis; it is the part of the cell cycle that comes before mitosis. Mitosis itself has exactly four stages, and you meet them on Day 4.
细胞周期 (cell cycle) 指的是：从一个细胞刚刚形成，到它自己分裂成两个细胞为止，按固定顺序不断重复的一整套事件。它分两大部分，这里的用词一定要准，全年都在考。第一部分是（分裂）间期 (interphase)，内部又按顺序分成三个亚期：G1 期（第一间隙期）、S 期（合成期）、G2 期（第二间隙期）。第二部分是 M 期（分裂期），它由两步组成：先是有丝分裂 (mitosis)，也就是细胞核的分裂；紧接着是胞质分裂 (cytokinesis)，把细胞质分开，形成两个独立的细胞。请特别注意这句话没说的部分：间期不是有丝分裂的一个时期，它是排在有丝分裂之前的那一段细胞周期。有丝分裂本身只有四个时期，第 4 天再讲。
Now look at the diagram and pay attention to the sizes of the slices, because they are not decoration. In almost every cell you will meet, interphase occupies roughly 90 per cent or more of the cycle. In the standard textbook example of a rapidly dividing human cell grown in the laboratory, one full cycle takes about 24 hours, of which M phase is roughly one hour — everything else is interphase. Different sources quote slightly different breakdowns, so always work from the figures the question gives you rather than a memorised percentage. Your lessons spend most of their time on mitosis because mitosis has lots of named stages and diagrams to learn, and that gives students a badly distorted mental picture of the timing. In terms of time, mitosis is the brief event at the end of a long preparation, not the main event of a cell's life.
现在看图，并且注意各个扇形的大小——那不是随便画的。你会遇到的绝大多数细胞里，间期占整个周期约 90% 甚至更多。课本上常用的例子是实验室培养的快速分裂的人体细胞：一个完整周期约 24 小时，其中 M 期大约只有 1 小时，其余全是间期。不同资料给出的具体比例略有差别，所以答题时一律以题目给的数据为准，不要背死一个百分比。课堂上之所以花大量时间讲有丝分裂，是因为它有一堆需要背的时期名称和图，这反而让同学在时间感上产生严重错觉。从时间上看，有丝分裂只是漫长准备之后那个短暂的收尾动作，而不是细胞一生中的主戏。
Two more features of the diagram matter. First, it is a cycle: the arrow goes all the way round, so each of the two daughter cells produced at cytokinesis begins its own G1 and the whole sequence starts again. Second, the length of a cycle is not fixed — it varies enormously between cell types and organisms. Some human cells, such as those lining the gut, divide roughly once a day, while others may go months or years between divisions. When a question says Construct or Label a cell cycle diagram, draw interphase as the large arc, subdivide it G1 to S to G2 in that order, keep M phase as a thin wedge, subdivide M into mitosis and cytokinesis, and write the words DNA replication next to S phase. Those labels are where the marks sit.
这张图还有两点要抓住。第一，它是个「循环」：箭头绕一整圈——胞质分裂产生的两个子细胞各自从自己的 G1 期重新开始，整套流程再来一遍。第二，周期的长度不是固定的，不同细胞类型、不同生物之间差别极大：人体肠道内壁的细胞大约一天分裂一次，而有些细胞可能几个月甚至几年才分裂一次。碰到 Construct / Label（画图并标注）的题目，请这样画：间期画成很大的一段弧，按 G1 → S → G2 的顺序细分；M 期画成很窄的一块，再细分成有丝分裂和胞质分裂；并在 S 期旁边写上「DNA replication」。分数就落在这些标注上。
A very common error is to list interphase as a fifth stage of mitosis, alongside prophase, metaphase, anaphase and telophase. Mitosis has four stages, full stop. Interphase belongs to the cell cycle and finishes before prophase begins. If a question asks you to name the stages of mitosis and you include interphase, you have shown the marker exactly which idea you have muddled.
非常常见的错误是把间期当成有丝分裂的「第五个时期」，和前期、中期、后期、末期并列。有丝分裂只有四个时期，没有第五个。间期属于细胞周期，而且在前期 (prophase) 开始之前就已经结束了。如果题目问「有丝分裂有哪几个时期」而你把间期写进去，等于直接告诉阅卷老师你把这两个概念搞混了。
Because lessons and textbook pages devote most of their space to mitosis, students often draw or interpret the cell cycle circle as though mitosis takes up most of the cycle. Time spent teaching a phase has nothing to do with how long the phase lasts. On a correctly drawn diagram the M phase wedge is thin and interphase is the overwhelming majority of the circle.
因为课堂和课本把大部分篇幅给了有丝分裂，同学画图或读图时常常把 M 期画成/看成占了大半个圆。请记住：讲课花多少时间和这个时期实际持续多久毫无关系。画对的细胞周期图上，M 期只是一小条窄扇形，间期占了圆的绝大部分。
Say it out loud until it is automatic: G1 grow, S copy, G2 check, M divide. Then add the tail: and cytokinesis splits the cytoplasm. That single line gives you the sequence, the key event of each phase, and reminds you that cytokinesis is separate from mitosis. And keep the two words apart: mitosis is the division of the NUCLEUS only, while 'cell division' means mitosis PLUS cytokinesis. Writing 'mitosis' when you mean the whole of cell division loses marks.
大声念到形成条件反射：G1 长大、S 复制、G2 检查、M 分裂；后面再补一句「胞质分裂把细胞质分开」。这一句话同时帮你记住顺序、每一期的核心事件，还提醒你胞质分裂是独立于有丝分裂的一步。另外，这两个词千万别混着用：有丝分裂只指「细胞核」的分裂，而「细胞分裂」= 有丝分裂 + 胞质分裂。把整个细胞分裂笼统写成「有丝分裂」是要扣分的。
Inside interphase: G1, S and G23
G1 stands for the first gap — and it is a 'gap' only in the sense that it sits between the last division and the start of DNA synthesis, because nothing about it is idle. During G1 the newly formed cell grows back towards a normal size, synthesises proteins, and builds new organelles such as ribosomes and mitochondria, so that there will eventually be two working sets of machinery to hand on. It is also getting on with its ordinary job: a liver cell in G1 is still processing chemicals, a leaf cell is still photosynthesising. For most cells G1 is the longest sub-phase, and it is where a cell spends most of its ordinary working life.
G1 期（第一间隙期，即 DNA 合成前期）里的 G 是 gap（间隙）——但它只是「排在上一次分裂和 DNA 合成之间」这个意义上的间隙，绝不是闲着。在 G1 期，刚形成的细胞重新长回正常大小，合成大量蛋白质，并制造新的细胞器 (organelle)，例如核糖体和线粒体，以便将来能把两套完整的「生产设备」分给两个子细胞。与此同时它还在照常干本职工作：肝细胞在 G1 期照样在处理化学物质，叶肉细胞照样在进行光合作用。对多数细胞来说，G1 是最长的一个亚期，细胞平时「过日子」的时间基本都在这里。
S stands for synthesis, and this is the one phase you must never misplace: S phase is when the DNA is replicated. Every chromosome is copied, so the cell now holds two identical copies of every chromosome — a complete second copy of its entire genome. The structural consequence is the sentence that turns up in exam answers again and again: a chromosome that was a single thread of DNA now consists of two identical sister chromatids, joined at a region called the centromere. Read the next line carefully, because it is where marks are won and lost. The amount of DNA in the nucleus has doubled, but the chromosome number has NOT changed. A human body cell still has 46 chromosomes at the end of S phase — it simply has 92 chromatids. How the copying is actually carried out is tomorrow's lesson.
S 期（合成期）的 S 是 synthesis（合成），这是全课最不能记错位置的一个时期：DNA 复制 (DNA replication) 就发生在 S 期。每一条染色体都被复制一份，于是细胞里每条染色体都有了两份完全相同的拷贝，即整个基因组多出了一份完全相同的副本。它带来的结构变化正是考试答案里反复出现的那句话：原本是一条 DNA 细丝的染色体，现在由两条完全相同的姐妹染色单体 (sister chromatid) 组成，二者在着丝粒 (centromere) 处连在一起。下面这句请逐字看清，得分失分全在这里：细胞核内 DNA 的「数量」翻倍了，但染色体的「条数」没有变。人的体细胞在 S 期结束时仍然是 46 条染色体，只不过有 92 条染色单体。至于复制到底是怎么进行的，是明天（第 3 天）的内容。
G2 is the second gap, and it works like a pre-flight check. The cell keeps growing, duplicates the remaining organelles, and synthesises the proteins that the division itself will need — including the protein subunits that will later be assembled into spindle fibres, the protein cables that attach to each chromosome's centromere during mitosis and pull the chromatids apart. Just as importantly, the newly copied DNA is checked and repairable errors are repaired, so the cell does not carry a damaged copy into division. Only when G2 is complete does the cell enter M phase. If you can produce one accurate sentence about each sub-phase — G1: the cell grows and makes organelles and proteins; S: the DNA is replicated so each chromosome becomes two sister chromatids joined at a centromere; G2: the cell grows further, checks the copied DNA and makes the proteins needed for division — you can answer almost any Describe question on interphase.
G2 期（第二间隙期）是第二个间隙期，作用相当于起飞前的最后检查。细胞继续长大，复制剩下的细胞器，并合成分裂本身要用到的蛋白质——包括日后组装成纺锤丝 (spindle fibre) 的蛋白质亚基；纺锤丝就是有丝分裂时附着在染色体着丝粒上、把染色单体拉开的那些蛋白质「缆绳」。同样重要的是：刚复制出来的 DNA 会被检查，能修复的错误会被修复，免得细胞带着一份有损伤的拷贝进入分裂。只有 G2 期全部完成，细胞才进入 M 期。只要你能给每个亚期各说出一句准确的话——G1：细胞长大、合成蛋白质和细胞器；S：DNA 被复制，每条染色体变成由着丝粒连着的两条姐妹染色单体；G2：继续长大、检查复制好的 DNA、合成分裂所需的蛋白质——间期的 Describe 题基本就都能答了。
Older books and lazy summaries call interphase the resting stage, and students then write that 'the cell rests before dividing'. That answer scores zero. Interphase is the longest and most metabolically active part of the cycle: the cell is growing, building organelles and proteins, doing its everyday job and copying an entire genome. If you are tempted to write 'rest', write G1, S and G2 with one event each instead.
老课本和偷懒的总结把间期叫「静止期／休息期」，同学于是写「细胞在分裂前休息一下」——这种答案零分。间期是整个周期里最长、代谢最活跃的一段：细胞在长大、造细胞器和蛋白质、照常干活，还要把整个基因组复制一遍。想写「休息」的时候，改成写 G1、S、G2 三个亚期各一件事。
'The DNA copies itself during prophase' is one of the most heavily penalised errors in this topic. Replication is finished before mitosis begins — that is precisely why the chromosomes that become visible in prophase already appear as two chromatids. If replication happened during prophase, there would be nothing for the spindle to separate at anaphase.
「DNA 在前期自我复制」是这一章被扣分最狠的错误之一。复制在有丝分裂开始之前就已经完成——正因为如此，前期显现出来的染色体一上来就是两条染色单体的样子。如果复制真发生在前期，那么后期 (anaphase) 时纺锤体就根本没有东西可分。
Textbook diagrams almost always draw the replicated, X-shaped chromosome, so students assume every chromosome has two chromatids all of the time. It does not. Throughout G1 — which is most of a cell's life — each chromosome is a single unreplicated thread with one chromatid and one centromere. The two-chromatid X form exists only from the end of S phase until anaphase, when the centromere divides and each chromatid becomes a chromosome in its own right. Remember too that these condensed, drawable shapes are only visible during division; for the whole of interphase the same DNA is present as long, uncondensed threads.
课本上画的几乎都是复制完的「X 形」染色体，于是同学以为染色体永远有两条染色单体。并非如此。在整个 G1 期——也就是细胞一生中最长的一段时间——每条染色体都只是一条尚未复制的细丝，只有一条染色单体、一个着丝粒。两条染色单体的「X 形」只存在于 S 期结束到后期之间；后期着丝粒一分裂，每条染色单体就各自成为一条独立的染色体。还要记住：课本上那种能画出形状的「粗短」染色体只有在分裂时才看得见，整个间期里同样的 DNA 都是松散伸展的细丝状态。
Not every cell keeps cycling. Some leave the cycle after G1 and enter a state called G0, where they carry out their function without preparing to divide. Mature neurons and cardiac muscle cells are normally in G0 permanently, which is part of why nerve and heart damage does not heal the way a cut in your skin does, while liver cells sit in G0 but can re-enter the cycle if the organ is damaged. This is extension in all three states — useful background, not a required dot point.
并不是所有细胞都一直转这个圈。有些细胞在 G1 期之后就离开细胞周期，进入所谓的 G0 期：只履行自己的功能，不再为分裂做准备。成熟的神经元和心肌细胞通常永久停留在 G0 期，这也是神经和心脏的损伤不像皮肤伤口那样能长好的原因之一；肝细胞平时也在 G0 期，但器官受损时能重新回到细胞周期。这一点在三个州的大纲里都属于拓展内容——当背景知识理解即可，不是必考点。
A teacher gives out a two-page worksheet, stapled in the corner, to 46 students. How many handouts left the room? Forty-six — not ninety-two — even though ninety-two sheets of paper did. You count staples, not sheets. That is exactly the count at the end of S phase. Every chromosome has been replicated, so each one is now two identical sister chromatids held together at a single centromere, and the centromere is the staple. The cell holds twice the DNA but still 46 chromosomes, because one centromere means one chromosome. The number only becomes 92 at anaphase, when each centromere divides and every chromatid gains one of its own. Where it breaks down, in two places: the two pages of a handout say different things, whereas sister chromatids carry identical DNA — and that identity is the entire point of copying. And a staple is a separate object clipped on afterwards, whereas the centromere is not added to the chromosome — it is a region of the chromosome's own DNA, copied during S phase along with everything else.
用「订书钉订起来的双页讲义」讲染色体 vs 染色单体。老师发一份两页、角上订了订书钉的讲义给 46 个学生：发出去的是 46 份讲义，不是 92 份，尽管确实有 92 张纸出了教室——数的是订书钉，不是纸张。S 期结束时的计数完全一样：每条染色体都复制了，现在由一个着丝粒连着的两条完全相同的姐妹染色单体组成，着丝粒就是那枚订书钉。DNA 的量翻倍，但仍然是 46 条染色体，因为「一个着丝粒 = 一条染色体」。只有到后期 (anaphase) 着丝粒分裂、每条染色单体各得一个着丝粒时，条数才变成 92。类比的边界（有两处）：第一，讲义的两页内容不同，而姐妹染色单体携带的 DNA 完全相同——这份「相同」正是复制的全部意义；第二，订书钉是事后夹上去的一个外来物件，而着丝粒并不是「加」到染色体上的，它本身就是染色体自身 DNA 上的一个区域，在 S 期里和其余部分一起被复制。
The key question: why must the DNA be copied BEFORE mitosis?453
This is the question your class asked, and it deserves a proper answer rather than 'because it just has to'. Start from what mitosis actually does. Mitosis is a separation process, not a copying process. Spindle fibres attach at the centromere of each chromosome and pull the two sister chromatids apart, one to each pole — and that is the whole of what mitosis is able to do. It has no mechanism for manufacturing new genetic material. So if the copies are not already sitting there when mitosis begins, there is nothing to separate, and no amount of careful dividing can invent the missing DNA.
这就是你们班上问的那个问题，它值得一个像样的回答，而不是「反正就得这样」。先想清楚有丝分裂到底在干什么：有丝分裂是一个「分开」的过程，不是「复印」的过程。纺锤丝附着在每条染色体的着丝粒上，把两条姐妹染色单体各拉向一极——有丝分裂的本事到此为止。它没有任何制造新遗传物质的机制。所以，如果分裂开始时拷贝还没准备好，就根本没有东西可分；分得再仔细，也变不出缺失的 DNA。
Do the counting and it becomes obvious. A human body cell is diploid and has 46 chromosomes. During S phase every chromosome is replicated, so the cell enters mitosis with 46 chromosomes made up of 92 chromatids. At anaphase the centromeres divide and one chromatid from each pair travels to each pole, so each new nucleus receives 46 chromosomes — a complete set, and identical to the set in the other nucleus. Now imagine the cell had skipped S phase. It would enter mitosis with 46 single-chromatid chromosomes, and the spindle could do nothing except share those 46 out between the two poles, giving roughly 23 each: a random, incomplete half-set. Those daughter cells would be missing the genes for essential proteins, so they could not function, and a second division would strip out more again. Copying first is what makes the daughter cells both complete and genetically identical.
把数字数一遍就一目了然了。人的体细胞是二倍体 (diploid)，有 46 条染色体。S 期里每条染色体都被复制，所以细胞进入有丝分裂时是 46 条染色体、共 92 条染色单体。到后期，着丝粒分裂，每对姐妹染色单体各有一条被拉向一极，于是每个新细胞核都拿到 46 条染色体——一整套，而且和另一边那套完全相同。现在设想细胞跳过了 S 期：它进入有丝分裂时是 46 条只有单条染色单体的染色体，纺锤体只能把这 46 条在两极之间「分家」，各得大约 23 条——那是随机而残缺的半套遗传物质。这样的子细胞会缺掉编码某些必需蛋白质的基因，根本无法正常工作；再分裂一次还会丢得更多。正因为先复制，子细胞才既「完整」又「遗传物质完全相同」。
Why does the copying happen during interphase rather than during mitosis itself? Two reasons you can quote. First, DNA can only be copied while it is uncoiled and accessible — long, loose threads of DNA and protein called chromatin — which is exactly its state throughout interphase; from prophase onwards it is coiled up tightly for transport and is not being read or copied. Second, replication takes time, and the new copy has to be checked before it is separated — which is precisely the job of G2. So the order of the cycle is not arbitrary. Replicate in S, check in G2, separate in M. Learn to say the whole chain in one breath: DNA is replicated in S phase, so each chromosome consists of two identical sister chromatids joined at a centromere; at anaphase the centromeres divide and one chromatid of each pair moves to each pole; therefore each daughter nucleus receives one complete, identical copy of every chromosome.
那为什么复制发生在间期，而不是在有丝分裂过程中？有两条理由可以直接写进答案。第一，DNA 只有在解螺旋、松散伸展成由 DNA 和蛋白质构成的细丝（这种形态叫染色质 chromatin）、可以被酶接触到的状态下才能被复制，而这正是它整个间期的状态；从前期开始，DNA 就高度螺旋化、压缩成便于搬运的形态，此时既不被读取也不被复制。第二，复制需要时间，而且复制出来的新拷贝必须先接受检查再分开——这正是 G2 期的任务。所以周期的顺序不是随便排的：S 期复制 → G2 期检查 → M 期分开。请练到能一口气把整条因果链说完：DNA 在 S 期完成复制，所以每条染色体由着丝粒连着的两条完全相同的姐妹染色单体组成；后期时着丝粒分裂，每对中的一条被拉向一极；因此每个子细胞核都得到每一条染色体的一份完整且相同的拷贝。
Justify needs evidence plus a stated conclusion. Try this shape: 'Mitosis separates the sister chromatids of each chromosome; it does not copy DNA. If the DNA were not replicated first, there would be no sister chromatids to separate, so the parent cell's 46 chromosomes could only be shared between the two poles, giving each daughter cell an incomplete set that is missing essential genes. Because DNA replication in S phase produces two identical chromatids per chromosome, mitosis can deliver one complete identical set to each daughter cell. Replication must therefore be completed before mitosis begins.' Three moves: what mitosis does, what would go wrong, conclusion.
Justify（论证）要求「证据 + 明确结论」。可以套这个结构：「有丝分裂只是把每条染色体的两条姐妹染色单体分开，它并不复制 DNA。如果事先没有复制，就没有姐妹染色单体可分，亲代细胞那 46 条染色体只能在两极之间分摊，每个子细胞得到的是残缺的一套、缺少必需的基因。正因为 S 期的 DNA 复制让每条染色体有了两条完全相同的染色单体，有丝分裂才能给每个子细胞送去一整套完全相同的遗传物质。所以复制必须在有丝分裂开始之前完成。」三步：有丝分裂做什么 → 不复制会怎样 → 结论。
This is the single most common weak answer in the whole topic. Fair sharing of the original DNA would give each daughter cell half a genome, not a whole one. The identity and completeness of the daughter cells comes from the COPYING step in S phase, not from careful division. If your sentence does not contain the word replicated or copied, it is not yet an explanation.
这是整章里最常见的「弱答案」。把原有的 DNA 平均分，两个子细胞各得半套基因组，而不是各得一整套。子细胞之所以完整且遗传物质完全相同，靠的是 S 期那一步「复制」，不是靠分得公平。你的句子里如果没有出现「复制 (replicated / copied)」这个词，它就还算不上一个解释。
After S phase a human cell has 46 chromosomes and 92 chromatids — not 92 chromosomes. A replicated chromosome is still ONE chromosome, because it still has one centromere; it simply now has two chromatids. Count centromeres, not arms. The chromosome number changes only at anaphase: when the centromeres divide, each chromatid gains its own centromere and so becomes a chromosome in its own right, and the cell momentarily contains 92 chromosomes. These are then shared equally between the two poles, so each new nucleus again receives 46 single-chromatid chromosomes — exactly the number the parent cell started with. Each daughter nucleus therefore ends up with the same chromosome number as the parent: in mitosis the chromosome number is never halved.
S 期之后，人的细胞是 46 条染色体、92 条染色单体，而不是 92 条染色体。复制后的染色体仍然算「一条」染色体，因为它只有一个着丝粒，只不过现在有两条染色单体。数着丝粒，不要数「胳膊」。染色体的条数只在后期才改变：着丝粒分裂后，每条染色单体都有了自己的着丝粒，因而各自成为一条独立的染色体，此时细胞内瞬时共有 92 条染色体；随后这 92 条被平均拉向两极，所以每个新细胞核又各得 46 条只含单条染色单体的染色体——和亲代细胞开始时完全一样。也就是说，子细胞核的染色体条数与亲代相同：有丝分裂过程中，染色体的条数不会减半。
Writing about the cell cycle: verbs, numbers, diagrams and quality control631
Most marks in this topic are lost to the verb, not to the biology, so match your answer to what is being asked. Outline the events of the cell cycle wants a short sequence in general terms: interphase (G1 growth, S DNA replication, G2 preparation and checking), then mitosis, then cytokinesis. Describe what occurs during S phase wants characteristics and features with no reasoning attached: DNA is replicated, so that each chromosome comes to consist of two identical sister chromatids joined at a centromere. Explain and Justify want the cause-and-effect links, with every sentence tied to the next by because, so that or therefore. A description dressed up as an explanation earns description marks only — and that is where the difference between a good answer and a top answer usually sits.
这一章丢分，多半丢在「动词」而不是生物学本身，所以答案要对准题目要求的动词。Outline（概述）细胞周期：给一条简短的顺序即可——间期（G1 生长、S 完成 DNA 复制、G2 准备与检查），然后有丝分裂，然后胞质分裂。Describe（描述）S 期发生了什么：只讲事实和特征，不讲因果——DNA 被复制，于是每条染色体由着丝粒连着的两条完全相同的姐妹染色单体组成。Explain（解释）和 Justify（论证）则要求因果链，句与句之间要用 because / so that / therefore 串起来。把「描述」包装成「解释」，只能拿到描述的分——好答案和满分答案的差距往往就在这里。
You will also be handed numbers, and the arithmetic is straightforward as long as you show it. If a cell completes one full cycle in 24 hours and M phase takes about 1 hour, then the time in interphase is 24 minus 1 equals 23 hours, and the percentage is 23 divided by 24, times 100, which is 95.8 per cent. Write the working and the per cent sign; Calculate questions award both. The reasoning also runs backwards: if a diagram or table shows that a phase occupies a large slice of the cycle, that phase lasts a long time. This is why a correctly drawn cell cycle circle has an enormous interphase arc and a thin M phase wedge, and why a circle drawn as four equal quarters is marked wrong even when every label on it is correct.
题目也常常直接给你数字，只要把过程写出来，算术并不难。比如一个细胞完成一整个周期用 24 小时，其中 M 期约 1 小时，那么处于间期的时间是 24 − 1 = 23 小时，占比 = 23 ÷ 24 × 100 = 95.8%。一定要写出算式并带上百分号，Calculate（计算）题两样都给分。这个推理反过来也成立：如果图或表显示某个时期占了很大一块，就说明这个时期持续时间长。这正是为什么画对的细胞周期圆图里间期是巨大的一段弧、M 期只是窄窄的一小条扇形，也是为什么把圆平均分成四份的图即使标签全对也要被判错。
Finally, build the habit of using technical vocabulary without being prompted. Under exam pressure the default answer is 'the cell splits in two', which earns nothing at all, because it names no phase, no structure and no process. Train yourself to include all three in every answer, like this: 'During S phase of interphase the DNA is replicated, so each chromosome comes to consist of two identical sister chromatids joined at a centromere.' That one sentence contains a phase, a process, a structure and a consequence. Keep this working vocabulary live for the rest of the topic: interphase, G1, S phase, G2, M phase, mitosis, cytokinesis, replicated, sister chromatid, centromere, chromatin, diploid, daughter cell, genetically identical.
最后，要养成不用提醒就使用专业术语的习惯。考试一紧张，最容易脱口而出的答案是「细胞分成了两个」——这句话零分，因为它没点出任何一个时期、结构或过程。训练自己每个答案里都同时出现这三样，例如：「在间期的 S 期，DNA 被复制，于是每条染色体由着丝粒连着的两条完全相同的姐妹染色单体组成。」这一句里同时有时期、过程、结构和结果。把这套「活词汇」在整章里一直用下去：（分裂）间期、G1 期、S 期、G2 期、M 期、有丝分裂、胞质分裂、复制、姐妹染色单体、着丝粒、染色质、二倍体、子细胞、遗传物质完全相同。
The cycle is not a free-running loop; a cell is only allowed to proceed when conditions are right, and this is assessed at checkpoints. At the G1/S checkpoint the cell effectively asks: am I big enough, are nutrients and growth signals available, and is my DNA undamaged? Only then does it commit to S phase. At the G2/M checkpoint: has all of my DNA been replicated, and is the copy free of damage? Only then does mitosis begin. At the spindle (M) checkpoint, during metaphase, the cell checks that every chromosome is attached to spindle fibres at its centromere before anaphase is allowed to start. Cell cycle regulation is required content for VCE Unit 1; for NSW and QLD treat it as valuable extension. You do not need the molecules that run the checkpoints.
细胞周期并不是一个自动空转的圈：只有条件合格，细胞才被「放行」进入下一个时期，而这种评估发生在检查点 (checkpoint)。G1/S 检查点上，细胞相当于在问：我够大了吗？营养和生长信号有没有？我的 DNA 有没有损伤？都合格才承诺进入 S 期。G2/M 检查点：我的 DNA 是否全部复制完成？复制出来的拷贝有没有损伤？合格才进入有丝分裂。纺锤体（M）检查点在中期，检查每一条染色体是否都已通过着丝粒与纺锤丝正确相连，然后才允许后期开始。细胞周期调控在维州 VCE Unit 1 属于必考内容；在新州和昆州属于很有价值的拓展。检查点背后的分子机制（各种蛋白）不在你的考纲内，不用背。
If checkpoints fail, cells can divide when they should not, and can carry damaged DNA into division instead of pausing to repair it. Uncontrolled, repeated division produces a growing mass of cells — a tumour — and these cells divide more often than normal cells, ignore signals to stop, and may lose their normal specialised function. This is the required consequence in VCE Unit 1 (loss of cell cycle regulation and cancer) and a strong extension for NSW and QLD. Keep the answer at this level: link cancer to a FAILURE OF CELL CYCLE CONTROL rather than describing it vaguely as 'cells growing wrongly'.
如果检查点失灵，细胞就可能在不该分裂的时候分裂，也可能带着受损的 DNA 直接进入分裂，而不是停下来修复。这种不受控制的反复分裂会形成不断增大的细胞团块，也就是肿瘤 (tumour)；这些细胞分裂频率高于正常细胞、无视「停止分裂」的信号，还可能丧失原有的特化功能。这在 VCE Unit 1 是必须掌握的后果（细胞周期调控失灵与癌症），在新州和昆州则是很好的拓展。答题时保持在这个层次：把癌症和「细胞周期调控失效」明确挂钩，别只笼统写「细胞乱长」。
Everything in the five main sections is core for NSW Module 5 and for VCE Unit 1. In QCAA Unit 1 the cell cycle appears mainly as background to stem cells and differentiation, and the detail is assessed in Unit 4 — so treat today as core preparation either way. The state differences show up at the edges: VCE Unit 1 requires the sub-phases of the cell cycle plus checkpoints, loss of regulation and cancer, but for S phase it asks only that 'DNA is replicated' — the mechanism belongs to Unit 3. NSW Module 5 and QCAA Unit 4 want the replication detail itself, which is tomorrow's lesson. If you are unsure which applies to you, learn the core plus the checkpoint idea: it is required in one state and useful background everywhere else.
正文五个部分的内容，对新州 Module 5 和维州 VCE Unit 1 都是核心必学。昆州 QCAA Unit 1 只把细胞周期作为干细胞与细胞分化的背景带过，具体内容放在 Unit 4 考——所以无论如何，今天的内容都是必要的基础。差别出现在边缘部分：维州 VCE Unit 1 要求掌握细胞周期各亚期，还要求检查点、调控失灵与癌症，但对 S 期只要求知道「DNA 被复制」这一句——具体机制放在 Unit 3。新州 Module 5 和昆州 Unit 4 则要求掌握复制的细节本身，那正是明天的课。如果你不确定自己属于哪一种，就按「核心 + 检查点」来学：检查点在一个州是必考，在其他州也是有用的背景知识。
- Cells divide for growth, for repair and replacement of worn-out cells, and for asexual reproduction (for example budding in yeast and Hydra, or a strawberry runner) — and division continues throughout adult life in skin, gut lining and blood.细胞分裂的三个理由：生长、修复与更新（替换用旧了的细胞）、无性生殖（例如酵母和水螅的出芽、草莓的匍匐茎）；而且皮肤、肠道内壁和血细胞终生都在持续更新，不是只有小孩才分裂。
- A cell cannot just keep growing: as it enlarges, volume increases faster than surface area, so the surface area to volume ratio falls and the membrane can no longer supply the interior — so the cell divides instead.细胞不能无限长大：体积增长比表面积快，表面积与体积比 (surface area to volume ratio) 下降，细胞膜供不起内部所需，所以只能靠分裂来解决。
- The cell cycle = interphase (G1, S, G2) followed by M phase (mitosis, then cytokinesis), and it repeats: each daughter cell starts again at G1. Mitosis is nuclear division only; cell division = mitosis + cytokinesis.细胞周期 = （分裂）间期（G1 期 → S 期 → G2 期）+ M 期（先有丝分裂，后胞质分裂），并且不断循环：每个子细胞都从自己的 G1 期重新开始。有丝分裂只指细胞核的分裂；细胞分裂 = 有丝分裂 + 胞质分裂。
- Interphase is the longest and busiest part of the cycle — roughly 90 per cent or more of it — and it is NOT a resting phase and NOT a stage of mitosis.间期是周期中最长、最忙的一段（约占 90% 以上）；它既不是「休息期」，也不是有丝分裂的某个时期。
- G1 grow (proteins and organelles); S copy (DNA replicated, so each chromosome becomes two identical sister chromatids joined at a centromere); G2 check the copy and make the proteins needed for division, including spindle components.G1 长大（合成蛋白质和细胞器）；S 复制（DNA 复制，每条染色体变成着丝粒相连的两条姐妹染色单体）；G2 检查复制结果并合成分裂所需蛋白质（包括纺锤体成分）。
- S phase doubles the amount of DNA but not the chromosome number: a human cell still has 46 chromosomes, now made of 92 chromatids. Count centromeres, not arms — and remember that for most of a cell's life a chromosome is a single unreplicated thread, not an X.S 期让 DNA 的数量翻倍，但染色体条数不变：人的细胞仍是 46 条染色体，只是有了 92 条染色单体。数着丝粒，不数「胳膊」；也要记住，细胞一生中大部分时间里染色体只是一条尚未复制的细丝，并不是「X 形」。
- Replication must finish before mitosis because mitosis only SEPARATES existing sister chromatids — without a prior copy, each daughter cell would receive an incomplete half-set and could not function. The identity of daughter cells comes from copying, not from fair sharing.复制必须在有丝分裂之前完成，因为有丝分裂只能「分开」已经存在的姐妹染色单体；没有事先复制，子细胞只会拿到残缺的半套遗传物质，无法正常行使功能。子细胞之所以遗传物质完全相同，靠的是「复制」这一步，不是靠「分得公平」。
Check Yourself
Answer from memory first — the explanation appears as soon as you choose.
During S phase the DNA of the cell is replicated, so that two identical copies of every chromosome are produced. As a result, each chromosome now consists of two identical sister chromatids joined together at the centromere. The amount of DNA doubles, but the chromosome number stays the same (a human cell still has 46 chromosomes, now made up of 92 chromatids). The verb is Describe, so do not explain why replication must come first — and do not omit that the chromosome number is unchanged, which is usually the second mark. With three marks available, credit is given separately for (i) DNA is replicated, (ii) each chromosome now consists of two identical sister chromatids joined at a centromere, and (iii) the amount of DNA doubles while the chromosome number does not change.
参考答案：S 期中细胞的 DNA 被复制，每条染色体都产生两份完全相同的拷贝；结果每条染色体由在着丝粒处相连的两条完全相同的姐妹染色单体组成。DNA 的数量翻倍，但染色体的条数不变（人的细胞仍是 46 条染色体，此时共有 92 条染色单体）。注意题目动词是 Describe（描述）：只写「发生了什么」，不要展开写「为什么必须先复制」——那是 Explain / Justify 的活。三个得分点分别是：① DNA 被复制；② 每条染色体由着丝粒相连的两条完全相同的姐妹染色单体组成；③ DNA 数量翻倍而染色体条数不变——第三点最容易漏，千万别省。
Mitosis is a separation process, not a copying process: spindle fibres attach at the centromere and pull the two sister chromatids of each chromosome to opposite poles. If the DNA had not already been replicated in S phase, there would be no sister chromatids to separate, so the parent cell's chromosomes could only be shared out between the two poles and each daughter cell would receive an incomplete set missing essential genes, making it non-functional. Because replication in S phase gives every chromosome two identical chromatids, mitosis can deliver one complete and identical set of chromosomes to each daughter cell. Replication must therefore be completed before mitosis begins. Structure the answer in three moves — what mitosis does, what would go wrong without replication, and the conclusion — linking the sentences with because, so that or therefore.
参考答案（Justify 要求「证据 + 明确结论」）：有丝分裂是「分开」而不是「复制」的过程——纺锤丝附着在着丝粒上，把每条染色体的两条姐妹染色单体拉向两极。如果 DNA 没有在 S 期事先复制好，就没有姐妹染色单体可分，亲代细胞的染色体只能在两极之间分摊，每个子细胞得到的将是缺少必需基因的残缺一套，无法正常工作。正因为 S 期的复制让每条染色体都有了两条完全相同的染色单体，有丝分裂才能给每个子细胞送去完整且相同的一套染色体。所以复制必须在有丝分裂开始之前完成。写作要点：三步走（有丝分裂做什么 → 不复制会怎样 → 结论），句子之间用 because / so that / therefore 连起来。
Time in interphase = 24 - 1 = 23 hours. Percentage = 23 / 24 x 100 = 95.8% (about 96%). This tells you that interphase must be drawn as by far the largest part of a cell cycle diagram, with M phase as a thin wedge; a circle divided into equal quarters would misrepresent the timing, even if all the labels were correct. Calculate questions award the working and the per cent sign separately, and the final 'so the diagram must be drawn this way' statement is the third mark. Note that other sources quote breakdowns such as G1 40%, S 30%, G2 20%, M 10% — always calculate from the data you are given rather than from a memorised percentage.
参考答案：间期时长 = 24 − 1 = 23 小时；百分比 = 23 ÷ 24 × 100 = 95.8%（约 96%）。这说明画细胞周期图时，间期必须占绝大部分，M 期只是窄窄的一小条扇形；把圆平均分成四份即使标签全对，也严重歪曲了时间比例。Calculate（计算）题一定要写出算式并带百分号，两处都给分；最后那句「所以图该怎么画」是第三个得分点，别省。另外要注意：有些资料给出的比例是 G1 40%、S 30%、G2 20%、M 10%，所以一律按题目给的数据算，不要背死一个百分比。