Part 2

Human chromosomes If you were to spread out all of your chromosomes from one nucleus in your body, they would look something like this. Humans have 23 pairs of chromosomes, 46 in total. One chromosome in each of the pairs comes from each of your biological parents. What are chromosomes? Chromosomes contain DNA, and they keep all the genetic information safe. Different organisms have different numbers of chromosomes. We have 46 chromosomes, guinea pigs have 64, and chimps have 48—the same number as potatoes! Chromosome pair 23 consists of the sex chromosomes. There are two types, called X and Y. These are special because they help to determine whether you are male or female. Usually, girls have two X chromosomes and boys have one X and one Y. CRAZY CHROMOSOMES The number of chromosomes in living things varies hugely and can be surprising. An elephant has 56 chromosomes, but a chicken has 78! Jumper ants These poisonous Australian ants only have one pair of chromosomes. Common garden ants have 15 pairs. Adder's tongue fern Perhaps surprisingly, plants often have lots of chromosomes. The adder's tongue fern has 1,262 chromosomes—that's the most found so far in any living thing! Atlas blue butterfly This butterfly from North Africa has 452 chromosomes. That's the largest-known number of chromosomes in an animal.

人类染色体 如果你把身体里一个细胞核中的所有染色体铺开，它们看起来大概就是这样。 人类一共有23对染色体，总共46条。 每对染色体中各有一条来自你的生父和生母。 染色体是什么？ 染色体包含DNA，能保护所有遗传信息的安全。 不同生物的染色体数量各不相同。 我们有46条染色体，豚鼠有64条，黑猩猩有48条——和土豆的染色体数量一样！ 第23对染色体是性染色体。 一共有两种，分别叫作X和Y。 它们很特殊，因为能决定你的性别是男还是女。 通常女孩有两条X染色体，男孩有一条X和一条Y。 疯狂的染色体 生物的染色体数量差异极大，结果往往出人意料。 大象有56条染色体，鸡却有78条！ 跳蚁 这种有毒的澳大利亚蚂蚁只有一对染色体。 普通花园蚂蚁有15对。 瓶尔小草 说来也许意外，植物通常拥有很多染色体。 瓶尔小草有1262条染色体——这是目前所有生物中发现的最多数量！ 阿特拉斯蓝蝶 这种产自北非的蝴蝶有452条染色体。 这是已知动物中染色体数量最多的。

Transcription When a protein needs to be made, the gene that makes it must be activated. This process is called transcription. DNA can't leave the nucleus of the cell, so instead, a temporary copy of the DNA code is transcribed into a molecule of messenger RNA (mRNA), which can leave the nucleus. Genes make proteins Each gene usually contains the instructions for making one protein. Proteins come in different shapes and sizes and do all the work in your cells. Some of them, such as insulin, are important hormones that travel around your body in the blood. Using genes Your DNA code is like an instruction manual made up of lots of different sentences, called genes. When you need to use a gene, its code is read. Each gene is marked with a start and a stop code, like a capital letter and a period in a sentence. 1 Genes The DNA code is made up of many genes. Genes are only read once they get a "go" signal from the cell. 2 Gene unwinds Once your gene gets the "go" signal, its DNA unwinds. This separates the base pairs so that a copy can be made. 3 mRNA is made A single-stranded copy of the gene sequence is made using RNA bases. These make an mRNA molecule that can be destroyed once the required protein has been made. 4 DNA winds back up After the mRNA copy of the code has been made, the DNA is wound back up again to keep the information safe. Any damage could result in a mutation. 5 Leaving the nucleus Now we have mRNA, but we still need to read it. For this to happen, the mRNA leaves the nucleus through a pore (hole).

转录 当需要合成一种蛋白质时，编码它的基因就会被激活。 这个过程叫作转录。 DNA无法离开细胞核，因此DNA编码会被临时复制，转录成可以离开细胞核的信使RNA（mRNA）分子。 基因制造蛋白质 每个基因通常都包含合成一种蛋白质的指令。 蛋白质有着不同的形状和大小，承担着细胞内所有的工作。 部分蛋白质，比如胰岛素，是重要的激素，会随着血液流经全身。 利用基因 你的DNA编码就像一本由许多不同“句子”组成的说明书，这些“句子”就是基因。 当你需要用到某个基因时，就会读取它的编码。 每个基因都用起始码和终止码标记，就像句子的大写首字母和句号。 1 基因 DNA编码由许多基因组成。 只有收到细胞发出的“开始”信号，基因才会被读取。 2 基因解旋 基因收到“开始”信号后，它的DNA就会解开螺旋。 这会让碱基对分开，以便进行复制。 3 合成mRNA 会利用RNA碱基合成出基因序列的单链拷贝。 这些组成了mRNA分子，在合成出需要的蛋白质后，mRNA就会被分解。 4 DNA重新螺旋 mRNA拷贝合成完成后，DNA会重新缠绕成螺旋，保护遗传信息的安全。 任何损伤都可能引发突变。 5 离开细胞核 现在我们已经得到了mRNA，但还需要读取它的编码。 为了完成这一步，mRNA会通过核孔（小孔）离开细胞核。

6 Codons Each group of three letters makes up a single word in the gene, called a codon. 7 Amino acids and tRNA Each codon codes for an amino acid and each amino acid is attached to a tRNA molecule. These pair with the codons on the mRNA. Reading the code The ribosome is incredibly fast at translating the mRNA sequence into protein. It can join up amino acids at a rate of 200 per minute. The biggest protein in the human body is called titin, and it takes almost three hours to make. Cracking the code Can you read the instructions written in the genetic code? No? Well, you'll need the help of a type of RNA called transfer RNA (tRNA) and a ribosome. They form a team that cracks the code and follows its instructions to make proteins. Ribosome The ribosome is a hard-working molecular machine. It matches the codons in the mRNA to tRNA, which brings in the amino acids needed to make proteins. This process is called translation. 8 Making proteins Amino acids make long chains, like beads on a string. The chain folds itself up into a 3-D protein and gets to work in the cell. Nearly all living things on Earth use the same genetic code to make proteins from the same animo acids. Twenty different amino acids are coded for by DNA. You can make 11 of these in your body. You must get the other nine from your food.

6 密码子 每三个碱基字母组成基因里的一个“单词”，叫做密码子。 7 氨基酸与转运RNA 每个密码子对应一种氨基酸，每种氨基酸都连接着一个转运RNA（tRNA）分子。 它们会和mRNA上的密码子配对。 读取编码 核糖体能极快地把mRNA序列翻译为蛋白质。 它连接氨基酸的速度可达每分钟200个。 人体内最大的蛋白质叫做肌联蛋白，合成它几乎需要三个小时。 破解遗传密码 你能读懂遗传密码里的指令吗？ 读不懂？ 那你需要转运RNA（tRNA）和核糖体来帮忙。 它们组成团队破解密码，按照指令合成蛋白质。 核糖体 核糖体是一台勤劳的分子机器。 它把mRNA上的密码子和转运RNA匹配，转运RNA就会带来合成蛋白质所需的氨基酸。 这个过程叫做翻译。 8 合成蛋白质 氨基酸连接成长链，就像串在绳子上的珠子。 长链会折叠成三维结构的蛋白质，然后在细胞里开始工作。 地球上几乎所有生物都用同一套遗传密码，从相同的氨基酸合成蛋白质。 DNA一共为20种不同的氨基酸编码。 其中11种你可以在体内自己合成。 另外9种你必须从食物中获取。

Copying your DNA A complete copy of your DNA is inside nearly every one of the 40 trillion cells in your body. Each time your body makes a new cell, you need to replicate, or make a copy of, all your DNA so that the new cell knows what to do. A human skin cell divides in two to form two daughter cells. Grown in a lab, these cells can help scientists learn about wound healing. DNA replication When DNA copies itself, it first unwinds into two separate strands. Then, a special protein called a DNA polymerase matches new DNA bases with the sequence on the two unwound halves of the helix. This completes the two new DNA strands. Each day, you make 330 billion feet (100 billion meters) of DNA! Why do we need new cells? Whenever you grow or repair any damage, such as a cut, you need new cells. New cells are made by cell division. Before a cell can divide, it first needs to copy all of its DNA. The "parent" cell then splits in two, making two new "daughter" cells, each with a complete set of DNA. STAGE 1 Copying chromosomes Each chromosome replicates itself to form two strands of DNA, which are stuck to each other in the middle. The membrane of the nucleus then breaks down. STAGE 2 Lining up The replicated chromosomes attach to threadlike fibers that help them line up in the center of the cell. STAGE 3 Pulled apart Pulling from the fibers separates the two halves of the replicated chromosomes. The two copies move into different halves of the cell as the cell begins to divide. STAGE 4 Daughter cells The parent cell will now become two daughter cells, each with a complete copy of DNA in a newly formed nucleus.

复制你的DNA 你身体内近40万亿个细胞中，几乎每个细胞都携带一整套完整的DNA拷贝。 每当身体生成一个新细胞，都需要复制整套DNA，这样新细胞才能知道该完成什么工作。 人类的一个皮肤细胞会分裂成两个，形成两个子细胞。 这类细胞在实验室培养后，能帮助科学家研究伤口愈合的原理。 DNA复制 DNA自我复制时，首先会解旋分开成两条独立单链。 接着，一种叫做DNA聚合酶的特殊蛋白质，会将新的DNA碱基和螺旋解开的两条单链上的序列一一配对。 这样就形成了两条全新的DNA链。 你每天合成的DNA总长度，足足有1000亿米！ 我们为什么需要新细胞呢？ 不管是身体成长，还是修复伤口（比如破皮划伤），都需要新细胞。 新细胞通过细胞分裂产生。 细胞开始分裂前，首先要复制好全部的DNA。 之后“母细胞”一分为二，生成两个新的“子细胞”，每个子细胞都带有一整套完整的DNA。 第一阶段 复制染色体 每条染色体都会自我复制，形成两条DNA链，中间连接在一起。 之后细胞核的核膜会解体消失。 第二阶段 排列 复制好的染色体会附着在丝状纤维上，这些纤维帮助它们排列在细胞中央。 第三阶段 分离 纤维产生拉力，把复制好的染色体的两半分开。 细胞开始分裂时，两份DNA分别移动到细胞的两个半区。 第四阶段 形成子细胞 母细胞最终会变成两个子细胞，每个子细胞新形成的细胞核里，都有一套完整的DNA拷贝。

Do all your cells have the same DNA? Scientists used to think that cells became specialized for a particular role in the body by losing part of their DNA. We now know that different cell types contain the same DNA, so why do they look so different? Heart muscle cells The muscles in your arms and legs need to rest after use. However, heart muscle is different. It never rests and keeps working every moment until you die. FACT FILE » Heart cells beat by themselves, even when they are grown in a Petri dish! » Heart cells are full of mitochondria to give them lots of energy. » Your heart beats 100,000 times a day. Neurons Neurons, or nerve cells, are very long cells that transmit signals in and out of your brain and spine. These signals travel as fast as 250 mph (400 kph)! FACT FILE » Our brains contain about 86 billion neurons. » Your longest neuron is 3 ft 4 in (1 m) long, from your spine to your toes. » Giraffes have neurons that are more than 16 ft 6 in (5 m) long! DIFFERENT CELLS, DIFFERENT GENES Your whole genome is not active in every cell. Instead, different cells just use the genes they need. In a heart muscle cell, only the genes that are needed to make and run heart muscle are active. Other specialized genes that make different cells, such as neurons, are tightly locked away and can't be read. Gene used only in heart muscle cells Locked-away gene not used by this cell type Gene used by many different cell types Bone cells The outside of your bones is made up of a hard tissue filled with long-lasting, star-shaped cells that help take care of the bones. FACT FILE » Bones also store minerals inside your body. » The smallest bone in your body is in your ear and helps you hear. » Birds have lightweight, hollow bones to help them fly. Lung cells The lining of your lungs is made up of cells that allow oxygen from the air into your blood—and carbon dioxide out— when you breathe. FACT FILE » Lung cells are covered with little brushlike structures to sweep out gunk. » The total surface area inside both of your lungs is about the size of a tennis court! » Lungs have 1,500 miles (2,400 km) of airways.

所有细胞的DNA都一样吗？ 过去科学家认为，细胞分化成身体里不同功能的特化细胞时，会丢失一部分DNA。 现在我们知道，不同类型的细胞其实带有相同的DNA，那为什么它们长得这么不一样呢？ 心肌细胞 你胳膊和腿上的肌肉，用过后需要休息。 不过心肌不一样哦。 它永远不会休息，从你出生到离世，每一刻都在不停工作。 知识库 » 心肌细胞就算放在培养皿里培养，也会自主跳动哦！ » 心肌细胞里充满了线粒体，能提供大量能量。 » 你的心脏一天会跳动十万次。 神经元 神经元也叫神经细胞，是一类很长的细胞，负责在大脑和脊髓之间传递进出信号。 这些信号的传递速度最快能达到每小时400公里！ 知识库 » 我们的大脑大约有860亿个神经元。 » 你身上最长的神经元足足有1米长，从脊髓一直延伸到脚趾头。 » 长颈鹿的神经元长度超过5米呢！ 不同细胞，开启不同基因 并不是每个细胞都激活了整套基因组。 实际上，不同细胞只会用到自身需要的基因。 在心肌细胞里，只有生成和运转心肌需要的基因才处于激活状态。 其他用来生成不同细胞（比如神经元）的特异性基因，都会被牢牢锁起来，无法被读取使用。 仅心肌细胞使用的基因 该细胞不用的封存基因 多种细胞通用的基因 骨细胞 骨头的外侧是坚硬组织，里面布满了长寿的星形细胞，负责养护骨骼。 知识库 » 骨头还会在你身体里储存矿物质。 » 你身体里最小的骨头在耳朵里，能帮助我们听见声音。 » 鸟类的骨头轻盈中空，能帮助它们飞行。 肺细胞 你肺脏的内层细胞，会在你呼吸时，让空气中的氧气进入血液，同时把二氧化碳排出去。 知识库 » 肺细胞表面覆盖着细小的刷毛状结构，能把脏东西扫出去 » 你两个肺的内表面积加起来，差不多有一个网球场那么大！ » 肺里的气道总长度有2400公里呢。

Passing on your genes Living things pass on their genes to the next generation. This means that children often look like their parents, since they share half of their genes with their mother and half with their father. This is called genetic inheritance. Two versions of every gene The cockerpoo puppies here have two copies of each of their chromosomes—one from Mom and one from Dad. This means that they, just like humans and many other animals, have two versions of every gene. Different versions of the same gene are called alleles (a-leelz). DOMINANT ALLELES If you have two alleles of the same gene, such as the gene that controls hair-shedding, which one wins? Some alleles are dominant— their instructions override other alleles—and they always win. Dominant gene None of the puppies shed their hair, which means the dad's allele is dominant. Scientists can compare the DNA of different dog breeds to find out which genes code for which characteristics.

传递基因 生物会把自己的基因传给下一代。 这就是为什么孩子往往长得像父母，因为他们一半基因来自妈妈，一半来自爸爸。 这叫做基因遗传。 每个基因都有两个版本 图中的可卡颇幼犬，每条染色体都有两份——一份来自妈妈，一份来自爸爸。 也就是说，它们和人类以及许多其他动物一样，每个基因都带有两个版本。 同一个基因的不同版本叫做等位基因。 显性等位基因 如果你同一个基因有两个等位基因，比如控制掉毛的基因，哪个会起作用呢？ 有些等位基因是显性的——它们的指令会压倒其他等位基因，永远占上风。 显性基因 这些幼犬都不掉毛，说明狗爸爸的等位基因是显性的。 科学家可以对比不同品种狗的DNA，找出哪些基因决定哪些性状。