Cell Communication and Cell Cycle
AP Biology Topic 4 7:34 English narration · English + 中文 subtitles burned in
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Transcript
Look closely.
仔细看。
These are real cells, photographed in the middle of dividing.
这些是真实的细胞，拍摄于它们正在分裂的时刻。
Inside your body, millions of new cells are made every second — skin, blood, gut lining, all replaced.
在你体内，每秒都有数百万个新细胞产生—— 皮肤、血液、肠道内壁，全都在被替换。
But a cell never decides this on its own. It waits to be told.
但细胞从不自己决定这件事，它在等待被告知。
And when that message goes wrong, a cell divides when it should not.
而当这个信息出错时，细胞就会在不该分裂的时候分裂。
This is Unit Four, in five steps.
这就是第四单元，分五步。
How cells talk to each other.
细胞之间如何交流。
How an outside message becomes an inside action.
外界的信息如何变成细胞内部的行动。
How feedback holds the body steady.
反馈如何让身体保持稳定。
Then the cell cycle itself.
然后是细胞周期本身。
And last, the controls that decide when a cell may divide.
最后是决定细胞何时可以分裂的调控。
Cells talk with chemicals.
细胞用化学物质交流。
One cell releases a signal molecule — a ligand.
一个细胞释放出一种信号分子——配体。
The ligand is carried away, sometimes across a tiny gap, sometimes right around the body in the blood.
配体被带走， 有时只是穿过一条很小的缝隙，有时随血液走遍全身。
It reaches a target cell and fits a receptor, a protein shaped to hold that one ligand.
它到达靶细胞，嵌进一个受体， 那是一种形状正好能容纳这一种配体的蛋白质。
Only then does the target cell respond.
只有到这时，靶细胞才会作出反应。
Signals travel three ways: by direct contact, locally to close neighbours, or long distance as hormones in the blood.
信号有三种传播方式：直接接触、就近传给邻居，或者作为激素随血液长距离传送。
Here is short-range signalling, close up.
这是近距离信号传递的特写。
An impulse arrives at the end of one nerve cell.
一个神经冲动到达一个神经细胞的末端。
Tiny sacs fuse with the membrane and spill their chemical into a gap far too small to see.
微小的囊泡与膜融合，把里面的化学物质倒进一条小到看不见的缝隙里。
The chemical drifts across, binds receptors on the next cell, and the message carries on.
这些化学物质飘过缝隙，结合到下一个细胞的受体上，信息就这样传了下去。
It only travels one way, because only one side holds the chemical.
它只能朝一个方向传，因为只有一侧储存着这种化学物质。
So why does a hormone change some cells and not others?
那么，为什么一种激素只改变某些细胞，而不改变另一些呢？
The blood carries it everywhere.
血液把它带到了每一个角落。
It washes past bone cells, muscle cells, liver cells, all of them.
它流过骨细胞、肌肉细胞、肝细胞，流过所有细胞。
But only a cell with the matching receptor can bind it.
但只有带着相匹配受体的细胞才能结合它。
The rest feel nothing at all.
其余的细胞什么也感觉不到。
The signal goes everywhere; the receptor decides who listens.
信号无处不在，是受体决定了谁在听。
Write that sentence down — it is the answer the exam wants.
把这句话记下来——这就是考试想要的答案。
The signal transduction pathway is the whole journey, in three named stages.
信号转导通路就是这一整段旅程，分为三个有名字的阶段。
Reception: the ligand binds, and the receptor changes shape.
接收：配体结合上去，受体改变形状。
Transduction: a relay of molecules passes the message inward.
转导：一连串分子把信息往细胞内部传递。
Response: the cell does something — a gene switched on, a product made.
反应：细胞做出某种行动——打开一个基因， 制造一种产物。
Learn those three words, in that order.
把这三个词按顺序记住。
Look inside a liver cell.
我们看看肝细胞内部。
Blood sugar has fallen, so glucagon binds its receptor.
血糖下降了，于是胰高血糖素结合到它的受体上。
The receptor switches on a G protein.
受体激活一个G蛋白。
That switches on an enzyme making cyclic A M P, a second messenger that spreads through the cell at once.
G蛋白激活一种酶，这种酶制造环腺苷酸， 一种能瞬间扩散到全细胞的第二信使。
Cyclic A M P wakes protein kinase A, which starts an enzyme cascade, and glycogen is broken into glucose.
环腺苷酸唤醒蛋白激酶A， 蛋白激酶A启动一串酶的级联反应，糖原被分解成葡萄糖。
Look at the branching: each step switches on many.
看这个分叉：每一步都激活很多个。
Let's put numbers on it.
我们来算一算。
One hormone binds one receptor.
一个激素分子结合一个受体。
That receptor switches on about one hundred relay proteins.
这个受体激活大约一百个中继蛋白。
Each of those makes about one thousand second messengers.
每一个中继蛋白又制造大约一千个第二信使。
Pause and work out the total.
先暂停，自己算出总数。
Ready?
好了吗？
One hundred times one thousand is one hundred thousand product molecules, from a single hormone.
一百乘以一千，等于十万个产物分子，全都来自一个激素分子。
Signals also keep you steady.
信号也让你保持稳定。
Temperature, blood glucose, water — each is held near a set point.
体温、血糖、水分——每一项都被维持在设定点附近。
Watch the loop.
看这个环路。
Something moves the level away from normal.
某种因素让这个数值偏离了正常。
A receptor detects the change.
感受器察觉到这个变化。
A nerve or a hormone carries the news to an effector.
神经或激素把消息传给效应器。
The effector responds, and the response pushes the level back to normal.
效应器作出反应，而这个反应把数值推回正常。
Because the response opposes the change, we call it negative feedback.
因为反应与变化方向相反，我们叫它负反馈。
Here is that loop drawn against time.
这是同一个环路随时间画出来的图。
Glucose is pushed above the set point, so the body brings it down.
血糖被推到设定点以上，于是身体把它降下来。
It dips below, so the body brings it up.
它又掉到设定点以下，于是身体把它升上去。
Each correction is smaller than the last.
每一次修正都比上一次小。
Notice it is never perfectly flat.
注意，它从来都不是完全平的。
Homeostasis is a small wobble around a value, not a fixed number.
稳态是围绕一个数值的小幅波动，而不是一个固定不变的数。
Positive feedback does the opposite.
正反馈正好相反。
It pushes the change further, so an event finishes fast.
它把变化推得更远，好让一件事情快点完成。
In childbirth, the baby's head presses on the cervix.
在分娩时，胎儿的头压迫子宫颈。
That releases the hormone oxytocin.
这会释放催产素这种激素。
Oxytocin makes the contraction stronger, which presses the head harder still.
催产素让子宫收缩得更强，于是头压得更用力。
Round and round, growing each turn, until the baby is born.
就这样一圈又一圈，每一圈都更强，直到孩子出生。
Negative feedback returns you to a set point; positive feedback runs away on purpose.
负反馈把你带回设定点； 正反馈则是有意地一路失控。
Now the second half of the topic.
现在进入这一单元的后半部分。
A cell's life is a cycle.
细胞的一生是一个循环。
Most of it is interphase, when the cell grows and prepares.
其中大部分时间是间期， 细胞在这时生长并做准备。
In G one it grows and builds organelles.
在G1期它长大，制造更多细胞器。
In S phase the DNA is copied — once, and only here.
在S期，DNA被复制—— 只复制一次，而且只在这里。
In G two it makes proteins and runs a final check.
在G2期它合成蛋白质，做最后的检查。
Then M, mitosis and cytokinesis: the short part where the cell splits.
然后是M期，有丝分裂和胞质分裂：细胞真正一分为二的那一小段。
Cells that stop dividing rest outside the cycle, in G zero.
停止分裂的细胞则停在周期之外，处于G0期。
Mitosis has four stages and you must name them in order.
有丝分裂有四个时期，你必须能按顺序说出它们。
Prophase: the chromosomes condense and the spindle forms.
前期：染色体凝缩，纺锤体开始形成。
Metaphase: they line up along the middle.
中期：染色体排列在细胞中央。
Anaphase: sister chromatids are pulled to opposite ends.
后期：姐妹染色单体被拉向两极。
Telophase: two new nuclei form.
末期：两个新的细胞核形成。
Cytokinesis then splits the cytoplasm, giving two identical daughter cells — and each round of division doubles their number.
接着胞质分裂把细胞质分开，得到两个完全相同的子细胞——每分裂一轮，细胞数量就翻一倍。
Watch it happen.
来看它发生的过程。
The chromosomes were copied earlier, so each one is two sister chromatids joined together.
染色体在更早的时候已经被复制过，所以每条染色体都是两条 连在一起的姐妹染色单体。
They line up across the middle.
它们排列在中央。
The spindle pulls the sisters apart, one copy to each end.
纺锤体把姐妹染色单体拉开，两极各得一份。
The cell pinches in two, and each daughter carries a full, identical set.
细胞从中间缢裂成两个，每个子细胞都带着完整而相同的一套。
That is the point of mitosis: growth and repair, with no change at all.
这就是有丝分裂的意义：生长和修复，而且完全不发生改变。
This is what it really looks like: an onion root tip, stained and photographed down a microscope.
这就是它真实的样子：一段洋葱根尖，染色后在显微镜下拍摄。
The dark threads are chromosomes.
那些深色的丝状物就是染色体。
Find a cell where they sit in a tight line across the middle — that cell is in metaphase.
找一个染色体紧紧排成一条线、横在中央的细胞——那个细胞正处于中期。
In the lab you count cells like these to work out how long each stage lasts.
在实验课上，你就是靠数这样的细胞，来算出每个时期各持续多久。
What decides when the cycle moves on?
是什么决定细胞周期能不能继续往下走？
Checkpoints.
检查点。
At fixed points the cell stops and asks: am I big enough, was the DNA copied correctly, are the chromosomes attached?
在几个固定的位置，细胞会停下来问： 我长得够大了吗？ DNA复制正确了吗？
If any answer is no, the cycle halts.
染色体都连好了吗？
Proteins called cyclins, working with their kinases, push the cycle past a checkpoint when all is well.
只要有一个答案是否定的， 周期就停在那里。 叫做周期蛋白的蛋白质，和它们的激酶一起工作， 在一切正常时推动周期越过检查点。
Normally a damaged cell stops here.
正常情况下，受损的细胞会在这里停下来。
But if a checkpoint fails, the cell divides with damaged DNA. Its daughters inherit the damage and divide again.
但如果某个检查点失灵，细胞就带着受损的DNA继续分裂。
Nothing stops them, so a lump of cells builds up: a tumour.
它的子细胞继承了这些损伤，又继续分裂。
Uncontrolled division like this is what underlies cancer.
没有东西能阻止它们，于是细胞越堆越多：形成肿瘤。
Guard genes such as p fifty-three normally halt the cycle for repair.
像p53这样的保护基因，本来会让周期停下来进行修复。
When they break, the damage is copied forward.
一旦它们坏了，损伤就被一代代复制下去。
Four marks students give away.
四个学生常送掉的分。
First, name all three stages in order: reception, transduction, response.
第一，按顺序说出三个阶段：接收、转导、反应。
Second, when asked why one cell responds, say the receptor is specific and complementary to the ligand.
第二，被问到为什么只有某个细胞会反应时，要说受体对配体具有特异性、形状互补。
Third, negative feedback reverses a change back to a set point; positive feedback amplifies it to finish an event.
第三，负反馈把变化逆转回设定点；正反馈把变化放大，好让一件事完成。
Fourth, the DNA is copied once, in S phase, so both daughter cells are identical.
第四，DNA只复制一次，发生在S期，所以两个子细胞完全相同。