Mitosis
A-Level Biology Topic 5 7:19 English narration · English + 中文 subtitles burned in
Chapters
Transcript
Right now, inside you, millions of cells are dividing.
此刻，在你体内，数百万个细胞正在分裂。
A cell grows, copies everything it contains, and splits into two — and the two daughter cells are not merely similar.
一个细胞长大，把它包含的一切复制一遍，然后分成两个—— 而这两个子细胞不只是相似。
They are genetically identical, carrying exactly the same genes as the parent.
它们在遗传上完全相同，带着和亲代一模一样的基因。
That precision is what lets you grow from a single fertilised egg, heal a cut, and replace worn-out cells every day.
正是这种精确，让你能从一个受精卵长大、让伤口愈合、让磨损的细胞每天都被替换。
The process is called mitosis.
这个过程叫做有丝分裂。
In this lesson we open up a chromosome and name its parts, follow the cell cycle around from growth to division, measure how fast a tissue is dividing with the mitotic index, walk through the four stages of mitosis, and finish with three ideas the exam loves: telomeres, stem cells, and what happens when division stops being controlled.
在这节课里，我们会打开一条染色体，说出它各部分的名字；沿着细胞周期从生长走到分裂； 用有丝分裂指数测量一块组织分裂得有多快；走过有丝分裂的四个时期； 最后以考试最爱的三个概念收尾：端粒、干细胞，以及当分裂失去控制时会发生什么。
Let's begin.
让我们开始吧。
Start with the chromosome itself.
从染色体本身说起。
It is one enormously long molecule of DNA, wound tightly around proteins called histones — that winding is what lets a huge length fit inside a tiny nucleus.
它是一条极长的DNA分子，紧紧缠绕在叫做组蛋白的蛋白质上—— 正是这种缠绕，让巨大的长度能装进小小的细胞核。
Before a cell divides, all that DNA is copied.
细胞分裂之前，所有DNA都会被复制。
So a replicated chromosome is two identical copies joined together: we call them sister chromatids, and the point holding them together is the centromere.
所以一条复制后的染色体，是两份完全相同的拷贝连在一起：我们把它们叫做姐妹染色单体， 而把它们连在一起的那个点叫着丝粒。
Finally, each tip is capped by a telomere, which protects the end.
最后，每个末端都由端粒封住，保护着末端。
So why does this matter?
那么这为什么重要？
Because mitosis produces two genetically identical daughter cells — the same number of chromosomes and the same genes, as each other and as the parent.
因为有丝分裂产生两个在遗传上完全相同的子细胞—— 染色体数目相同、基因相同，彼此之间以及与亲代之间都一样。
That sameness is exactly what a body needs in four situations: growth, making more cells so a multicellular organism gets bigger; replacement of cells that die or wear out; repair of a damaged tissue; and asexual reproduction, where one parent produces identical offspring.
这种完全相同， 正是身体在四种情形下所需要的：生长，制造更多细胞让多细胞生物长大； 替换死去或磨损的细胞；修复受损的组织；以及无性生殖，一个亲本产生完全相同的后代。
Mitosis is only a small part of a cell's life.
有丝分裂只是细胞一生中很小的一部分。
The whole life, from one division to the next, is the cell cycle.
从一次分裂到下一次分裂的完整过程，叫做细胞周期。
Most of it is interphase, and interphase has three parts: in G one the cell grows; in S phase, S for synthesis, the DNA is replicated; and in G two the cell grows again and checks the copy before dividing.
其中大部分是间期，而间期又分三段：在G一期，细胞长大；在S期，S代表合成，DNA被复制； 在G二期，细胞再次长大，并在分裂前检查复制的结果。
Then comes the short M phase: mitosis divides the nucleus into two, and cytokinesis splits the rest of the cell, giving two separate daughter cells.
接着是短短的M期： 有丝分裂把细胞核一分为二，胞质分裂把细胞其余部分分开，得到两个独立的子细胞。
How fast is a tissue dividing?
一块组织分裂得有多快？
We measure it with the mitotic index — the fraction of cells that are caught in mitosis.
我们用有丝分裂指数来衡量——也就是正处于有丝分裂中的细胞所占的比例。
Look down a microscope at a stained slide and count: how many cells are visibly dividing, and how many cells are there in total.
在显微镜下看一张染色的切片，数一数：有多少个细胞明显正在分裂，一共又有多少个细胞。
Suppose eight of the fifty cells you can see are in mitosis.
假设你看到的五十个细胞中有八个在分裂。
Then the mitotic index is eight over fifty, which is zero point one six.
那么有丝分裂指数就是八比五十，也就是零点一六。
A high index means cells are dividing rapidly — perfectly normal in a growing root tip, but in adult animal tissue it can be a warning sign.
指数高意味着细胞分裂很快——在生长中的根尖里这完全正常， 但在成年动物组织里，它可能是一个警示信号。
Now watch the whole thing happen.
现在来看整个过程发生一遍。
The nucleus breaks down, the copied chromosomes line themselves up in the middle of the cell, and then the two halves of each chromosome are dragged to opposite ends.
细胞核解体，复制好的染色体在细胞中央排成一列， 然后每条染色体的两半被拉向相反的两端。
Two new nuclei form, the cell pinches in two, and where there was one cell there are now two.
两个新的细胞核形成，细胞从中间收缩分开， 原来一个细胞的地方，现在有了两个。
Let's slow that down and name the four stages.
让我们把它放慢，给这四个时期命名。
Four stages, in order, and one key event each.
四个时期，按顺序，每个都有一个关键事件。
In prophase, the chromosomes coil up until you can see them as two chromatids, the nuclear envelope breaks down, and a spindle of fibres forms across the cell.
在前期，染色体盘绕收缩，直到你能看见它由两条染色单体组成， 核膜解体，纺锤丝在细胞中张开。
In metaphase, the chromosomes line up along the middle — the equator — and spindle fibres attach to each centromere.
在中期，染色体沿着中央——赤道板——排成一列， 纺锤丝连到每个着丝粒上。
In anaphase, the centromeres split and the sister chromatids are pulled to opposite poles.
在后期，着丝粒分开，姐妹染色单体被拉向相反的两极。
And in telophase, a set of chromosomes has reached each pole, and a new nuclear envelope forms around each set, giving two nuclei.
而在末期，每一极都到达了一套染色体，新的核膜在每一套周围形成，得到两个细胞核。
Telophase gives you two nuclei, but still only one cell.
末期给了你两个细胞核，但仍然只有一个细胞。
Splitting the rest of it is cytokinesis, and it happens in two different ways depending on what the cell is wrapped in.
把细胞的其余部分分开，就是胞质分裂， 而它的方式有两种，取决于细胞外面裹着什么。
In an animal cell the surface membrane pinches inwards all the way round, like pulling a drawstring, until the two halves separate.
在动物细胞里，表面的细胞膜沿着一圈向内收缩，像拉紧一根抽绳，直到两半分开。
A plant cell cannot do that: it is enclosed in a rigid cellulose wall, and you cannot pinch a box.
植物细胞做不到这一点：它被一层坚硬的纤维素细胞壁包着，而盒子是捏不拢的。
Instead vesicles line up across the middle and fuse, building a new section of cell wall and membrane straight across the equator until it meets the old wall on both sides.
于是囊泡在中间排成一列并融合， 沿着赤道直接搭出一段新的细胞壁和细胞膜，直到两边都接上原来的壁。
Either way the outcome is the same, and it is the outcome the exam wants named: two genetically identical daughter cells.
不管哪一种方式，结果都是一样的，而这个结果正是考试要你说出来的： 两个基因完全相同的子细胞。
If a question shows you a dividing cell and asks how you know it is a plant, the new wall across the middle is the giveaway.
如果题目给你一张正在分裂的细胞图，问你怎么知道那是植物细胞， 中间那道新长出来的细胞壁，就是最明显的证据。
Here is a problem hiding inside replication.
复制里藏着一个问题。
When DNA is copied, the copying machinery cannot quite reach the very end of the molecule, so a little is lost every time.
当DNA被复制时，复制的机器无法完全到达分子的最末端，所以每次都会丢失一点。
The solution is the telomere: short, repeated stretches of DNA that cap each end and carry no genes at all.
解决办法就是端粒：一段段重复的短DNA，把每个末端封住，而且完全不携带基因。
Because it is the telomeres that get shortened, division after division, the genes in the middle are never lost.
因为被缩短的是端粒，一次分裂又一次分裂，中间的基因就永远不会丢失。
But telomeres do not last forever — and that puts a limit on how many times a cell can divide.
但端粒不会永远撑下去——这也就限制了一个细胞能分裂多少次。
Where do replacement cells come from?
用来替换的细胞从哪里来？
From stem cells.
来自干细胞。
A stem cell is unspecialised — it has not yet become any particular cell type — and it can keep dividing by mitosis for a very long time.
干细胞是未分化的——它还没有变成任何特定的细胞类型—— 而且它能通过有丝分裂持续分裂很长时间。
Its second power is that it can differentiate: the same stem cell can become a red blood cell, a muscle cell, or a nerve cell, depending on the signals it receives.
它的第二种本领是能够分化： 同一个干细胞可以变成红细胞、肌细胞或神经细胞，取决于它收到的信号。
That is why stem cells are the body's source of new cells for replacing losses and repairing tissue.
这就是为什么干细胞是身体补充损失、修复组织的新细胞来源。
Normally the cell cycle is tightly controlled: a cell divides only when the body needs it to, and then it stops.
正常情况下，细胞周期受到严格控制：细胞只在身体需要时才分裂，然后就停下来。
But if that control is lost, a cell can divide again and again without ever stopping.
但如果这种控制丢失了，一个细胞就可能一次又一次地分裂，永不停止。
The daughter cells divide too, and the result is a growing lump of cells — a tumour.
它的子细胞也继续分裂， 结果就是一团不断长大的细胞——肿瘤。
So the same process that builds and repairs your body becomes dangerous the moment its brakes fail.
所以，同一个建造和修复你身体的过程， 一旦刹车失灵，就变得危险。
Uncontrolled mitosis is the link between this topic and cancer.
失控的有丝分裂，正是这个专题与癌症之间的联系。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, learn the stages in order — prophase, metaphase, anaphase, telophase — with one key event each, and remember interphase is not part of mitosis.
第一，按顺序记住四个时期——前期、中期、后期、末期—— 每个配一个关键事件，并记住间期不属于有丝分裂。
Second, always say mitosis gives two genetically identical diploid cells.
第二，永远要写有丝分裂产生两个在遗传上 完全相同的二倍体细胞。
Third, the mitotic index is cells in mitosis divided by total cells — a plain fraction.
第三，有丝分裂指数就是处于分裂的细胞数除以细胞总数——一个普通的分数。
Fourth, link uncontrolled division to a tumour, and telomere shortening to the limit on how many divisions a cell gets.
第四，把失控的分裂和肿瘤联系起来，把端粒缩短和细胞能分裂的次数上限联系起来。