📚 IGCSE Edexcel Biology: Gene Expression Exam Essentials | IGCSE Edexcel 生物：基因表达 考点精讲
Gene expression is the process by which the genetic code stored in DNA is used to synthesise functional gene products, mainly proteins. It involves two key stages: transcription, where a segment of DNA is copied into messenger RNA (mRNA), and translation, where ribosomes decode the mRNA to assemble amino acids into a polypeptide chain. Understanding how genes are switched on and off, and how mutations can alter protein structure, is central to mastering this IGCSE Edexcel topic. This article breaks down every essential concept you need, with clear explanations and bilingual notes.
基因表达是指储存在 DNA 中的遗传密码被用来合成功能性基因产物（主要是蛋白质）的过程。它包含两个关键阶段：转录（DNA 片段被复制成信使 RNA）和翻译（核糖体解码 mRNA，将氨基酸组装成多肽链）。理解基因如何开启和关闭、突变如何改变蛋白质结构，是掌握 IGCSE Edexcel 这一主题的核心。本文为你梳理每一个必备概念，配以清晰的中英双语讲解。
1. DNA, Genes and Chromosomes | DNA、基因和染色体
The genetic material in almost all living organisms is deoxyribonucleic acid (DNA). A gene is a short section of DNA that carries the code for a particular protein. In eukaryotic cells, DNA is organised into long structures called chromosomes, which are found inside the nucleus. Each chromosome contains many genes, and a complete set of chromosomes carries the organism’s entire genome.
几乎所有生物的遗传物质都是脱氧核糖核酸（DNA）。基因是 DNA 上携带特定蛋白质编码信息的短片段。在真核细胞中，DNA 被组织成称为染色体的长结构，位于细胞核内。每条染色体包含许多基因，一整套染色体携带了生物体的全部基因组。
2. The Structure of DNA | DNA 的结构
DNA is a double helix formed by two antiparallel strands. Each strand is a polymer of nucleotides, and each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The four bases in DNA are adenine (A), thymine (T), cytosine (C) and guanine (G). A always pairs with T through two hydrogen bonds, and C pairs with G through three hydrogen bonds. This complementary base pairing is fundamental to DNA replication and transcription.
DNA 是由两条反向平行的链构成的双螺旋结构。每条链是由核苷酸聚合而成的，每个核苷酸包含一个脱氧核糖、一个磷酸基团和一个含氮碱基。DNA 中的四种碱基分别是腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。A 总是与 T 通过两个氢键配对，C 与 G 通过三个氢键配对。这种互补碱基配对是 DNA 复制和转录的基础。
3. The Genetic Code | 遗传密码
The sequence of bases along a gene forms a triplet code. Each set of three bases, called a codon, codes for one specific amino acid. There are 64 possible codons, but only 20 standard amino acids, so the genetic code is described as degenerate. The code is also universal — the same codons specify the same amino acids in almost all organisms, which is evidence for a common ancestor.
基因上的碱基序列构成了三联体密码。每三个碱基（称为一个密码子）对应一种特定的氨基酸。共有 64 种可能的密码子，但标准氨基酸只有 20 种，因此遗传密码具有简并性。密码还是通用的——相同的密码子在几乎所有生物中都编码相同的氨基酸，这为共同祖先的存在提供了证据。
4. Transcription: Making mRNA | 转录：合成 mRNA
Transcription is the first step of gene expression and occurs in the nucleus (in eukaryotes). The enzyme RNA polymerase binds to a specific region of the DNA called the promoter. It unwinds the double helix and uses one strand — the template strand — to synthesise a complementary molecule of messenger RNA (mRNA). RNA uses uracil (U) instead of thymine, so adenine pairs with uracil. Transcription stops when RNA polymerase reaches a terminator sequence.
转录是基因表达的第一步，在（真核生物的）细胞核中进行。RNA 聚合酶与 DNA 上称为启动子的特定区域结合，解旋双螺旋，并以其中一条链——模板链——为模板合成一条互补的信使 RNA（mRNA）。RNA 中用尿嘧啶（U）代替胸腺嘧啶，因此腺嘌呤与尿嘧啶配对。当 RNA 聚合酶到达终止子序列时，转录停止。
5. RNA Processing in Eukaryotes | 真核生物的 RNA 加工
In eukaryotic cells, the initial RNA transcript (pre-mRNA) undergoes modifications before it is ready for translation. A guanine cap is added to the 5′ end and a poly-A tail is added to the 3′ end to protect the mRNA and aid its export from the nucleus. Introns (non-coding regions) are removed by splicing, leaving only the exons that will be translated. This mature mRNA then travels to the ribosome through a nuclear pore.
在真核细胞中，初始 RNA 转录本（前体 mRNA）在翻译前要经过修饰。在 5′ 端加上一个鸟嘌呤帽，在 3′ 端加上一个 poly-A 尾，以保护 mRNA 并帮助其从细胞核输出。内含子（非编码区域）通过剪接被切除，仅留下将被翻译的外显子。随后，成熟的 mRNA 通过核孔来到核糖体。
6. Translation: Building Proteins | 翻译：构建蛋白质
Translation occurs at ribosomes in the cytoplasm. The ribosome reads the mRNA codons one by one. Each codon is recognised by a specific transfer RNA (tRNA) molecule carrying the corresponding amino acid. The tRNA has an anticodon that is complementary to the mRNA codon. When the anticodon and codon pair, the amino acid is added to the growing polypeptide chain. Translation begins at the start codon (AUG) and ends at a stop codon, where the polypeptide is released.
翻译发生在细胞质中的核糖体上。核糖体逐个读取 mRNA 上的密码子。每个密码子被一种携带相应氨基酸的特定转运 RNA（tRNA）分子所识别。tRNA 上有一个与 mRNA 密码子互补的反密码子。当反密码子与密码子配对时，该氨基酸就会被添加到正在延伸的多肽链上。翻译从起始密码子（AUG）开始，到终止密码子结束，此时多肽链被释放。
7. The Role of Ribosomes and tRNA | 核糖体和 tRNA 的作用
Ribosomes are made of ribosomal RNA (rRNA) and proteins, and consist of a large and a small subunit. The small subunit binds to the mRNA, while the large subunit has sites for tRNA binding (A site, P site and E site). The tRNA molecules act as adaptors, linking the genetic code to the correct amino acid. Enzymes called aminoacyl-tRNA synthetases attach each amino acid to its corresponding tRNA in an energy-requiring step, ensuring fidelity.
核糖体由核糖体 RNA（rRNA）和蛋白质组成，包含一个大亚基和一个小亚基。小亚基与 mRNA 结合，大亚基则具有 tRNA 结合位点（A 位、P 位和 E 位）。tRNA 分子充当适配器，将遗传密码与正确的氨基酸连接起来。氨酰-tRNA 合成酶以耗能的方式将每种氨基酸连接到其相应的 tRNA 上，确保了翻译的准确性。
8. Protein Folding and Function | 蛋白质的折叠和功能
Once the polypeptide chain is released, it folds spontaneously into a specific three-dimensional shape determined by the sequence of amino acids. This shape is critical for the protein’s function. Proteins can act as enzymes, hormones, antibodies, structural components or membrane carriers. A change in just one amino acid (as in sickle cell anaemia) can drastically alter the protein’s shape and render it non-functional.
多肽链被释放后，会自发折叠成由氨基酸序列决定的特定三维形状。这种形状对蛋白质的功能至关重要。蛋白质可以作为酶、激素、抗体、结构成分或膜载体发挥作用。仅仅一个氨基酸的改变（如在镰刀型细胞贫血症中）就可能极大地改变蛋白质的形状并使其丧失功能。
9. Gene Mutations: Types | 基因突变：类型
A gene mutation is a permanent change in the base sequence of DNA. Substitution involves swapping one base for another, which may lead to a single amino acid change (missense) or produce an early stop codon (nonsense). Insertion and deletion mutations cause a frameshift: the reading frame of codons shifts, leading to a completely different sequence of amino acids after the mutation point, often resulting in a non-functional protein.
基因突变是指 DNA 碱基序列的永久性改变。替换突变是将一个碱基换成另一个碱基，可能导致单个氨基酸改变（错义突变）或提前产生终止密码子（无义突变）。插入和缺失突变会引起移码：密码子阅读框发生移动，导致突变点之后的氨基酸序列完全不同，通常产生无功能的蛋白质。
10. Effects of Mutations | 突变的影响
Not all mutations are harmful. Some are neutral and have no noticeable effect on the phenotype, especially if they occur in non-coding regions or if the amino acid change does not affect protein function. Beneficial mutations can increase an organism’s chance of survival and may be selected for during evolution. For example, a mutation in a human gene may confer resistance to a disease. Only mutations in gametes can be passed on to offspring.
并非所有突变都是有害的。有些是中性的，对表型没有明显影响，特别是如果它们发生在非编码区域，或者氨基酸的改变不影响蛋白质功能。有利突变可以提高生物体的生存机会，并可能在进化过程中被选择。例如，人类基因的某个突变可赋予对某种疾病的抵抗力。只有发生在配子中的突变才能传给后代。
11. Control of Gene Expression | 基因表达的调控
Not all genes are expressed all the time. In specialised cells, many genes are switched off so that only the proteins needed for that cell type are made. Gene expression can be controlled at the transcriptional level by proteins called transcription factors that bind to the promoter or enhancer regions. Environmental signals, hormones and developmental cues all influence which genes are transcribed, allowing cells to differentiate and respond to changes.
并不是所有基因都一直表达。在特化细胞中，许多基因处于关闭状态，仅制造该细胞类型所需的蛋白质。基因表达可以在转录水平上被调控：称为转录因子的蛋白质与启动子或增强子区域结合。环境信号、激素和发育信号都会影响哪些基因被转录，使细胞能够分化并对变化作出反应。
12. Enzymes and Gene Expression | 酶与基因表达
Enzymes control every step of gene expression. Helicase unwinds DNA during transcription, RNA polymerase synthesises mRNA, and various enzymes modify pre-mRNA. In translation, aminoacyl-tRNA synthetases and peptidyl transferase (a ribosomal RNA enzyme) catalyse the formation of peptide bonds. The availability and activity of these enzymes directly affect the rate of protein synthesis, linking metabolism to gene expression.
酶控制着基因表达的每一步。解旋酶在转录过程中解开 DNA，RNA 聚合酶合成 mRNA，各种酶修饰前体 mRNA。在翻译中，氨酰-tRNA 合成酶和肽基转移酶（一种核糖体 RNA 酶）催化肽键的形成。这些酶的可用性和活性直接影响蛋白质合成速率，将代谢与基因表达联系起来。
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