DNA & Protein Synthesis
A-Level Biology Topic 6 9:34 English narration · English + 中文 subtitles burned in
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Inside almost every one of your cells sits a molecule about two metres long, folded up so tightly you cannot see it.
在你几乎每一个细胞里，都装着一条大约两米长的分子，被折叠得极其紧密，你根本看不见它。
It is DNA, and it is written in an alphabet of just four letters: A, T, C and G.
它就是DNA，用只有四个字母的字母表写成：A、T、C 和 G。
The order of those letters is the complete instruction manual for building you — every protein, every enzyme, every cell.
这些字母的顺序， 就是构建你的完整说明书——每一种蛋白质、每一种酶、每一个细胞。
In this lesson we find out how that code is stored, copied, and read.
在这节课里，我们要弄清这套密码是如何被储存、复制和读取的。
We will build a nucleotide from its three parts, assemble the double helix and its base-pairing rules, copy it semi-conservatively, then follow the flow of information: DNA to messenger RNA by transcription, and messenger RNA to a polypeptide by translation.
我们会用三个部分搭出一个核苷酸，装配出双螺旋以及它的碱基配对规则，用半保留的方式复制它， 然后跟随信息的流动：通过转录从DNA到信使RNA，再通过翻译从信使RNA到多肽。
We finish with what goes wrong — gene mutations.
最后我们看看哪里会出错——基因突变。
Let's begin.
让我们开始吧。
Nucleic acids — DNA and RNA — are polymers of nucleotides.
核酸——DNA和RNA——是核苷酸的聚合物。
Every nucleotide is built from the same three parts: a phosphate group, a five-carbon sugar, and a nitrogen-containing base.
每个核苷酸都由同样的三部分构成：一个磷酸基团、 一个五碳糖，和一个含氮碱基。
One special nucleotide is worth knowing by name: ATP.
有一个特别的核苷酸值得记住它的名字：ATP。
It carries the base adenine, the sugar ribose, and three phosphates — and breaking off that last phosphate releases the energy a cell runs on.
它带着腺嘌呤这个碱基、核糖这个糖，以及三个磷酸——而断开最后一个磷酸， 就释放出细胞赖以运转的能量。
The bases themselves fall into two groups by shape.
碱基本身按形状分成两组。
Purines, adenine and guanine, have a double ring.
嘌呤，也就是腺嘌呤和鸟嘌呤，有双环。
Pyrimidines — cytosine, thymine and uracil — have a single ring.
嘧啶——胞嘧啶、胸腺嘧啶和尿嘧啶——只有单环。
That is why a purine always pairs with a pyrimidine.
这就是为什么嘌呤总是和嘧啶配对。
Now build the molecule.
现在来搭这个分子。
Each strand has a backbone of alternating sugar and phosphate, joined by phosphodiester bonds, with the bases pointing inwards.
每条链都有一个由糖和磷酸交替组成的骨架，用磷酸二酯键连接，碱基朝内。
Two strands meet in the middle, and the pairing is exact — this is complementary base pairing.
两条链在中间相遇，配对是精确的——这就是碱基互补配对。
A always pairs with T, held by two hydrogen bonds.
A 总是与 T 配对，由两个氢键维系。
C always pairs with G, held by three — which makes a C-G pair harder to pull apart.
C 总是与 G 配对，由三个氢键维系——这让 C-G 配对更难被拉开。
One more detail the exam always wants: the two strands run in opposite directions.
还有一个考试总要考的细节： 两条链的方向相反。
One goes five prime to three prime, the other three prime to five prime.
一条从五撇走向三撇，另一条从三撇走向五撇。
We say they are antiparallel.
我们说它们是反平行的。
Before a cell divides, all of that has to be copied. That copying is called DNA replication, and it must be perfect.
细胞分裂之前，这一切都必须被复制——而且复制必须完美。
Watch what happens: the double helix unwinds, the hydrogen bonds break, and the two strands come apart.
看看发生了什么：双螺旋解开， 氢键断裂，两条链分开。
Each old strand then acts as a template, and free nucleotides pair up along it by complementary base pairing.
然后每条旧链都充当模板，游离的核苷酸沿着它按碱基互补配对结合上去。
Where there was one double helix, there are now two.
原来是一条双螺旋的地方，现在有了两条。
Let's name the parts.
我们来给各部分命名。
First the helix unwinds and the strands separate.
首先，螺旋解开，两条链分离。
Each old strand is a template, and free nucleotides pair with the exposed bases.
每条旧链都是模板，游离核苷酸与暴露的碱基配对。
Then the enzyme DNA polymerase joins those nucleotides into a strand — but it can only work in one direction, five prime to three prime.
然后，DNA聚合酶这种酶把这些核苷酸连成一条链——但它只能朝一个方向工作，从五撇到三撇。
That one rule forces the two new strands to be made differently: the leading strand is built continuously, following the fork, while the lagging strand is built backwards in short pieces, which the enzyme DNA ligase then seals together.
就这一条规则，迫使两条新链以不同的方式合成：前导链沿着复制叉连续合成， 而后随链则倒着以短片段合成，再由DNA连接酶把它们缝在一起。
And notice the result: every new molecule keeps one old strand and one new one.
注意结果：每一个新分子都保留一条旧链和一条新链。
That is why we call it semi-conservative.
这就是我们称它为半保留复制的原因。
To use a gene, the cell first copies it into RNA.
要使用一个基因，细胞先把它抄成RNA。
RNA is like DNA with three differences: it is a single strand, not a double helix; its sugar is ribose, not deoxyribose; and it uses uracil wherever DNA would use thymine.
RNA像DNA，但有三点不同：它是单链，不是双螺旋； 它的糖是核糖，不是脱氧核糖；而且凡是DNA用胸腺嘧啶的地方，它都用尿嘧啶。
The most important type here is messenger RNA, written mRNA.
这里最重要的类型是信使RNA。
Now, how does a sequence of bases specify a protein?
那么，一串碱基是怎样决定一种蛋白质的呢？
The code is read in triplets — three bases at a time.
密码是按三联体读的——一次三个碱基。
Each triplet codes for one amino acid, or acts as a start or stop signal.
每个三联体编码一个氨基酸，或者作为起始或终止信号。
There are sixty-four possible triplets but only twenty common amino acids, so most amino acids have more than one triplet: we say the code is degenerate.
可能的三联体有六十四个，而常见的氨基酸只有二十个，所以大多数氨基酸不止一个三联体： 我们说这套密码是简并的。
And it is universal — nearly every living thing uses the same code.
而且它是通用的——几乎所有生物都用同一套密码。
Here is the whole flow of information, from gene to protein.
这就是从基因到蛋白质的完整信息流。
In the nucleus, one strand of the DNA is copied into messenger RNA.
在细胞核里，DNA的一条链被抄成信使RNA。
That message leaves the nucleus and travels to a ribosome.
这条信息离开细胞核，前往核糖体。
There it is read three bases at a time, and transfer RNA molecules — tRNA — bring the matching amino acids, which are joined into a chain.
在那里它被一次三个碱基地读取， 转运RNA把对应的氨基酸带过来，连成一条链。
DNA to RNA to protein.
DNA 到 RNA 再到蛋白质。
Let's take those two steps one at a time.
我们把这两步分开来看。
Transcription happens in the nucleus.
转录发生在细胞核里。
The strand of DNA that gets copied is called the template strand; its partner is the non-transcribed strand.
被抄写的那条DNA链叫做模板链；它的搭档叫做非转录链。
The enzyme RNA polymerase moves along the template and joins RNA nucleotides that pair with the exposed bases — remembering that uracil pairs with adenine.
RNA聚合酶这种酶沿着模板移动，把与暴露碱基配对的RNA核苷酸连接起来—— 记住尿嘧啶与腺嘌呤配对。
In eukaryotes there is one more step.
在真核生物里还多一步。
The first RNA made, the primary transcript, contains coding stretches called exons and non-coding stretches called introns.
最先合成的RNA，也就是初级转录本， 含有叫做外显子的编码片段，和叫做内含子的非编码片段。
The introns are cut out and the exons joined together — splicing — and what leaves the nucleus is the finished messenger RNA.
内含子被剪掉，外显子被连在一起—— 这叫剪接——离开细胞核的，就是成熟的信使RNA。
Translation happens at the ribosome.
翻译发生在核糖体上。
The messenger RNA attaches, and the ribosome reads it three bases at a time — each triplet of messenger RNA is called a codon.
信使RNA附着上来，核糖体一次读三个碱基—— 信使RNA的每一个三联体叫做一个密码子。
For each codon, a molecule of transfer RNA arrives carrying one amino acid, and it fits because its anticodon is complementary to that codon.
对每一个密码子，都有一个转运RNA带着一个氨基酸前来， 它之所以能配上，是因为它的反密码子与那个密码子互补。
As the ribosome moves along, codon by codon, the amino acids are joined together by peptide bonds, and the polypeptide chain grows.
随着核糖体一个密码子一个密码子地前进， 氨基酸被肽键连接起来，多肽链不断增长。
When a stop codon is reached, the chain is released.
遇到终止密码子时，链就被释放出来。
Let's do one.
我们来做一道。
A DNA template strand reads T A C, G G A, C T T.
一条DNA模板链读作 T A C，G G A，C T T。
To get the messenger RNA, pair each base up — and remember RNA uses uracil, not thymine.
要得到信使RNA，把每个碱基配上—— 并记住RNA用尿嘧啶，不用胸腺嘧啶。
T A C gives A U G.
T A C 给出 A U G。
G G A gives C C U.
G G A 给出 C C U。
C T T gives G A A.
C T T 给出 G A A。
So the messenger RNA reads A U G, C C U, G A A.
所以信使RNA读作 A U G，C C U，G A A。
Now, A U G is the start codon, so this codes for three amino acids.
而 A U G 是起始密码子，所以它编码三个氨基酸。
Two mistakes cost most of the marks here: pairing A with T instead of U, and transcribing the coding strand instead of the template.
这里有两个错误会丢掉大部分分数：把 A 与 T 而不是 U 配对，以及抄写编码链而不是模板链。
Before we break the code, three details that decide marks.
在讲密码出错之前，先说三个决定分数的细节。
First, the code is non-overlapping.
第一，密码是不重叠的。
Read A U G, then C C U, then G A A — each base belongs to exactly one triplet, and the reader never backs up to re-use a base it has already read.
读作 AUG，再读 CCU，再读 GAA—— 每一个碱基只属于一个三联体，读取时绝不会退回去重复使用已经读过的碱基。
That is why a single inserted base wrecks everything after it: the frame simply shifts along.
这正是为什么插入一个碱基就会毁掉它后面的一切：读码框整个错位了。
Second, there is a check that saves the mark on every transcription question.
第二，每道转录题都有一个能保住分数的自查方法。
The messenger RNA is complementary to the template strand — but that means it is identical to the other strand, the coding strand, apart from uracil standing in for thymine.
信使RNA与模板链互补——但这就意味着它与另一条链，也就是编码链，是一样的， 只是用尿嘧啶代替了胸腺嘧啶。
So if your mRNA does not read like the coding strand with U for T, you copied the wrong strand.
所以如果你写出的mRNA读起来不像"编码链把T换成U"，那你就抄错了链。
Third, A U G does two jobs at once: it is the start signal, and it also codes for the amino acid methionine.
第三，AUG同时做两件事：它是起始信号，同时也编码氨基酸甲硫氨酸。
So a three-codon message builds three amino acids, but only two remain in the finished polypeptide once that starting methionine is cut away — read the question carefully to see which number it wants.
所以一个三密码子的信息会连上三个氨基酸， 但当起始的甲硫氨酸被切掉之后，成熟多肽里只剩两个—— 要看清题目问的是哪一个数。
Finally, what happens when the code changes?
最后，密码改变时会发生什么？
A gene mutation is a change in the base sequence of DNA.
基因突变就是DNA碱基序列的改变。
Watch one occur: a single base is altered, and from that point the message the cell reads may no longer say what it used to.
看一次突变发生： 一个碱基被改变，从那一点起，细胞读到的信息可能就不再是原来的意思了。
Sometimes the protein is unchanged.
有时蛋白质毫无变化。
Sometimes it is ruined.
有时它彻底被毁掉。
There are three kinds.
有三种。
Start from the original sequence, read as triplets.
从原始序列开始，按三联体读。
A substitution swaps one base for a different one.
替换把一个碱基换成另一个。
It changes at most one triplet, so at most one amino acid — and often none at all, because the code is degenerate.
它最多改变一个三联体，也就最多改变一个氨基酸——而且常常一个也不改，因为密码是简并的。
A deletion removes a base, and an insertion adds one.
缺失去掉一个碱基，插入加上一个碱基。
Both of those shift every triplet after that point, so almost every amino acid downstream is wrong.
这两种都会让那一点之后的每一个三联体都移位， 所以下游几乎每一个氨基酸都是错的。
We call that a frameshift, and its effect on the polypeptide is usually severe.
我们把这叫做移码，它对多肽的影响通常很严重。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, get the bonds right: A to T is two hydrogen bonds, C to G is three, and the strands are antiparallel.
第一，把键写对：A 与 T 是两个氢键，C 与 G 是三个， 而且两条链是反平行的。
Second, always say where each step happens — transcription in the nucleus, translation at the ribosome.
第二，永远说清每一步发生在哪里——转录在细胞核，翻译在核糖体。
Third, describe the code with all four words: triplet, non-overlapping, degenerate and universal.
第三，描述密码要用全四个词：三联体、非重叠、简并、通用。
Fourth, for any mutation, explain the effect on the protein — and why a substitution can be silent.
第四，对任何突变，都要说明它对蛋白质的影响——以及为什么替换可能是沉默的。