📚 Introducing DNA: The Molecule of Life | DNA 入门：生命分子
Every living thing on Earth, from the tallest tree to the smallest bacterium, carries a set of instructions that controls how it grows, develops and functions. These instructions are written in a chemical code called DNA. In this KS3 guide, you will learn what DNA is, where it is found, how its structure allows it to store information, and why it matters for inheritance and variation.
地球上每一种生物，从最高的树木到最小的细菌，都携带一套控制其生长、发育和功能的指令。这些指令以一种叫做 DNA 的化学密码书写。在这份 KS3 指南中，你将学习 DNA 是什么、它在哪里、它的结构如何储存信息，以及它为何对遗传和变异如此重要。
1. What is DNA? | 什么是 DNA？
DNA stands for deoxyribonucleic acid. It is the molecule that carries the genetic instructions used by living organisms. DNA is often called the 'blueprint of life' because it contains the information needed to build and maintain an organism.
DNA 代表脱氧核糖核酸。它是携带生物体遗传指令的分子。DNA 常被称为 '生命的蓝图'，因为它包含构建和维持一个生物体所需的信息。
At KS3 level, you do not need to memorise every chemical detail. The key idea is that DNA is a very long molecule made of smaller repeating units, and the order of those units forms a code.
在 KS3 阶段，你不需要记住每一个化学细节。关键思想是 DNA 是一个由较小的重复单位组成的很长分子，这些单位的排列顺序构成了一种密码。
Nearly all cells in an organism contain the same DNA. This means DNA can be thought of as the instruction manual shared by almost every cell, even though different cells use different parts of the manual.
生物体中几乎所有细胞都含有相同的 DNA。这意味着 DNA 可以被看作是几乎所有细胞共享的说明书，尽管不同细胞使用说明书的不同部分。
2. Where is DNA found? | DNA 在哪里？
In plant and animal cells, DNA is found mainly inside the nucleus. The nucleus is a membrane-bound structure that acts as the control centre of the cell.
在植物和动物细胞中，DNA 主要存在于细胞核内。细胞核是一个有膜包围的结构，充当细胞的控制中心。
In bacterial cells, which do not have a nucleus, DNA is found free in the cytoplasm. Even without a true nucleus, bacteria still use DNA to store their genetic information.
在细菌细胞中，没有真正的细胞核，DNA 游离在细胞质中。即使没有真正的细胞核，细菌仍然使用 DNA 来储存遗传信息。
Some organelles, such as mitochondria and chloroplasts, also contain small amounts of DNA. At KS3 you mainly need to link DNA with the nucleus, but this extra DNA is useful evidence for endosymbiotic theory.
一些细胞器，如线粒体和叶绿体，也含有少量 DNA。在 KS3 阶段你主要需要将 DNA 与细胞核联系起来，但这些额外的 DNA 是内共生理论的有用证据。
3. DNA Structure: The Double Helix | DNA 的结构：双螺旋
DNA has a famous shape called a double helix. This shape looks like a twisted ladder or a spiral staircase.
DNA 有一个著名的形状，叫做双螺旋。这个形状看起来像一个扭曲的梯子或螺旋楼梯。
The double helix was first described by James Watson and Francis Crick in 1953, building on work by Rosalind Franklin and Maurice Wilkins. Their discovery revolutionised biology.
双螺旋结构于 1953 年由詹姆斯·沃森和弗朗西斯·克里克首次描述，他们建立在罗莎琳德·富兰克林和莫里斯·威尔金斯的工作基础上。这一发现彻底改变了生物学。
The two strands of DNA twist around each other. The sides of the ladder are made of sugar and phosphate groups, while the rungs are made of pairs of nitrogen-containing bases.
DNA 的两条链相互缠绕。梯子的两侧由糖和磷酸基团构成，而梯子的横档由含氮碱基对构成。
This structure is important because it allows DNA to be copied accurately and to store a huge amount of information in a small space.
这种结构很重要，因为它使 DNA 能被准确复制，并在很小的空间内储存大量信息。
4. Nucleotides: The Building Blocks | 核苷酸：基本组成单位
DNA is a polymer, which means it is made of many repeating units joined together. The repeating unit in DNA is called a nucleotide.
DNA 是一种聚合物，这意味着它由许多重复单元连接而成。DNA 中的重复单元叫做核苷酸。
Each nucleotide has three parts: a phosphate group, a sugar called deoxyribose, and one nitrogen-containing base. You can remember this as 'phosphate - sugar - base'.
每个核苷酸由三部分组成：一个磷酸基团、一种叫做脱氧核糖的糖和一个含氮碱基。你可以记为 '磷酸—糖—碱基'。
Many nucleotides join together to form a long chain. The phosphate of one nucleotide links to the sugar of the next, creating a strong sugar-phosphate backbone.
许多核苷酸连接在一起形成长链。一个核苷酸的磷酸与下一个核苷酸的糖相连，形成坚固的糖-磷酸骨架。
It is the sequence of bases along the chain that carries the genetic code. The sugar and phosphate parts are the same in every nucleotide, but the base can change.
携带遗传密码的是链上碱基的顺序。每个核苷酸中的糖和磷酸部分都相同，但碱基可以不同。
5. The Four Bases and Base Pairing | 四种碱基与碱基配对
There are four different bases in DNA: adenine (A), thymine (T), cytosine (C) and guanine (G). These bases are often represented by their single letters.
DNA 中有四种不同的碱基：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。这些碱基通常用单个字母表示。
The bases pair up in a specific way: A always pairs with T, and C always pairs with G. This is called complementary base pairing.
碱基以特定方式配对：A 总是与 T 配对，C 总是与 G 配对。这叫做互补碱基配对。
Complementary base pairing is the reason DNA can be copied accurately. If one strand has the sequence A-T-G-C, the other strand must have T-A-C-G.
互补碱基配对是 DNA 能被准确复制的原因。如果一条链的序列是 A-T-G-C，那么另一条链必定是 T-A-C-G。
You do not need to know the full chemical names of the bases, but you should be able to use the pairing rule to predict a matching strand.
你不需要知道碱基的完整化学名称，但你应该能够使用配对规则来预测互补链。
6. Genes: Short Sections of DNA | 基因：DNA 的短片段
A gene is a short section of DNA that contains the instructions for making a specific protein. Proteins do most of the work in cells, so genes control many characteristics.
基因是 DNA 的一小段，包含制造特定蛋白质的指令。蛋白质承担细胞内的大部分工作，因此基因控制许多特征。
For example, there are genes that affect eye colour, blood type, and the production of enzymes. Not all of your DNA is made of genes; some regions have other functions.
例如，有影响眼睛颜色、血型和酶生成的基因。并非所有的 DNA 都由基因组成；有些区域有其他功能。
Each gene has a particular order of bases. A change in this order, called a mutation, can alter the protein and may change a characteristic.
每个基因都有特定的碱基顺序。这种顺序的变化叫做突变，可能改变蛋白质，并可能改变某个特征。
Genes can be turned on or off in different cells, which is why a muscle cell and a nerve cell can contain the same DNA but look and behave very differently.
基因可以在不同细胞中开启或关闭，这就是为什么肌肉细胞和神经细胞虽然含有相同的 DNA，但外观和行为却截然不同。
7. Chromosomes: Packaged DNA | 染色体：包装好的 DNA
DNA molecules are extremely long, so cells need a way to organise them. Inside the nucleus, DNA is wrapped around proteins and coiled tightly to form structures called chromosomes.
DNA 分子非常长，因此细胞需要一种方式来组织它们。在细胞核内，DNA 缠绕在蛋白质上并紧密螺旋，形成叫做染色体的结构。
Human body cells normally contain 46 chromosomes, arranged in 23 pairs. One chromosome in each pair comes from the mother and the other from the father.
人类体细胞通常含有 46 条染色体，排列成 23 对。每对染色体中一条来自母亲，另一条来自父亲。
Different species have different numbers of chromosomes. For example, fruit flies have 8 chromosomes, while a garden pea has 14. Chromosome number is not related to how 'advanced' an organism is.
不同物种的染色体数目不同。例如，果蝇有 8 条染色体，而豌豆有 14 条。染色体数目与生物体的 '高级' 程度无关。
When a cell divides, chromosomes become shorter and thicker, which makes them visible under a light microscope. This is why chromosomes are often drawn as X-shaped structures.
当细胞分裂时，染色体会变短变粗，这使得它们在光学显微镜下可见。这就是为什么染色体常被画成 X 形结构。
8. How DNA Copies Itself | DNA 如何自我复制
Before a cell divides, its DNA must be copied so that each new cell receives a complete set of instructions. This process is called DNA replication.
在细胞分裂之前，其 DNA 必须被复制，以便每个新细胞获得一整套指令。这个过程叫做 DNA 复制。
Replication begins when the two strands of the double helix unzip. Each original strand then acts as a template for building a new complementary strand.
复制开始时，双螺旋的两条链解开。每条原始链随后充当构建新互补链的模板。
Because of the base-pairing rule, A attracts T and C attracts G. New nucleotides are added in the correct order, producing two identical DNA molecules.
由于碱基配对规则，A 吸引 T，C 吸引 G。新的核苷酸按正确顺序加入，产生两个相同的 DNA 分子。
Each new DNA molecule contains one original strand and one new strand. This is called semi-conservative replication, and it helps reduce copying errors.
每个新的 DNA 分子都包含一条原始链和一条新链。这叫做半保留复制，有助于减少复制错误。
9. Why DNA Matters: Inheritance and Variation | DNA 为何重要：遗传与变异
DNA is passed from parents to offspring during reproduction, which explains why children resemble their parents but are not identical to them.
DNA 在生殖过程中从父母传递给后代，这就解释了为什么孩子与父母相似但并不完全相同。
In asexual reproduction, offspring are genetically identical to the parent unless a mutation occurs. In sexual reproduction, offspring receive a mixture of DNA from two parents, creating variation.
在无性生殖中，除非发生突变，否则后代在遗传上与亲本相同。在有性生殖中，后代获得来自两个亲本的 DNA 混合，从而产生变异。
Variation within a species is important because it gives populations a better chance of surviving environmental changes. DNA is therefore central to evolution by natural selection.
物种内的变异很重要，因为它使种群更有机会在环境变化中生存。因此，DNA 是自然选择进化的核心。
By studying DNA, scientists can determine paternity, identify genetic disorders, trace ancestry and even help solve crimes using forensic evidence.
通过研究 DNA，科学家可以确定亲子关系、识别遗传疾病、追溯祖先，甚至利用法医证据帮助破案。
10. Extracting DNA in the Lab | 在实验室提取 DNA
You can extract DNA from fruit such as strawberries or bananas using simple equipment. This experiment helps you see that DNA is a real physical material, not just a diagram.
你可以使用简单的设备从草莓或香蕉等水果中提取 DNA。这个实验帮助你认识到 DNA 是一种真实的物理物质，而不仅仅是图表。
A common method is to mash the fruit, add a detergent to break open cell membranes, and then add cold ethanol. DNA is not soluble in cold ethanol, so it appears as a white stringy layer.
一种常见方法是捣碎水果，加入洗涤
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