📚 Genome and Gene – DNA Coding for Proteins | 基因组与基因 – DNA编码蛋白质
In IGCSE Biology, one of the most fundamental concepts is understanding how genetic information is stored and used in living organisms. The entire DNA of an organism is called its genome, and within that genome, specific sections known as genes carry the instructions to make proteins. Mastering this topic not only helps you succeed in your Edexcel exam but also gives you a deep insight into how life works at the molecular level.
在IGCSE生物学中，最基本的观念之一是理解遗传信息如何在生物体内储存并加以利用。一个生物体的全部DNA被称为基因组，而基因组中一些特定的区段——也就是基因——则携带了制造蛋白质的指令。掌握这个主题不仅有助于你在Edexcel考试中取得好成绩，也能让你从分子层面深入了解生命的运作方式。
1. Introduction to DNA | DNA简介
Deoxyribonucleic acid, or DNA, is the molecule that carries the genetic blueprint of almost all living organisms. DNA is a long polymer made up of repeating units called nucleotides. Each nucleotide consists of a phosphate group, a sugar called deoxyribose, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The double helix structure, discovered by Watson and Crick, allows DNA to store vast amounts of information safely and to be copied accurately before cell division.
脱氧核糖核酸（DNA）是携带几乎所有生物遗传蓝图的分子。DNA是一种长链聚合物，由称为核苷酸的重复单元组成。每个核苷酸包括一个磷酸基团、一个叫做脱氧核糖的糖以及四种含氮碱基之一：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。沃森和克里克发现的双螺旋结构使DNA能够安全地储存大量信息，并且在细胞分裂前被精确地复制。
2. What Is a Genome? | 什么是基因组？
The genome is defined as the entire DNA sequence of an organism. In humans, the genome contains around 3 billion base pairs organised into 23 pairs of chromosomes inside the nucleus, plus a small amount of DNA in the mitochondria. The genome includes not only the genes that code for proteins but also non-coding regions that regulate gene expression or have other functions. In Edexcel IGCSE, you need to understand that the genome is the complete set of genetic material, unique to each species (and individual).
基因组被定义为生物体的全部DNA序列。人类基因组包含约30亿个碱基对，它们分布在细胞核内的23对染色体上，此外线粒体中还有少量DNA。基因组不仅包含编码蛋白质的基因，还包括调控基因表达或具有其他功能的非编码区域。在Edexcel IGCSE中，你需要明白基因组是一整套遗传物质，每个物种（以及每个个体）都有其独特的基因组。
3. What Is a Gene? | 什么是基因？
A gene is a specific segment of DNA that contains the instructions to make a particular protein. Each gene has a unique sequence of bases that determines the order of amino acids in the protein. In classical genetics, a gene was thought of as a unit of heredity, but at the molecular level, it is a section of DNA that codes for a functional product, usually a polypeptide chain. Some genes also code for functional RNA molecules that do not become proteins, but the Edexcel specification emphasises the protein-coding role of genes.
基因是DNA上的一段特定序列，它含有制造某种特定蛋白质的指令。每个基因都具有独特的碱基序列，这个序列决定了蛋白质中氨基酸的排列顺序。在经典遗传学中，基因被视为遗传的单位，但在分子水平上，基因就是编码功能性产物（通常是多肽链）的一段DNA。有些基因也可以编码不翻译成蛋白质的功能性RNA分子，但Edexcel考纲重点强调基因的蛋白质编码功能。
4. Genes Code for Proteins | 基因编码蛋白质
Proteins are essential macromolecules that carry out most of the work in cells. They function as enzymes, structural components, hormones, antibodies, and transporters. The sequence of bases in a gene is transcribed into messenger RNA (mRNA) and then translated into a chain of amino acids. This chain folds into a specific three-dimensional shape to become a functional protein. Thus, one gene essentially specifies one polypeptide, a concept often summarised as 'one gene – one protein' (with some exceptions).
蛋白质是细胞中执行大部分工作的重要大分子。它们可以作为酶、结构成分、激素、抗体和运输蛋白发挥作用。基因中的碱基序列首先被转录成信使RNA（mRNA），然后被翻译成氨基酸链。这条链折叠成特定的三维形状，从而成为有功能的蛋白质。因此，一个基因基本上对应着一条多肽——这一概念常被概括为“一个基因–一种蛋白质”（存在某些例外）。
5. The Genetic Code | 遗传密码
The genetic code is the set of rules by which the sequence of bases in DNA is translated into proteins. Each three bases, called a codon, specify one amino acid. For example, the codon TTT in DNA corresponds to the amino acid phenylalanine. The code is universal, meaning that the same codons code for the same amino acids in almost all organisms, which is powerful evidence for common ancestry. It is also degenerate, meaning that most amino acids are encoded by more than one codon.
遗传密码是将DNA中的碱基序列翻译为蛋白质的一套规则。每三个碱基（称为一个密码子）规定一种氨基酸。例如，DNA中的密码子TTT对应苯丙氨酸。遗传密码是通用的，也就是说，几乎所有生物体中相同的密码子都编码相同的氨基酸，这是共同祖先的有力证据。密码子还具有简并性，即大多数氨基酸由不止一个密码子编码。
6. From Gene to Protein: Transcription | 从基因到蛋白质：转录
Transcription is the first stage of protein synthesis. It takes place in the nucleus. The enzyme RNA polymerase binds to a region just before the gene, called the promoter, and unwinds the DNA double helix. It then uses one of the DNA strands as a template to synthesise a complementary strand of mRNA. Adenine in DNA pairs with uracil (U) in RNA instead of thymine. Once the gene has been transcribed, the mRNA molecule is edited and then exits the nucleus through a nuclear pore into the cytoplasm.
转录是蛋白质合成的第一个阶段，发生在细胞核中。RNA聚合酶结合到基因前方称为启动子的区域，使DNA双螺旋解开。然后它以一条DNA链为模板，合成一条互补的mRNA链。DNA中的腺嘌呤在RNA中与尿嘧啶（U）配对，而不是与胸腺嘧啶配对。基因被转录后，mRNA分子经过加工，然后通过核孔从细胞核进入细胞质。
7. Translation: Building the Protein | 翻译：构建蛋白质
Translation occurs in the cytoplasm on ribosomes. The ribosome reads the mRNA codons one by one. Transfer RNA (tRNA) molecules, each carrying a specific amino acid and having an anticodon complementary to the mRNA codon, bring the correct amino acids to the ribosome. Peptide bonds form between adjacent amino acids, creating a polypeptide chain. The process continues until a stop codon is reached, after which the polypeptide is released and folds into its functional shape.
翻译发生在细胞质中的核糖体上。核糖体逐一阅读mRNA上的密码子。转运RNA（tRNA）分子各自携带一个特定的氨基酸，并有一个与mRNA密码子互补的反密码子，它们将正确的氨基酸带到核糖体上。相邻氨基酸之间形成肽键，从而生成一条多肽链。这个过程一直持续到遇到终止密码子，之后多肽被释放并折叠成有功能的形状。
8. The Role of Proteins in Cells | 蛋白质在细胞中的作用
Proteins are incredibly versatile. Enzymes like amylase speed up digestion, haemoglobin carries oxygen in red blood cells, collagen provides strength to skin and bones, insulin regulates blood sugar, and antibodies defend against pathogens. Every protein's function is determined by its shape, which is in turn dictated by the gene sequence. Therefore, the link between genes and proteins is the foundation of all cellular activities and physiological traits.
蛋白质的功能极为多样。淀粉酶等酶加速消化，血红蛋白在红细胞中运输氧气，胶原蛋白赋予皮肤和骨骼强度，胰岛素调节血糖，抗体抵御病原体。每种蛋白质的功能都是由其形状决定的，而形状又由基因序列决定。因此，基因与蛋白质之间的联系是所有细胞活动和生理特征的基础。
9. Genotype and Phenotype Connection | 基因型与表现型的联系
The genotype is the genetic constitution of an organism, i.e., the alleles it possesses. The phenotype is the observable characteristics, which result from the interaction of the genotype with the environment. Since genes code for proteins that produce specific traits, differences in genes (alleles) lead to differences in proteins and ultimately to variation in phenotypes. For example, a single gene mutation can alter the haemoglobin protein, causing sickle cell anaemia.
基因型是生物体的遗传组成，也就是它所拥有的等位基因。表现型是可以观察到的性状，它是基因型与环境相互作用的结果。由于基因编码产生特定性状的蛋白质，基因的差异（等位基因）会导致蛋白质的差异，最终造成表现型的变异。例如，单个基因突变就能改变血红蛋白，导致镰状细胞贫血。
10. Mutations: Changes in Genes | 突变：基因的变化
A mutation is a random change in the DNA base sequence. Mutations can occur during DNA replication or be caused by mutagens such as radiation or chemicals. If a mutation occurs within a gene, it may change the codon and incorporate a different amino acid into the protein. This can alter the protein's shape and function, sometimes leading to genetic disorders or, rarely, a beneficial adaptation. Mutations are the ultimate source of genetic variation.
突变是DNA碱基序列发生的随机改变。突变可以在DNA复制过程中发生，也可能由辐射或化学物质等诱变剂引起。如果突变发生在一个基因内部，它可能改变密码子，从而在蛋白质中掺入不同的氨基酸。这会改变蛋白质的形状和功能，有时导致遗传病，或者极少数情况下产生有益的适应。突变是遗传变异的最终来源。
11. Genome Projects and Medicine | 基因组计划与医学
Sequencing an organism's entire genome allows scientists to identify all its genes. The Human Genome Project, completed in 2003, mapped all human genes and has revolutionised medicine. Knowledge of the genome helps in diagnosing genetic diseases, developing gene therapies, and creating personalised medicine. In agriculture, genome studies of crops and livestock lead to improved varieties. For Edexcel IGCSE, you should appreciate how understanding the genome and genes opens up new possibilities in healthcare and biotechnology.
对生物体进行全基因组测序可以让科学家识别其所有基因。2003年完成的人类基因组计划绘制了全部人类基因图谱，彻底改变了医学。对基因组的了解有助于诊断遗传病、开发基因疗法以及实现个性化医疗。在农业方面，作物和牲畜的基因组研究可以培育出优良品种。就Edexcel IGCSE而言，你应该认识到理解基因组和基因为医疗保健和生物技术开辟了新的可能性。
12. Key Takeaways for IGCSE Edexcel Biology | IGCSE Edexcel生物要点总结
The genome is the entire DNA of an organism, while a gene is a section of DNA that codes for a specific protein. This relationship lies at the heart of molecular biology. You must be able to define both terms precisely, explain that genes determine protein structure, and link genes to phenotypic traits. Remember the steps of protein synthesis (transcription and translation) and the universality of the genetic code. Use diagrams and flowcharts to visualise the processes – these will help you answer exam questions with confidence.
基因组是生物体的全部DNA，而基因是编码特定蛋白质的一段DNA。这一关系是分子生物学的核心。你必须能精确地定义这两个术语，解释基因如何决定蛋白质结构，并将基因与表现型性状联系起来。记住蛋白质合成的步骤（转录和翻译）以及遗传密码的通用性。运用示意图和流程图来可视化这些过程——这将帮助你自信地回答考试题目。
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