The Structure of DNA

What DNA is and what its job is874

Start with the job, because the structure only makes sense once you know what the molecule has to do. DNA — deoxyribonucleic acid — is the chemical that stores the instructions a cell uses to make proteins. That matters more than it sounds, because proteins do almost all of the real work in a living thing: enzymes that speed up reactions, keratin in your hair and nails, haemoglobin that carries oxygen, receptors sitting in a membrane. If DNA carries the instructions for the proteins, then DNA effectively controls what a cell can build, what it looks like and what it can do. You do not need to know this year exactly how those instructions are read — that is a separate topic later — but you do need to be able to state the job in one clean sentence: DNA stores the genetic instructions for making proteins, and it passes those instructions on when a cell divides.

先讲“用途”，因为只有知道这个分子要干什么，它的结构才讲得通。DNA（脱氧核糖核酸 deoxyribonucleic acid）是储存“造蛋白质的说明书”的化学物质。为什么这很重要？因为在生物体内几乎所有实际干活的都是蛋白质：加速化学反应的酶、头发和指甲里的角蛋白、运氧的血红蛋白、细胞膜上的受体等等。既然 DNA 掌握着造蛋白质的说明书，它实际上就决定了一个细胞能造什么、长什么样、能做什么。今年你不需要知道说明书具体是怎么被“读取”的（那是后面的课题），但必须能用一句话说清它的工作：DNA 储存制造蛋白质的遗传指令，并在细胞分裂时把这套指令传下去。

Where is it? In a eukaryotic cell — a plant, animal or fungal cell — nearly all of the DNA sits inside the nucleus, which is exactly why the nucleus is described as the control centre of the cell. (Small amounts of DNA are also found in mitochondria and, in plants, in chloroplasts.) Prokaryotes such as bacteria have no nucleus at all: their DNA is a single circular chromosome sitting free in the cytoplasm. The important point for you is that it is the same molecule, built to the same design, in every living organism on Earth. Learn the structure once and it applies to a bacterium, to a strawberry plant and to you. That universality is itself a piece of biological evidence, and it is the reason a diagram of DNA in a Year 11 textbook is never labelled human DNA or plant DNA — it is just DNA.

它在哪里？在真核细胞（eukaryotic cell，即植物、动物、真菌细胞）里，绝大部分 DNA 都在细胞核 (nucleus) 里——这正是我们把细胞核叫做“细胞控制中心”的原因。（线粒体里也有少量 DNA，植物的叶绿体里也有。）原核生物（如细菌）根本没有细胞核：它们的 DNA 是一条环状染色体，直接漂在细胞质里。对你来说关键的一点是：地球上所有生物体内的 DNA 都是同一种分子、同一套构造。这套结构只要学一次，细菌、草莓、你自己身上都通用。这种“普遍性”本身就是一条生物学证据，也是为什么课本上的 DNA 结构图从来不标“人的 DNA”或“植物的 DNA”——它就叫 DNA。

A useful way into the structure is to ask what this molecule has to be good at. First, it has to store an enormous amount of information: a single human body cell contains about two metres of DNA, packed into a nucleus only a few micrometres across. Second, it has to be copyable — exactly, every single time a cell divides, with no drift. Everything in today's lesson is the structural answer to those two demands. And here is the headline fact that the rest of the day builds on: DNA is a polymer. A polymer is a very long molecule made by joining many small repeating units, called monomers, in a chain — the way a paper chain is built from identical loops. The monomer of DNA is called a nucleotide, and that is where we start.

想读懂结构，可以先问：这个分子必须擅长什么？第一，它要装下海量信息——人的一个体细胞里的 DNA 拉直大约有两米长，却要塞进直径只有几微米的细胞核里。第二，它必须能被复制，而且每次细胞分裂都要一模一样地复制，不能走样。今天这节课讲的全部结构，其实都是对这两个要求的“结构性回答”。今天最核心的一句话是：DNA 是一种多聚体 (polymer)。多聚体就是把许多相同的小单位（单体 monomer）像纸链圈一样一个接一个连起来形成的长分子。DNA 的单体叫核苷酸 (nucleotide)，我们就从它讲起。

The nucleotide: the repeating building block117

Every nucleotide has exactly three parts, and you should be able to name all three without hesitating: a phosphate group, a deoxyribose sugar, and one nitrogenous base. Look at the diagram and trace them with your finger. Notice how they are arranged — the deoxyribose sugar sits in the middle and acts as the connecting piece: the phosphate group attaches to one side of the sugar and the base attaches to the other side. That arrangement is not decoration, it is the whole reason the molecule can build a backbone on the outside and hold the bases on the inside. Exam papers frequently show an unlabelled nucleotide and ask you to label the three components, so learn it as a picture as well as a list. If you are ever unsure which shape is which in a diagram, the sugar is the one in the middle with two neighbours.

每个核苷酸 (nucleotide) 恰好有三个部分，你要能不假思索地说出来：磷酸基团 (phosphate group)、脱氧核糖 (deoxyribose sugar)、一个含氮碱基 (nitrogenous base)。看着图用手指顺一遍。注意它们的排列方式——脱氧核糖在中间，起“连接件”的作用：磷酸基团接在糖的一边，碱基接在糖的另一边。这个排列不是装饰，正因为糖在中间，分子才能在外侧连成骨架、把碱基留在内侧。考试常给一张没有标注的核苷酸结构图，让你标出三个组成部分的名称，所以要把它当“图”记，而不只是当“三个词”背。图上分不清谁是谁时记住：中间那个、两边都有邻居的，就是糖。

There are only four bases in DNA: adenine (A), thymine (T), cytosine (C) and guanine (G). Now the key idea. The phosphate group is identical in every nucleotide, and the deoxyribose sugar is identical in every nucleotide. The only thing that differs from one nucleotide to the next is which of the four bases is attached. So there are only four kinds of nucleotide in the entire molecule, and a strand of DNA is a message written in a four-letter alphabet.

DNA 中只有四种碱基：腺嘌呤 (adenine, A)、胸腺嘧啶 (thymine, T)、胞嘧啶 (cytosine, C)、鸟嘌呤 (guanine, G)。关键来了：每个核苷酸里的磷酸基团都一模一样，脱氧核糖也一模一样，唯一不同的就是接上去的是四种碱基中的哪一种。所以整条 DNA 里其实只有四种核苷酸，一条 DNA 链就是用四个字母写成的信息。

Nucleotides join up in a line: the sugar of one nucleotide bonds to the phosphate group of the next, over and over, for millions of units. That produces a continuous chain of alternating sugar–phosphate–sugar–phosphate running down the outside of the molecule, and it is called the sugar-phosphate backbone. Two things to lock in about it. First, these joins are strong covalent bonds — remember that, because tomorrow the contrast between these strong bonds and some much weaker ones becomes the whole point. Second, the bases are not part of the backbone; they hang off the side of it, pointing inwards. If you picture a single strand as one half of a ladder that has been sawn down the middle, the backbone is the upright side rail and the bases are half-rungs sticking out sideways, each one waiting for a partner.

核苷酸是这样连成链的：前一个核苷酸的糖与后一个核苷酸的磷酸基团相连，如此重复几百万次。于是在分子外侧形成一条“糖—磷酸—糖—磷酸”交替的连续长链，叫做磷酸—脱氧核糖骨架 (sugar-phosphate backbone)。有两点必须记牢。第一，骨架上的这些连接是强共价键 (covalent bond)——一定记住，明天讲到它与另一种弱得多的键的对比时，这就是全部重点。第二，碱基不属于骨架，它们挂在骨架侧面、朝内伸出。你可以把一条单链想成一把梯子从中间锯开后的一半：骨架是竖着的边框，碱基是横着伸出来的“半截横档”，每一根都在等一个搭档。

Base pairing and the double helix7118

A real DNA molecule is two strands, not one. They lie alongside each other with their backbones on the outside and their bases facing each other in the middle. The bases are what hold the two strands together, and they do it by a fixed rule called complementary base pairing: adenine always pairs with thymine (A–T), and cytosine always pairs with guanine (C–G). No other combination occurs. Stop on the consequence, because it is the most useful fact in the whole topic — if you know the base order of one strand, you can work out the other strand exactly. A strand reading A–T–G–C must be facing T–A–C–G. The two strands are not identical; they are complementary, each one a mirror-image record of the other. That is precisely what makes an exact copy possible.

真正的 DNA 分子是两条链，不是一条。两条链并排贴在一起，骨架在外侧，碱基朝内彼此相对。把两条链连在一起的正是碱基，而且遵循一条固定的原则——碱基互补配对原则 (complementary base pairing)：腺嘌呤总是与胸腺嘧啶配对（A—T），胞嘧啶总是与鸟嘌呤配对（C—G），绝无其他组合。这里要停一下，因为这是整个课题里最有用的一条：只要知道一条链的碱基顺序，就能准确推出另一条链。一条链是 A—T—G—C，对面必然是 T—A—C—G。注意，两条链不是“相同”，而是“互补 (complementary)”，彼此是对方的镜像记录——这正是精确复制得以可能的原因。

The pairs are joined by hydrogen bonds, and NESA-style questions want the numbers: two hydrogen bonds between A and T, three hydrogen bonds between C and G. A single hydrogen bond is weak — far weaker than the covalent bonds running along each backbone. But a DNA molecule contains millions of base pairs, and millions of weak bonds added together hold the two strands together firmly. This combination is the cleverest thing about the structure, and it is worth writing out as a full sentence in your notes: the two strands are held together by many weak hydrogen bonds that can be broken one at a time, while each strand itself is held together by strong covalent bonds that stay intact. That is exactly why DNA can be unzipped down the middle without either strand falling to pieces — and why the zipper analogy works so well.

碱基对之间靠氢键 (hydrogen bond) 相连，而 NESA 风格的题目要的就是数字：A 与 T 之间是 2 个氢键，C 与 G 之间是 3 个氢键。单个氢键很弱，比每条骨架上的共价键弱得多；但一个 DNA 分子有几百万个碱基对，几百万个弱键加在一起，就把两条链牢牢地拉住了。这套“强弱搭配”是这个结构里最巧妙的地方，值得在笔记上写成一整句：两条链之间靠大量弱氢键相连，这些氢键可以一个一个地被打断；而每条链自身靠强共价键连接，这些键始终不会断。这正是 DNA 能从中间像拉链一样被拉开、而两条链本身不会散架的原因——这也是“拉链”比喻好用的地方。

Finally, the whole thing is twisted. The two strands wind around each other into a spiral shape called a double helix, the model published by Watson and Crick in 1953, built on X-ray diffraction images produced by Rosalind Franklin and Maurice Wilkins. The two strands also run in opposite directions to each other; the word for that is antiparallel, and at Year 11 level you only need to state it, not explain it. Because the pairing rule is fixed, the helix stays the same width all the way along, like a spiral staircase whose steps are all the same length. The twisted-ladder analogy is a good one to hold in your head — the two sugar-phosphate backbones are the side rails and each base pair is a rung. Just be clear about where it breaks down: a real ladder rung is one solid piece of wood, whereas each rung of DNA is two half-rungs held together by nothing stronger than two or three hydrogen bonds (two for A–T, three for C–G), which is the entire reason the molecule can open.

最后，整个分子是拧着的。两条链互相缠绕成螺旋形，叫双螺旋 (double helix)。这就是沃森 (Watson) 和克里克 (Crick) 在 1953 年发表的模型，其依据是 Rosalind Franklin 与 Maurice Wilkins 拍摄的 X 射线衍射图。两条链的走向彼此相反，这个术语叫反向平行 (antiparallel)——11 年级只要求你说出这个词，不要求解释原因。由于配对规则是固定的，螺旋从头到尾粗细一致，就像每级台阶长度都相同的旋转楼梯。“拧过的梯子”这个比喻可以一直用：两条磷酸—脱氧核糖骨架是梯子的两根边框，每一个碱基对是一根横档。但要清楚它在哪里失效：真梯子的横档是一整根实木，而 DNA 的每根“横档”其实是两根半截横档拼起来的，中间只靠两三个氢键连着（A—T 两个，C—G 三个）——正因为如此，这个分子才打得开。

From base pair to genome: getting the sizes in order9108

This is where students lose marks that are genuinely easy to keep, because they use DNA, gene and chromosome as if the words were interchangeable. They are not interchangeable, and they are not three different substances either — they are the same material described at different levels of size, and you must be able to rank them and say how each relates to the next. Try the cookbook picture first: the individual letters are the bases, one recipe is a gene, one whole volume is a chromosome, and the complete set of volumes on the shelf is the genome. That picture is excellent for the ranking. Do not push it further than that, though — a cookbook has only one recipe per page and is read straight through, while a cell reads only the few genes it needs at any moment and ignores the rest.

这一段是“本来能拿、却常常丢掉”的分数所在，因为很多同学把 DNA、基因 (gene)、染色体 (chromosome) 当成可以随便替换的词。它们既不能互换，也不是三种不同的物质——而是同一种物质在不同尺度上的名称。你必须能把它们从小到大排序，并说清每一层和上一层的关系。先用“菜谱书”这个画面：一个个字母是碱基，一道菜谱是一个基因，一整本书是一条染色体，书架上整套书就是基因组 (genome)。这个画面用来排序非常好用，但别推得太远——菜谱书是一页一道、从头读到尾，而细胞在任何时刻只读它需要的那几个基因，其余的都不理会。

Now the real terms, smallest first. A base pair is one rung of the ladder: two bases joined by hydrogen bonds. A gene is a length of DNA — typically hundreds or thousands of base pairs long — that carries the instructions for making one protein. A DNA molecule is one enormously long double helix carrying many genes along its length.

现在用真正的术语，从小到大。碱基对 (base pair) 就是梯子的一根横档：两个碱基靠氢键相连。基因 (gene) 是一段 DNA——通常几百到几千个碱基对长——携带制造一种蛋白质的指令（本阶段按“一个基因对应一种蛋白质”处理）。DNA 分子是一条极长的双螺旋，上面沿途排着许多基因。

Most of the time that molecule is not the neat X-shaped chromosome you see in textbooks. Wrapped around packaging proteins called histones, it exists as chromatin: long, thin, tangled threads spread throughout the nucleus. Only when a cell prepares to divide does the chromatin coil and fold up tightly into a condensed chromosome, thick and short enough to be visible under a light microscope. The genome is the complete set of DNA in a cell or organism. Each level sits inside the next: base pairs make up a gene, genes sit along a DNA molecule, the DNA molecule is packaged as a chromosome, chromosomes sit inside the nucleus, and the nucleus sits inside the cell.

多数时候，这条分子并不是课本上那种整齐的 X 形染色体。它缠绕在包装蛋白（组蛋白 histone）上，以染色质 (chromatin) 的形式存在：细长、缠绕的丝状物，散布在整个细胞核中——这才是 DNA 在细胞一生中大部分时间的样子。只有当细胞准备分裂时，染色质才高度螺旋化、缩短变粗，形成染色体 (chromosome)，短而粗，在光学显微镜下才看得见。基因组 (genome) 是一个细胞或一个生物体内全部 DNA 的总和。每一层都包在更大的一层里：碱基对组成基因，基因排在 DNA 分子上，DNA 分子被包装成染色体，染色体在细胞核里，细胞核在细胞里。

Some numbers will anchor the scale for you. A human somatic (body) cell contains 46 chromosomes, arranged as 23 pairs, which is what we mean when we call the cell diploid. Note the word body cell. Egg and sperm cells are not diploid — they are haploid, carrying just 23 chromosomes, one from each pair. So 'every human cell has 46 chromosomes' is wrong; every human body (somatic) cell does. How haploid cells are made is a later topic. One complete set — the human genome — is about three billion base pairs long and carries roughly 20,000 protein-coding genes; a body cell holds two such sets, which is the two metres of DNA mentioned earlier, folded into a nucleus only a few micrometres across. So if an exam asks where a gene is found, the full-mark answer walks down the levels: a gene is a section of a DNA molecule, that DNA molecule is packaged with proteins as a chromosome, the chromosomes are inside the nucleus, and the nucleus is inside the cell. Practise saying that chain out loud until it is automatic.

用几个数字来固定尺度感。人的一个体细胞 (somatic cell) 含 46 条染色体，排成 23 对，这就是我们说这个细胞是二倍体 (diploid) 的意思。注意“体细胞”三个字。卵细胞和精子不是二倍体，而是单倍体 (haploid)，只有 23 条染色体，每一对里各取一条。所以“人的每个细胞都有 46 条染色体”是错的——是每个体细胞才有 46 条。单倍体细胞怎么来的，是后面的课题。完整的一套——人类基因组——约有 30 亿个碱基对，携带大约 2 万个编码蛋白质的基因；一个体细胞里有两套，加起来就是前面说的约两米长的 DNA，全部折叠进只有几微米宽的细胞核。所以当考题问“基因在哪里”，满分答案是顺着层级往下说：基因是 DNA 分子上的一段，这条 DNA 分子与蛋白质一起包装成染色体，染色体在细胞核内，细胞核在细胞内。把这一串层级关系大声念到脱口而出为止。

How to answer your worksheet boxes811

Your class worksheet has a box labelled DNA, with the prompt telling you it is made of NUCLEOTIDES. A box like that is asking for a compact definition, not an essay, and the marks sit in specific words. Write something close to this: DNA (deoxyribonucleic acid) is the molecule that stores the genetic instructions for making proteins. It is a polymer made of repeating units called nucleotides. Each nucleotide is made of a phosphate group, a deoxyribose sugar and one of four nitrogenous bases (A, T, C or G). The nucleotides join into two strands with a sugar-phosphate backbone on the outside, and the two strands are twisted into a double helix. That is four sentences and it covers function, monomer name, the three components, the four bases and the overall shape — every idea a marker could be looking for.

你课上学案里有一个标着 DNA 的方框，提示写着它由“NUCLEOTIDES（核苷酸）”构成。这种框要的是紧凑的定义，不是作文，而分数就落在几个具体的词上。写成接近这样：DNA（脱氧核糖核酸）是储存制造蛋白质的遗传指令的分子。它是由重复单位——核苷酸 (nucleotide)——构成的多聚体。每个核苷酸由一个磷酸基团、一个脱氧核糖和四种含氮碱基之一（A、T、C 或 G）组成。核苷酸连成两条链，外侧是磷酸—脱氧核糖骨架，两条链拧成双螺旋。四句话，涵盖了功能、单体名称、三个组成部分、四种碱基和整体形状——阅卷老师想找的点全在里面。

The second box is labelled BASE PAIRING, with the prompt that the strands are held together by base pairing. Careful here, because the prompt itself is where most students stop, and stopping there loses the mark. Write: The two strands are held together by complementary base pairing. Adenine always pairs with thymine (A–T) and cytosine always pairs with guanine (C–G). Each pair is joined by weak hydrogen bonds — two between A and T, three between C and G. There are millions of these bonds, so together they hold the helix firmly, but because each one is weak they can be broken so the strands can separate. Notice what that adds to the prompt: the name of the rule, the two specific pairs, the bond type, the numbers, and the weak-but-many point. Held together by base pairing on its own is just repeating the question back.

第二个框标着 BASE PAIRING（碱基配对），提示说两条链靠碱基配对连在一起。这里要当心：大多数同学就停在这句提示上，而停在这里就拿不到分。要写成：两条链靠碱基互补配对原则 (complementary base pairing) 连在一起。腺嘌呤总是与胸腺嘧啶配对（A—T），胞嘧啶总是与鸟嘌呤配对（C—G）。每一对由弱氢键 (hydrogen bond) 相连——A 与 T 之间 2 个，C 与 G 之间 3 个。这样的氢键有几百万个，合起来把双螺旋牢牢拉住；但因为每个都很弱，可以被打断，两条链就能分开。注意这比提示多了什么：原则的名称、两组具体配对、键的种类、数目，以及“弱但多”这一点。只写 held together by base pairing，等于把题目原样抄回去。

Two habits will lock this in tonight. First, cover both model answers, write them from memory, then compare word by word and circle anything you left out — missing words are almost always the technical ones (nucleotide, complementary, hydrogen, backbone). Second, drill the pairing rule until it is reflex: write out a random strand such as T–A–C–G–G–A–T–C and produce its complementary partner, then check every letter. If you can produce both worksheet answers from memory and complete a complementary strand without pausing, you have Day 1. One last warning about wording: never say the strands are held together by the backbone. The backbone runs along each strand separately and holds that strand together; it is base pairing that holds the two strands to each other.

今晚用两个习惯把它们固定下来。第一，把上面两段范文遮住，凭记忆默写，然后逐字对照，把漏掉的圈出来——漏掉的几乎总是那些术语（核苷酸、互补、氢键、骨架）。第二，把配对规则练成条件反射：随手写一条链，比如 T—A—C—G—G—A—T—C，写出它的互补链，再逐个字母检查。如果两个学案框都能默写出来、互补链也能不假思索地写完，Day 1 就过关了。最后提醒一个措辞陷阱：绝不能说“两条链靠骨架连在一起”。骨架是沿着每一条链各自延伸、把那一条链连成整体的；把两条链彼此拉在一起的是碱基配对。