Mitosis and Why It Matters

Mitosis is nuclear division — "cell division" means mitosis plus cytokinesis754

Picture a human body cell as it finishes G2 and steps into M phase. Every one of its 46 chromosomes is now made of two identical sister chromatids joined at a centromere, because S phase copied every DNA molecule in the nucleus. The cell's problem from here on is a sorting problem, not a copying problem: it is holding two complete sets of genetic information inside one nucleus, and it must get exactly one complete set into each of the two cells it is about to become. Mitosis is the process that solves that problem. Notice the strict meaning of the word: mitosis is the division of the nucleus. The chromosomes are separated and two new nuclei are formed. Mitosis does not divide the cell.

先想象一个**人体细胞**刚走完 G2 期（第二间隙期）、踏进 M 期（分裂期）的那一刻。因为 S 期已经把细胞核里每个 DNA 分子都复制过一遍，此时它的 46 条染色体，每一条都由着丝粒 (centromere) 连着的两条完全相同的姐妹染色单体 (sister chromatid) 组成。接下来细胞面对的是“分配”问题，而不是“复制”问题：一个细胞核里现在装着两整套遗传信息，它必须让即将形成的两个细胞各拿到完整的一套。解决这个问题的过程就是有丝分裂 (mitosis)。请注意这个词严格的含义：有丝分裂分的是**细胞核** (nucleus)——染色体被分开，形成两个新的细胞核。有丝分裂本身并不把细胞分成两个。

Splitting the cell itself is a separate event called cytokinesis, in which the cytoplasm is divided so that two physically separate daughter cells exist. Put them together and you get the equation your markers are looking for: mitosis + cytokinesis = M phase = cell division. This is not fussiness about words. If you use "mitosis" to mean the whole of cell division, you will have no vocabulary left when a question asks you to compare how plant and animal cells divide their cytoplasm, and you would not be able to explain a real biological observation (beyond the syllabus, but it settles the point): in some human tissues, notably the liver, mitosis occasionally occurs without cytokinesis, leaving one cell with two nuclei. That would be impossible if the two processes were the same thing.

把细胞本身一分为二，是另一件事，叫胞质分裂 (cytokinesis)——细胞质被分开，才真正出现两个各自独立的子细胞 (daughter cell)。两件事合起来，就是阅卷老师想看到的那个等式：有丝分裂 + 胞质分裂 = M 期 = 细胞分裂。这不是抠字眼。如果你用“mitosis”笼统指整个细胞分裂，那么遇到“比较植物细胞和动物细胞怎样分细胞质”这类题，你会发现自己没词可用；而且你也解释不了一个真实的生物学现象（超纲，但足以说明问题）：在人体某些组织（最典型的是肝脏）里，偶尔会出现只做有丝分裂、不做胞质分裂的情况，结果是一个细胞里有两个细胞核。如果两者是同一件事，这种现象根本不可能存在。

Here is the outcome statement, worth learning almost word for word: mitosis followed by cytokinesis produces two genetically identical daughter cells, each with the same chromosome number as the parent cell. In a human somatic cell that means each daughter cell has 46 chromosomes, arranged as 23 pairs — diploid, just like the cell it came from. Mitosis happens in somatic cells — ordinary body cells such as skin, liver, root-tip and gut-lining cells — and also in the germ-line cells that multiply before meiosis begins. In animals it is not the division that produces eggs and sperm; that is meiosis, and we will contrast the two at the end of today. One last thing to keep in proportion: mitosis takes up most of the lesson, but only a small slice of the actual cell cycle. Interphase is far longer.

下面这句“结论句”值得几乎逐字背下来：有丝分裂加上胞质分裂，产生两个遗传物质完全相同 (genetically identical) 的子细胞，每个子细胞的染色体数目与母细胞相同。对人的体细胞来说，就是每个子细胞都有 46 条染色体、23 对，属于二倍体 (diploid)，和它的来源细胞一模一样。有丝分裂发生在体细胞 (somatic cell) 里——皮肤、肝脏、根尖、肠道内壁这些普通的身体细胞——也发生在进入减数分裂之前不断增殖的生殖细胞前体中。在动物里，产生卵细胞和精子的不是它，而是减数分裂 (meiosis)，今天最后一节会专门对比。还有一点要有比例感：有丝分裂占了课堂的大半时间，但在真实的细胞周期 (cell cycle) 里只占很小一段，（分裂）间期 (interphase) 要长得多。

PMAT: what happens in each stage, and how to recognise it563

Mitosis is one continuous process, but biologists cut it into four named stages: prophase, metaphase, anaphase and telophase — PMAT. The stages are named for what you can see down a microscope, and a real cell slides smoothly from one into the next, so the boundaries are a convenience for describing and for examining. (Some textbooks slip prometaphase in between prophase and metaphase. Australian senior-biology courses examine the four PMAT stages, so learn those four; prometaphase is background only.)

有丝分裂其实是一个连续的过程，但生物学家把它切成四个有名字的时期：前期 (prophase)、中期 (metaphase)、后期 (anaphase)、末期 (telophase)，缩写 PMAT。这些名字是按“显微镜下看得见什么”来分的；真实细胞是从一期平滑滑进下一期的，所以这些界线主要是为了方便描述和考试。（有些教材会在前期和中期之间插一个 prometaphase（前中期）。澳洲高中生物课程考查的就是 PMAT 这四期，记这四个就够了；prometaphase 只当背景知识了解。）

Prophase. The chromatin — the long, thin, tangled form the DNA has been in all through interphase — coils up and condenses until each chromosome is short, thick and visible with a stain. Because S phase has already been and gone, each chromosome appears as two sister chromatids joined at a centromere: the familiar X shape. Do not let that picture become your definition of a chromosome, though. A chromosome only looks like an X between S phase and anaphase. For most of a cell's life — including in both daughter cells immediately after division — each chromosome is a single, unreplicated thread of chromatin, too thin to see. The nucleolus disappears, the nuclear envelope breaks down so the chromosomes are no longer fenced off from the cytoplasm, and spindle fibres — protein threads — form and extend from opposite poles of the cell. In animal cells a pair of centrioles moves to each pole; plant cells build a perfectly good spindle without centrioles.

前期 (prophase)。染色质 (chromatin)——DNA 在整个间期里那种细长、缠绕的形态——开始高度螺旋化、凝缩，直到每条染色体变得又短又粗、染色后在显微镜下清晰可见。因为 S 期早已完成，此时每条染色体呈现为由着丝粒连着的两条姐妹染色单体，也就是我们熟悉的“X 形”。但别把这个 X 形当成染色体的定义。染色体只有在 S 期之后、后期之前才呈 X 形。细胞一生的大部分时间里——包括刚分裂完的两个子细胞里——每条染色体都只是一条未复制的、单条的染色质细丝，细到根本看不见。核仁 (nucleolus) 消失，核膜 (nuclear envelope) 解体，染色体不再被“围”在核里；纺锤丝 (spindle fibre)——一种蛋白质细丝——从细胞两极形成并延伸出来。动物细胞里有一对中心粒 (centriole) 移向每一极；植物细胞没有中心粒，照样能组装出纺锤体。

Metaphase. The spindle fibres attach to each chromosome at the kinetochore, a protein structure built on the centromere — not at the ends of the chromosome, which is one of the most common diagram-drawing errors — and manoeuvre the chromosomes until they are all lined up in a single plane across the middle of the cell. That plane is the equator, also called the metaphase plate. Metaphase is the easiest stage of all to recognise: a tidy line of chromosomes across the centre of the cell and nothing at the poles. Think of it as the cell checking its work — every chromosome is gripped from both sides before anything is pulled apart.

中期 (metaphase)。纺锤丝附着在每条染色体的着丝粒区域——更准确地说，是附着在着丝粒上组装起来的一种蛋白质结构，叫动粒 (kinetochore)——而不是附着在染色体两端，后者是画图时最常见的错误之一。纺锤丝牵引染色体，直到所有染色体都排列在细胞中央的同一个平面上。这个平面就是赤道板 (equator / metaphase plate)。中期是最容易辨认的一期：细胞正中一条整整齐齐的染色体“横队”，两极什么都没有。可以把它理解成细胞在“验货”：每条染色体都被两侧的纺锤丝同时抓住之后，才允许开始分开。

Anaphase. The centromere of each chromosome divides, and the two sister chromatids are finally separated. The spindle fibres shorten and drag one chromatid of every pair towards one pole and the other towards the opposite pole. The instant the centromere splits, each chromatid stops being a chromatid and becomes a chromosome in its own right. Under the microscope, anaphase looks like two clusters of V-shaped or J-shaped chromosomes moving apart with a widening gap between them — V-shaped because each one is being dragged centromere-first, with its arms trailing behind.

后期 (anaphase)。每条染色体的着丝粒分裂，两条姐妹染色单体终于被分开。纺锤丝缩短，把每一对中的一条拉向这一极、另一条拉向那一极。着丝粒一分开的那一瞬间，每条染色单体就不再叫染色单体，而各自成为一条独立的染色体。显微镜下的后期长这样：两簇 V 形或 J 形的染色体正在向两边移动，中间的空隙越来越大——之所以呈 V 形，是因为它们都被着丝粒“领着走”，两条臂在后面拖着。

Telophase. The chromosomes reach the poles and mitosis reverses its opening moves: the chromosomes uncoil and decondense back into chromatin, a nuclear envelope forms around each of the two groups, the nucleolus reappears and the spindle breaks down. At the end of telophase there are two complete, identical nuclei — inside what is still, for the moment, a single cell. Mitosis is over. Cytokinesis is what happens next.

末期 (telophase)。染色体到达两极，有丝分裂开始“倒放”前期的动作：染色体解螺旋、松散回染色质状态，核膜在两组染色体外重新形成，核仁重新出现，纺锤体解体。末期结束时，出现了两个完整且完全相同的细胞核——但此刻它们仍然待在**同一个**细胞里。有丝分裂到此结束，接下来才轮到胞质分裂。

Cytokinesis: dividing the cytoplasm, and why plants do it differently57

Cytokinesis usually begins in late anaphase, is well under way through telophase, and finishes just after mitosis ends. In an animal cell, a ring of protein filaments lying just beneath the cell membrane around the equator contracts, like a drawstring being pulled tight. The membrane is drawn inwards, forming a groove called a cleavage furrow, and the furrow deepens until the two halves are pinched right apart into two separate cells. The drawstring analogy is a good one, with a caution: the "string" is inside the cell, made of protein filaments, and nothing has to be tied off — the membrane simply meets in the middle and fuses.

胞质分裂 (cytokinesis) 通常在后期后段就已开始，贯穿末期，在有丝分裂结束后不久完成。在动物细胞里，紧贴细胞膜内侧、沿赤道板环绕细胞一圈的蛋白质细丝会收缩，就像把一根抽绳慢慢拉紧。细胞膜被向内拉，形成一道凹槽，叫分裂沟 (cleavage furrow)；凹槽越陷越深，最后把细胞“掐”成两个独立的细胞。抽绳这个比喻不错，但要注意两点：这根“绳”在细胞**内**，由蛋白质细丝构成；而且不需要打结收口——细胞膜在中间相遇后直接融合封闭。

A plant cell cannot be pinched. It is surrounded by a rigid cellulose cell wall that will not deform like that, so plant cells build a partition instead. Small membrane-bound vesicles carrying wall material gather along the equator and fuse with one another to form a structure called the cell plate. The cell plate grows outwards from the centre of the cell until it reaches and joins the existing cell wall, and the material inside it becomes a new cell wall with a new membrane on either side. The direction is the giveaway you should quote in an exam: animal cytokinesis works from the outside inwards, plant cytokinesis from the inside outwards.

植物细胞“掐不动”。它外面包着一层坚硬的纤维素细胞壁 (cell wall)，不会那样变形，所以植物细胞改用“砌墙”的办法。携带细胞壁材料的具膜小囊泡聚集到赤道板位置，彼此融合，形成一个叫细胞板 (cell plate) 的结构。细胞板从细胞中央**向外**生长，直到与原有的细胞壁相接；板内的材料形成新的细胞壁，两侧各形成一层新的细胞膜。考试要写的“定案特征”就是方向：动物的胞质分裂是由外向内，植物的胞质分裂是由内向外。

One more accurate detail worth having. The cytoplasm and its organelles are shared out roughly, not precisely — one daughter cell may end up with a few more mitochondria than the other, and that does not matter. The genetic identity of the daughter cells comes from the nucleus, and specifically from replication in S phase followed by the precise separation of chromatids at anaphase. Cytokinesis in plant and animal cells is named content in VCE Unit 1, and it is a favourite comparison question, so learn both mechanisms with their proper terms.

还有一个准确的小细节值得记住。细胞质和其中的细胞器 (organelle) 是**大致**平分的，不是精确平分——某个子细胞可能多分到几个线粒体，这没关系。子细胞“遗传物质完全相同”这件事来自细胞核，具体说是来自 S 期的复制加上后期染色单体的精确分离。植物与动物细胞的胞质分裂在 VCE Unit 1 里是点名要求的内容，而且是各地都爱考的比较题，所以两套机制都要用规范术语记牢。

How two genetically identical daughter cells are actually produced745

This is the big question of the whole four-day chain, and the answer is a chain of causes. One: in S phase every DNA molecule is replicated semi-conservatively — the helix unzips as the hydrogen bonds between the bases break, each old strand acts as a template, and free nucleotides already present in the nucleus pair complementarily, A with T and C with G. Two: because the pairing rules are fixed, the sequence of each new strand is dictated by the old one, so the two DNA molecules produced are identical. Three: those two identical molecules stay attached to each other as sister chromatids, joined at a centromere.

这是这四天内容里最大的一个问题，答案是一条因果链。第一步：S 期里每个 DNA 分子都进行半保留复制 (semi-conservative replication)——碱基之间的氢键 (hydrogen bond) 断裂，双螺旋解开，每条母链作为模板链 (template strand)，细胞核中原本就存在的游离核苷酸 (nucleotide) 按碱基互补配对 (complementary base pairing) 原则配上去，A 配 T、C 配 G。第二步：因为配对规则是固定的，新链的序列完全由母链决定，所以生成的两个 DNA 分子完全相同。第三步：这两个相同的分子并不分开，而是作为姐妹染色单体，由着丝粒连在一起。

So much for making the copies; the next three steps are about delivering them. Four: at metaphase every chromosome is held at its centromere by spindle fibres from both poles. Five: at anaphase every centromere divides, so one chromatid of each pair goes to each pole — each pole receives one copy of every chromosome, never a random half. Six: telophase and cytokinesis wrap each set in its own nuclear envelope and cytoplasm.

复制这一半讲完了；接下来三步讲的是怎么把拷贝送出去。第四步：中期时，每条染色体的着丝粒都被来自两极的纺锤丝抓住。第五步：后期每个着丝粒分裂，每一对中的一条去一极——每一极拿到的是**每一条**染色体的一份拷贝，绝不是随机的一半。第六步：末期和胞质分裂把每一套分别包进各自的核膜和细胞质里。

Notice which step does the real work. Students very often write that the daughter cells are identical "because the DNA is divided evenly between them". Markers do not accept that, and they are right not to: dividing something evenly gives each side half of it, not a complete copy of it. The identity comes from copying first, in S phase, and separating precisely afterwards, at anaphase. If you remember one sentence for the exam, make it this one: replication makes the copies, anaphase distributes them. This is also why replication has to be finished in S phase before mitosis begins. Mitosis only separates chromatids that already exist; it never makes new DNA. If a human cell entered mitosis without copying its DNA first, its 46 single-chromatid chromosomes would simply be split between the two poles, each daughter cell would receive about 23 chromosomes and an incomplete set of genes, and the chromosome number would halve again at every division.

注意真正“干活”的是哪一步。很多同学会写：子细胞之所以相同，是“因为 DNA 被平均分给了它们”。阅卷老师不接受这种说法，而且这是对的：把一样东西平分，每边得到的是它的**一半**，而不是它的**完整拷贝**。相同性来自两件事——先在 S 期复制，再在后期精确分离。如果考试只让你记一句话，就记这句：复制负责“做出拷贝”，后期负责“把拷贝分出去”。这也正是复制必须在 S 期完成、在有丝分裂开始之前就结束的原因。有丝分裂只负责把已经存在的染色单体分开，它本身不制造任何新的 DNA。如果一个人类细胞没复制就进入分裂，那 46 条单染色单体的染色体只会被分到两极去，每个子细胞只拿到约 23 条、基因残缺不全，而且每分裂一次染色体数就再减半一次。

Finally, be careful about what "genetically identical" does and does not claim. It means the daughter cells carry the same DNA sequence — the same genes — as each other and as the parent cell. It does not mean they will always look alike or do the same job. As a multicellular organism develops, cells differentiate: a muscle cell and a nerve cell in your body contain identical DNA but switch on different genes, so they end up with completely different shapes and functions. Same instruction book, different pages open.

最后，要弄清楚“遗传物质完全相同 (genetically identical)”这句话主张了什么、没有主张什么。它的意思是：两个子细胞彼此之间、以及与母细胞之间，携带相同的 DNA 序列，也就是相同的基因 (gene)。它**不**意味着它们永远长得一样、干一样的活。多细胞生物在发育过程中，细胞会分化 (differentiation)：你体内的肌细胞和神经细胞含有完全相同的 DNA，但开启的基因不同，于是形态和功能天差地别。同一本说明书，只是翻开的页码不同。

Why it matters: growth, repair and replacement, and asexual reproduction67

Growth. You began as a single cell, a fertilised egg, and every one of the tens of trillions of cells in your body descends from it by mitosis. Growth in a multicellular organism is mostly an increase in the number of cells (cells also enlarge and differentiate), and because mitosis copies faithfully, every one of those cells inherits the same complete set of instructions. In plants, growth is concentrated in regions called meristems, which is exactly why a root tip is the standard slide for seeing mitosis: it is one of the few places where a large proportion of cells are dividing at any given moment.

**生长 (growth)。** 你曾经只是一个细胞——一个受精卵；如今身体里数十万亿个细胞，全都是它经有丝分裂一代代增殖而来的。多细胞生物的生长主要是细胞**数目**的增加（同时也伴随细胞体积增大和分化），而由于有丝分裂复制得极其忠实，每个细胞都继承了同样完整的一套指令。植物的生长集中在分生组织 (meristem) 区域——这正是为什么观察有丝分裂的标准材料是根尖：那是少数“任一瞬间都有大量细胞在分裂”的部位之一。

Repair and replacement. Mitosis does not stop when you stop growing, and this is the point students most often miss. Your skin is shed and replaced continuously. The lining of your gut is replaced every few days because it takes such a battering from digestive enzymes. Your bone marrow makes new red and white blood cells constantly, partly because a mature red blood cell has no nucleus and cannot divide, and survives only about four months. And when you cut yourself, mitosis is what closes the wound: cells at the edges of the cut divide to fill the gap, and normally stop dividing again once they meet neighbouring cells.

**修复与替换 (tissue repair)。** 停止长个子并不代表有丝分裂就停了，这一点最容易被忽略。皮肤在不断脱落、不断更新；肠道内壁因为长期受消化酶的“摧残”，每隔几天就要更换一批；骨髓持续生产新的红细胞和白细胞，部分原因是成熟红细胞没有细胞核、不能分裂，寿命大约只有四个月。当你划破手指，正是有丝分裂把伤口合上：切口边缘的细胞分裂来填补缺口，并且在与邻近细胞接触后通常就停止分裂。

Asexual reproduction. Some organisms use mitosis to produce whole new individuals. Yeast and Hydra reproduce by budding: a small outgrowth produced by mitosis eventually detaches and lives independently. A strawberry plant sends out runners that root and become new plants, and a gardener taking a cutting is using the same ability — both are vegetative propagation. A planarian flatworm cut in two can regenerate the missing half, and some starfish can regenerate from a fragment. In every case the offspring are produced by mitosis, so they are genetically identical to the parent: clones.

**无性生殖 (asexual reproduction)。** 有些生物直接用有丝分裂产生新个体。酵母和水螅 (Hydra) 靠出芽生殖：有丝分裂长出的小芽体最终脱落、独立生活。草莓通过匍匐茎 (runner) 生根长成新植株，园丁扦插用的也是同一种能力——两者都属于营养繁殖 (vegetative propagation)。涡虫被切成两段可以再生出缺失的一半，某些海星也能由断片再生。所有这些情况下，后代都由有丝分裂产生，因此与亲本遗传物质完全相同，即克隆 (clone)。

Binary fission. Bacteria also reproduce asexually, but they do not use mitosis — they use binary fission, and that contrast is named content in VCE Unit 1. A bacterium has no nucleus and no spindle, and its genetic material is a single circular chromosome. In binary fission that chromosome is copied, the two copies move apart as the cell elongates, and then the membrane and wall grow inwards to divide the cell into two. It is far simpler and much faster than the eukaryotic cell cycle: under ideal laboratory conditions E. coli can divide roughly every 20 minutes. If a question asks how bacteria reproduce, the word mitosis is wrong.

**二分裂 (binary fission)。** 细菌也是无性生殖，但它们**不做有丝分裂**，而是二分裂——这个对比在 VCE Unit 1 里是点名要求的内容。细菌没有细胞核、没有纺锤体，遗传物质是一条环状 DNA（单个环状染色体）。二分裂过程中，这条染色体先被复制，两份拷贝随细胞伸长而分向两端，随后细胞膜和细胞壁向内生长，把细胞一分为二。它比真核细胞周期简单得多、也快得多：在理想的实验室条件下，大肠杆菌 (E. coli) 大约每 20 分钟就能分裂一次。所以题目问细菌怎么繁殖时，写“mitosis”就是错的。

Mitosis is not meiosis: the contrast you must be able to write7

A safe rule is that Compare should cover both similarities and differences, so start with the similarities, which are real and often forgotten. Both mitosis and meiosis are forms of nuclear division. Both are preceded by one round of DNA replication in S phase. Both use spindle fibres to move chromosomes. Both involve chromosomes or chromatids being pulled to opposite poles. Both are followed by cytokinesis. If a question says Contrast, give differences only — the two command words are not interchangeable.

一条稳妥的原则是：Compare（比较）应当同时覆盖**相同点和不同点**，所以先说相同点——它们是真实存在的，却常被忽略。有丝分裂和减数分裂 (meiosis) 都是细胞核的分裂；两者之前都只进行**一轮** S 期的 DNA 复制；都靠纺锤丝牵引染色体；都涉及染色体或染色单体被拉向两极；之后都跟着胞质分裂。但如果题目用的是 Contrast（对比），那就只写不同点，上面这些一句都不要写——这两个命令动词不能互换。

Now the differences, criterion by criterion — and always apply each criterion to both processes in the same sentence. Number of divisions: mitosis is one nuclear division, whereas meiosis is two in succession. Number of daughter cells: mitosis gives two, whereas meiosis gives four. Chromosome number: mitosis keeps it the same, so a diploid human cell with 46 chromosomes gives two diploid cells with 46, whereas meiosis halves it, giving haploid cells with 23. Genetic outcome: mitosis produces daughter cells genetically identical to the parent, whereas meiosis produces cells that are genetically different from one another and from the parent. Function: mitosis occurs in body cells for growth, repair and asexual reproduction, and also in the germ-line cells that multiply before meiosis, whereas meiosis occurs in reproductive tissue and, in animals, produces the gametes used in sexual reproduction.

接下来逐条讲不同点——记住每一条标准都要在**同一句话**里同时套用到两个过程上。**分裂次数**：有丝分裂是一次核分裂，而减数分裂是连续两次。**子细胞数目**：有丝分裂产生 2 个，而减数分裂产生 4 个。**染色体数目**：有丝分裂保持不变，46 条的人类二倍体细胞产生两个 46 条的二倍体细胞；而减数分裂使其减半，产生 23 条的单倍体 (haploid) 细胞。**遗传结果**：有丝分裂的子细胞与母细胞遗传物质完全相同，而减数分裂产生的细胞彼此之间、以及与母细胞之间在遗传上都不相同。**功能**：有丝分裂发生在体细胞中，用于生长、修复和无性生殖，也发生在进入减数分裂之前不断增殖的生殖细胞前体中；而减数分裂发生在生殖组织中，在动物里产生用于有性生殖的配子 (gamete)。

You do not need the internal machinery of meiosis for this topic — the two divisions, crossing over and how variation arises come later. In some Australian courses meiosis and crossing over come later in the Year 11 sequence; in others they are met in Year 12, so check where your own course places them. For now, the job is simply to keep the two processes apart in your writing, because the classic way to lose marks here is to describe mitosis using facts that only apply to meiosis.

本章不需要你掌握减数分裂的内部细节——两次分裂的具体过程、交叉互换 (crossing over)、变异是怎么产生的，都在后面。在澳洲不同的课程里，减数分裂和交叉互换出现的时间不一样：有的安排在 11 年级后半段，有的要到 12 年级，具体以你自己的课程大纲为准。现在的任务只有一个：在你的答案里把这两个过程分得清清楚楚——因为这里最经典的失分方式，就是用只适用于减数分裂的事实去描述有丝分裂。