Part 1

The DNA Book

《DNA之书》

Introduction Life is the greatest show on Earth, and we humans have the extraordinary privilege of having brains large enough to begin to understand it. Some life- forms are microscopic, while others are so big they can be seen from space. There are plants that eat animals, and bacteria that live in scalding-hot water. Some animals spend their lives entirely in the dark, while many others live underwater. The secret of all this amazing diversity is DNA. DNA is the molecule that holds the code for life, from bats to beetles, and from mushrooms to mammoths. We all rely on it to grow, survive, and reproduce. Read on to find out what DNA is and what it can do—and what happens when it gets mutated! Professor Alison Woollard Dr. Sophie Gilbert

介绍 生命是地球上最奇妙的奇迹，我们人类拥有格外难得的优势：我们的大脑足够发达，已经可以开始读懂生命的奥秘了。 有些生物微小到肉眼看不见，有些生物却大到从太空都能观测到。 有吃动物的植物，也有能在滚烫热水里生存的细菌。 有些动物一辈子都生活在黑暗里，还有很多动物住在水底。 这些神奇多样的生命，秘密都藏在DNA里。 DNA是承载生命密码的分子，从蝙蝠到甲虫，从蘑菇到猛犸象，全都有它的参与。 所有生命都要靠它才能生长、存活和繁衍。 继续读下去，你就能知道DNA是什么、有什么本领，还有它变异之后会发生什么！ 艾莉森·伍拉德教授 索菲·吉尔伯特博士

What is DNA? DNA stands for deoxyribonucleic acid (dee-ox-ee-ry-boe-new-klay-ik a-sid). It's the chemical that makes up your genes, which contain the instructions that make you who you are, from the color of your eyes to the size of your feet. DNA DNA is made up of three different things: sugar, phosphate, and bases. These are joined together to make one really long chain that forms a shape called a double helix. DNA has a strong structure and can store information for a long time. Double helix The twisted shape of DNA is called a double helix. Bases Pairs of bases run along the inside of DNA. Genome A complete set of your DNA is called your genome. The genomes of different organisms, or living things, can be very different sizes. Genes Certain sections of DNA make up your genes, which are responsible for all your characteristics. We have about 20,000 genes—and so do worms! FRIEDRICH MIESCHER DNA was discovered by a Swiss scientist named Friedrich Miescher. He was investigating white blood cells, which he collected from pus-covered bandages at a nearby hospital. From these he extracted a substance he called "nuclein." Nuclein later became nucleic acid, and today it is known as deoxyribonucleic acid (DNA). Miescher discovered DNA in the late 1860s, but he didn't know what it did in the body. DNA code lives forever! Even when you're gone, your DNA code will live forever in your family. Backbone Sugar and phosphate units make up the outer backbone of DNA. Bases are the code of life The order, or sequence, of the bases makes up the information in the DNA. There are four different bases in DNA, called A, T, C, and G. They make up the code for all life on Earth. When the two halves of the DNA helix zip together, A always pairs with T, and C with G.

什么是DNA？ DNA的全称是脱氧核糖核酸。 它是构成基因的化学物质，基因里藏着塑造你的指令，从你眼睛的颜色到脚的大小，都由它决定。 DNA由三种不同的物质组成：脱氧核糖、磷酸和碱基。 它们连接在一起形成一条很长的链，最终盘成叫做双螺旋的形状。 DNA的结构十分稳固，可以长时间储存信息。 双螺旋 DNA扭转而成的形状叫做双螺旋。 碱基 一对一对的碱基排列在DNA的内侧。 基因组 你体内全套的DNA就叫做基因组。 不同生物的基因组，大小可以相差非常大。 基因 DNA上特定的片段就是你的基因，它决定了你所有的特征。 我们人类大约有两万个基因，小小的蛔虫也一样多！ 弗里德里希·米歇尔 DNA是由瑞士科学家弗里德里希·米歇尔发现的。 他当时研究白细胞，这些白细胞是从附近医院沾了脓的绷带上收集来的。 他从里面提取出了一种物质，将它命名为“核素”。 核素后来改名为核酸，也就是我们今天说的脱氧核糖核酸（DNA）。 米歇尔在19世纪60年代末发现了DNA，但他当时不知道DNA在人体内有什么作用。 DNA密码会永远留存！ 哪怕你离开人世，你的DNA密码也会在你的家人身上一直延续下去。 骨架 脱氧核糖和磷酸构成了DNA的外侧骨架。 碱基就是生命密码 碱基的排列顺序，承载了DNA里的全部信息。 DNA里一共有四种不同的碱基，分别叫做A、T、C、G。 它们共同构成了地球上所有生命的密码。 当DNA螺旋的两条链结合在一起时，A永远和T配对，C永远和G配对。

Where is DNA in my body? Each tiny cell in your body contains a complete copy of your genome. Amazingly, if you stretched out your genome it would be 6 ft 6in (2 m) long, so it has to be packed up very tightly to fit inside your cells. Tracking down your DNA We need to delve deep inside your cells to find the DNA. It is kept inside a special compartment of the cell called the nucleus. Here, your DNA is wound up very tightly to keep it safe. Cells We have about 40 trillion cells in our bodies—that's 40,000,000,000,000! You can fit 8,000 of these cells on the head of a pin. Nucleus The nucleus is the command center of the cell because it contains your genome. Chromosomes Your genome is shared out into separate packages, called chromosomes. Humans have 23 different chromosomes. There are two copies of each, so that's 46 in total. Gene A gene is a length of DNA that contains the code for a particular characteristic, such as hair color. Are there cells without DNA? Unusually, red blood cells that carry oxygen around your body don't have a DNA-containing nucleus. They live for only four months and have to be constantly replaced. DNA in food Most of our food comes from living things. When you eat a tomato, for example, you're also eating all of the DNA in its cells. Your body chops up this DNA during digestion. DNA Surprisingly, 98 percent of DNA isn't used for genes! Scientists haven't yet worked out exactly what it all does. IS ALL OUR DNA IN THE NUCLEUS? No! Some of our DNA lives outside the nucleus in mitochondria, which are tiny batteries that power cells. Plant cells also have DNA inside their chloroplasts, which they use to change sunlight into food. Mitochondria There are lots of mitochondria in animal cells, but only one nucleus. Chloroplasts Plant cells have chloroplasts, which contain a chemical that makes them green.

DNA在我身体的哪里？ 你身体里每一个小小的细胞，都携带了一整套你的基因组。 神奇的是，如果把你的基因组拉直，足足有2米长，所以它必须折叠得非常紧凑，才能挤进小小的细胞里。 寻找DNA 我们需要潜入细胞深处，才能找到DNA。 DNA被储存在细胞里一个叫做细胞核的特殊区域。 DNA在这里被紧紧地缠绕起来，得到很好的保护。 细胞 我们人体内大约有40万亿个细胞——也就是40000000000000个！ 针尖上就能放下八千个这样的细胞。 细胞核 细胞核是细胞的指挥中心，因为它储存了你的基因组。 染色体 你的基因组被分成了一个个独立的小包，这就是染色体。 人类一共有23种不同的染色体。 每种染色体都有两份，所以人体内总共有46条染色体。 基因 基因是一段DNA，它承载着某个特定特征的密码，比如头发颜色。 有没有不含DNA的细胞？ 很特别的是，负责给身体运送氧气的红细胞，没有携带DNA的细胞核。 红细胞只能存活四个月，需要不断地更新替换。 食物里的DNA 我们吃的大多数食物都来自生物。 比如你吃西红柿的时候，也吃下了它细胞里所有的DNA。 你的身体在消化过程中会把这些DNA分解掉。 DNA 神奇的是，我们体内有98%的DNA都不是基因！ 科学家至今还没完全搞清楚这些DNA到底有什么作用。 我们所有的DNA都在细胞核里吗？ 不是的！ 有一部分DNA在细胞核外，存在于线粒体里，线粒体就是给细胞供能的小电池。 植物细胞的叶绿体里也有DNA，叶绿体可以把阳光转化成养分。 线粒体 动物细胞里有很多个线粒体，但细胞核只有一个。 叶绿体 植物细胞有叶绿体，里面含有让植物变绿的化学物质。

What does DNA do? DNA is a code that makes all life on Earth—from humans to fish, and daffodils to dinosaurs. The main purpose of the DNA code is to make proteins. All living things use proteins to make and repair themselves. Different codes Living things on Earth vary a lot because of differences in their DNA code. Protists are single-celled organisms. They contain a nucleus, like the cells of fungi, plants, and animals. Mushrooms, molds, and yeasts are all fungi. Most fungi feed on the remains of dead plants and animals. WHERE DID DNA COME FROM? DNA probably developed from a molecule called RNA, which may have made up life billions of years ago. RNA looks a bit like DNA and can also do some of the things that proteins do. Some viruses still use RNA to encode their genes. There are about 400,000 different species of plant. Most of them are green. There are more than a million known species of animal, with many new ones discovered each year! IS DNA BORING? Even after the discovery of DNA, scientists thought it was boring for a long time. This was because DNA is made of only three things: sugar, phosphate, and bases. People thought that it was too simple to hold the code for life. I'm NOT boring! Inheritance Before the discovery of DNA, people were puzzled about how certain characteristics were passed on to the next generation. They set out to find a molecule that could be passed on, or inherited. People thought the molecule would be a protein— but they were wrong! In 1944, scientists discovered that if DNA from DNA can change characteristics smooth bacteria was mixed with rough bacteria, the rough bacteria became smooth. Only DNA could pass on this characteristic, not proteins. This was the proof that DNA was the important stuff!

DNA是做什么的？ DNA是一串密码，造就了地球上所有生命——从人类到鱼类，从水仙花到恐龙，无一例外。 DNA密码的主要作用是合成蛋白质。 所有生物都依靠蛋白质构建和修复自身。 不同的密码 地球上的生物千差万别，都是因为它们的DNA密码各不相同。 原生生物是单细胞生物。 它们和真菌、植物、动物的细胞一样，都含有细胞核。 蘑菇、霉菌和酵母菌都属于真菌。 大多数真菌以死亡动植物的残骸为食。 DNA从哪里来？ DNA很可能由名为RNA的分子演化而来，数十亿年前的生命可能就是由RNA构成的。 RNA长得有点像DNA，还能完成蛋白质承担的部分工作。 有些病毒至今仍用RNA来编码自己的基因。 地球上大约有40万种不同的植物。 它们大多是绿色的。 目前已知的动物物种超过一百万种，每年还会发现许多新物种！ DNA很无聊吗？ 即便DNA已经被发现，科学家也曾在很长一段时间里认为它很无趣。 这是因为DNA只由三种物质构成：糖类、磷酸盐和碱基。 人们曾觉得它结构太简单，不可能承载生命的密码。 我才不无聊！ 遗传 在DNA被发现之前，人们一直困惑于为什么生物的某些特征会传给下一代。 他们开始寻找这种可以传递，也就是遗传的分子。 人们原本以为这种分子是蛋白质——但他们错了！ 1944年，科学家发现：DNA能够改变生物特征，如果将光滑细菌的DNA与粗糙细菌混合，粗糙细菌就会变成光滑型。 只有DNA能传递这种特征，蛋白质做不到。 这就证明了DNA才是关键的遗传物质！

Let's experiment! Would you like to try and see DNA for yourself? Perform this fun experiment, which you can do at home. You'll be getting DNA out of strawberries. Can I see DNA? It's very hard to know what something is like unless you can see it. DNA is very tiny and hidden in our cells, but we have some tricks for getting it out and taking a look at it! WARNING! Rubbing alcohol contains a lot of alcohol. It is used as an antiseptic to kill germs, but it's also poisonous, so you must never drink it. It can also easily catch fire. Use it only with the help of an adult in an airy or well-ventilated area, and make sure you do not breathe in its fumes. YOU WILL NEED » 2 beakers or plastic cups » 2 tsp dishwashing liquid » 1 tsp salt » ½ cup water » 2 strawberries » 1 resealable plastic bag » Strainer or coffee filter » ½ cup cold rubbing alcohol » Tweezers » 1 scientist (that's you!) 1 Preparing the DNA-extraction mixture In a beaker, mix together 2 teaspoons of dishwashing liquid, 1 teaspoon of salt, and ½ cup of water. 2 Breaking open the cells Put the strawberries into the plastic bag, seal the bag, and crush the berries well with your fingers. Add 2 teaspoons of the DNA-extraction mixture, reseal the bag, and continue crushing for another minute. 3 Separating the DNA Strain the strawberry liquid into the other clean beaker. Next, gently add the rubbing alcohol, pouring it down the side of the beaker. Use an amount equal to the amount of strawberry liquid. DO NOT mix or stir. 4 Seeing the DNA Look for a white, cloudy layer at the top of the mixture. You can tilt the cup and pick up this layer using the tweezers. That's the DNA! Seeing chromosomes It's often hard to see DNA in cells because the chromosomes are all spread out. When cells get ready to divide, however, the chromosomes pack up and copy themselves (see Copying your DNA, page 24). UNDER THE MICROSCOPE We still haven't seen the DNA in much detail, have we? Very powerful microscopes can help us here… Viral DNA An electron microscope uses a beam of incredibly tiny particles to show microscopic objects, such as this DNA (blue) from a virus. Bacterial DNA This DNA (pink) from a bacterium has been made visible by an atomic force microscope, which builds up an image of the DNA by probing it with a minute instrument.

一起来做实验吧！ 你想不想亲手看一看DNA长什么样？ 这个有趣的实验你在家就能做。 我们要从草莓里提取出DNA。 我能看见DNA吗？ 没亲眼见过，就很难弄清一件东西到底是什么样的。 DNA非常微小，一直藏在细胞里，但我们有办法把它提取出来看一看！ 注意！ 外用酒精含有高浓度酒精。 它可以用作消毒剂杀灭细菌，但本身有毒，绝对不能饮用。 它还非常容易燃烧。 一定要在成年人的帮助下，在通风良好的地方使用，千万不要吸入它的蒸汽。 你需要准备：» 2个烧杯或塑料杯 » 2茶匙洗洁精 » 1茶匙盐 » 半杯水 » 2颗草莓 » 1个可密封塑料袋 » 滤网或咖啡滤纸 » 半杯冷藏外用酒精 » 镊子 » 1位小科学家（就是你啦！）1 配制DNA提取液：在烧杯中混合2茶匙洗洁精、1茶匙盐和半杯水。 2 破碎细胞：把草莓放进塑料袋，封好袋口，用手指把草莓充分捏碎。 加入2茶匙配好的DNA提取液，重新封好袋子，再继续捏碎一分钟。 3 分离DNA：把草莓汁液过滤到另一个干净的烧杯中。 接下来沿着烧杯内壁缓缓倒入外用酒精。 倒入的量和草莓汁液的量相等即可。 千万不要搅拌混合。 4 观察DNA：在混合液的上层寻找一团白色浑浊的物质。 你可以倾斜杯子，用镊子夹起这层物质。 这就是DNA呀！ 观察染色体 我们通常很难在细胞中看到DNA，因为染色体都分散开了。 不过当细胞准备分裂时，染色体会打包并且复制自己（参见《复制DNA》，第24页）。 显微镜下观察 我们到现在还没看清DNA的细节，对不对？ 高倍显微镜可以帮我们… 病毒DNA 电子显微镜利用一束极微小的粒子给微观物体成像，比如图中这种来自病毒的DNA（蓝色）。 细菌DNA 这张来自细菌的DNA（粉色），由原子力显微镜成像，这种显微镜用微型探针探测DNA，从而构建出DNA的图像。