📚 Gene Expression in IGCSE CIE Biology | IGCSE CIE 生物：基因表达 考点精讲
Gene expression is the process by which the genetic code in DNA is used to synthesise proteins. It underpins every cellular function and is central to the IGCSE CIE Biology syllabus. This article breaks down transcription, translation, and the role of mutations, providing a clear, exam-focused revision guide.
基因表达是指利用 DNA 中的遗传密码合成蛋白质的过程，它奠定了所有细胞功能的基础，也是 IGCSE CIE 生物课程的核心。本文详细解析转录、翻译以及突变的作用，为你提供清晰、贴合考点的复习指南。
1. What is Gene Expression? | 什么是基因表达？
Gene expression is the process of converting the information stored in a gene into a functional product, usually a protein. A gene is a specific length of DNA that codes for a particular polypeptide. The sequence of nucleotide bases determines the sequence of amino acids in the protein, which in turn determines its shape and function.
基因表达是将基因中储存的信息转化为功能性产物（通常是蛋白质）的过程。基因是一段编码特定多肽的 DNA 序列。核苷酸碱基的排列顺序决定了蛋白质中氨基酸的排列顺序，进而决定了其形状和功能。
Gene expression involves two main stages: transcription, which occurs in the nucleus, and translation, which occurs in the cytoplasm on ribosomes. The central dogma of molecular biology describes the flow of information: DNA → mRNA → protein.
基因表达包括两个主要阶段：发生在细胞核中的转录，以及发生在细胞质核糖体上的翻译。分子生物学的中心法则描述了这一信息流向：DNA → mRNA → 蛋白质。
Only certain genes are expressed in a given cell at a given time. This differential expression allows cells to specialise, an idea closely linked to differentiation in multicellular organisms.
特定细胞在特定时间只表达部分基因。这种差异表达使细胞能够特化，这与多细胞生物的分化概念密切相关。
2. DNA and RNA: The Key Molecules | DNA 和 RNA：关键分子
DNA (deoxyribonucleic acid) is a double-stranded helix composed of nucleotides. Each nucleotide contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The two strands are held together by hydrogen bonds between complementary bases: A pairs with T, and C pairs with G.
DNA（脱氧核糖核酸）是由核苷酸组成的双链螺旋。每个核苷酸含有一个脱氧核糖、一个磷酸基团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）。两条链通过互补碱基间的氢键结合在一起：A 与 T 配对，C 与 G 配对。
RNA (ribonucleic acid) is single-stranded and contains the sugar ribose instead of deoxyribose. The base uracil (U) replaces thymine, so in RNA, adenine pairs with uracil. Three main types of RNA participate in gene expression: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
RNA（核糖核酸）是单链结构，含有核糖而非脱氧核糖，碱基尿嘧啶（U）取代了胸腺嘧啶，因此在 RNA 中腺嘌呤与尿嘧啶配对。参与基因表达的 RNA 主要有三种：信使 RNA（mRNA）、转运 RNA（tRNA）和核糖体 RNA（rRNA）。
A comparison of DNA and RNA helps clarify their distinct roles in the cell.
比较 DNA 和 RNA 有助于理解它们在细胞中的不同作用。
| Feature | DNA | RNA |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Double-stranded helix | Single-stranded |
| Location | Mainly in nucleus | Nucleus and cytoplasm |
| Function | Stores genetic information | Transfers and translates code |
Comparison of DNA and RNA | DNA 与 RNA 的比较
3. Transcription: From DNA to mRNA | 转录：从 DNA 到 mRNA
Transcription is the first stage of gene expression. It takes place inside the nucleus. An enzyme called RNA polymerase binds to a specific region of the DNA called the promoter, which signals the start of a gene. The DNA double helix unwinds and the hydrogen bonds between the two strands break, exposing the template strand.
转录是基因表达的第一阶段，发生在细胞核内。一种叫做 RNA 聚合酶的酶结合到 DNA 上称为启动子的特定区域，该区域标志着基因的开始。DNA 双螺旋解旋，两条链之间的氢键断裂，暴露出模板链。
RNA polymerase moves along the template strand in the 3′ to 5′ direction, adding free RNA nucleotides that are complementary to the DNA bases. Adenine on the DNA template pairs with uracil on the growing mRNA strand; cytosine pairs with guanine; thymine pairs with adenine; and guanine pairs with cytosine. The nucleotides are joined by phosphodiester bonds to form a single-stranded mRNA molecule.
RNA 聚合酶沿模板链从 3′ 端向 5′ 端移动，添加与 DNA 碱基互补的游离 RNA 核苷酸。DNA 模板上的腺嘌呤与正在延长的 mRNA 链上的尿嘧啶配对；胞嘧啶与鸟嘌呤配对；胸腺嘧啶与腺嘌呤配对；鸟嘌呤与胞嘧啶配对。这些核苷酸通过磷酸二酯键连接形成单链 mRNA 分子。
As the RNA polymerase passes, the DNA helix reforms behind it. Transcription stops when the polymerase reaches a terminator sequence. The newly formed pre-mRNA then undergoes processing before it leaves the nucleus.
当 RNA 聚合酶通过后，DNA 螺旋在后方重新形成。当聚合酶到达终止子序列时，转录停止。新形成的前体 mRNA 在离开细胞核前需要经过加工。
4. RNA Processing in Eukaryotes | 真核细胞中的 RNA 加工
In eukaryotic cells, the initial transcript, called pre-mRNA, contains both exons (coding regions) and introns (non-coding regions). Introns must be removed and exons joined together to form a mature mRNA molecule — a process known as splicing.
在真核细胞中，最初的转录产物称为前体 mRNA，同时含有外显子（编码区）和内含子（非编码区）。必须切除内含子并将外显子连接起来形成成熟的 mRNA 分子，这一过程称为剪接。
Additionally, a modified guanine cap is added to the 5′ end, and a poly-A tail is added to the 3′ end. These modifications protect the mRNA from degradation and assist in the export of the mature mRNA from the nucleus to the cytoplasm through a nuclear pore.
此外，5′ 端会加上一个经过修饰的鸟嘌呤帽，3′ 端会加上多聚腺苷酸尾。这些修饰保护 mRNA 免遭降解，并帮助成熟 mRNA 通过核孔从细胞核转运到细胞质。
5. Translation: mRNA to Polypeptide | 翻译：从 mRNA 到多肽
Translation is the synthesis of a polypeptide chain from the mRNA code. It occurs in the cytoplasm on ribosomes. The ribosome binds to the mRNA at the start codon (usually AUG). A tRNA molecule carrying the amino acid methionine recognises this start codon through its complementary anticodon and binds to the mRNA.
翻译是从 mRNA 密码合成多肽链的过程，在细胞质的核糖体上进行。核糖体结合到 mRNA 的起始密码子（通常是 AUG）上。携带甲硫氨酸的 tRNA 分子通过其互补的反密码子识别该起始密码子并与 mRNA 结合。
The ribosome has two sites for tRNA molecules: the A site (aminoacyl-tRNA site) and the P site (peptidyl-tRNA site). The initiator tRNA enters the P site. A second tRNA, carrying a different amino acid, binds to the A site if its anticodon is complementary to the mRNA codon.
核糖体有两个 tRNA 结合位点：A 位（氨酰 tRNA 位点）和 P 位（肽基 tRNA 位点）。起始 tRNA 进入 P 位。第二个携带不同氨基酸的 tRNA，如果其反密码子与 mRNA 密码子互补，则结合到 A 位。
A peptide bond forms between the two amino acids. The ribosome then moves one codon along the mRNA; the tRNA in the P site is released, and the tRNA in the A site is now in the P site. The process repeats, adding amino acids one by one to form a growing polypeptide chain.
两个氨基酸之间形成肽键。随后核糖体沿 mRNA 移动一个密码子的距离；P 位的 tRNA 被释放，A 位的 tRNA 现在位于 P 位。这一过程不断重复，逐个添加氨基酸，形成不断延长的多肽链。
Translation ends when a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA molecule carries an anticodon for a stop codon; instead, a release factor binds, causing the polypeptide to be released and the ribosomal subunits to dissociate.
当终止密码子（UAA、UAG 或 UGA）进入 A 位时翻译结束。没有 tRNA 分子携带与终止密码子对应的反密码子；取而代之的是释放因子结合，导致多肽被释放，核糖体亚基解离。
6. The Role of Ribosomes and tRNA | 核糖体和 tRNA 的作用
Ribosomes are the molecular machines that carry out translation. Each ribosome is composed of a large subunit and a small subunit made of rRNA and proteins. They provide a platform where mRNA, tRNA, and amino acids can interact with precision.
核糖体是执行翻译的分子机器。每个核糖体由一个大亚基和一个小亚基组成，由 rRNA 和蛋白质构成。它们提供一个平台，使 mRNA、tRNA 和氨基酸能够精确相互作用。
tRNA molecules have a characteristic cloverleaf shape. At one end they have an anticodon — a triplet of unpaired bases that is complementary to an mRNA codon. At the other end is an attachment site for a specific amino acid, where an enzyme called aminoacyl-tRNA synthetase attaches the correct amino acid using ATP energy.
tRNA 分子具有典型的三叶草形状。一端是反密码子——三个未配对的碱基，与 mRNA 密码子互补。另一端是特定氨基酸的连接位点，一种叫作氨酰 tRNA 合成酶的酶利用 ATP 能量将正确的氨基酸连接在此处。
The specificity of translation is therefore achieved by the complementary base pairing between the mRNA codon and the tRNA anticodon, ensuring that amino acids are added in the correct order determined by the gene.
因此，翻译的特异性由 mRNA 密码子和 tRNA 反密码子之间的互补碱基配对实现，确保氨基酸按照基因决定的正确顺序加入。
7. The Genetic Code and Codons | 遗传密码和密码子
The genetic code is the set of rules by which the sequence of bases in DNA is translated into the amino acid sequence of a protein. Each amino acid is specified by a sequence of three bases on the mRNA, called a codon. There are 64 possible codons (4³), but only 20 amino acids, meaning the code is degenerate — most amino acids are coded for by more than one codon.
遗传密码是一套规则，规定了 DNA 中的碱基序列如何翻译成蛋白质的氨基酸序列。每种氨基酸由 mRNA 上的三个碱基序列决定，称为一个密码子。共有 64 种可能的密码子（4³），但只有 20 种氨基酸，这意味着密码具有简并性——大多数氨基酸可由不止一个密码子编码。
The genetic code is also universal — the same codons specify the same amino acids in almost all organisms, which is strong evidence for a common evolutionary origin.
遗传密码还具有通用性——几乎所有生物体都使用相同的密码子编码相同的氨基酸，这是共同进化起源的有力证据。
It is non-overlapping and read in triplets: the reading frame is set by the start codon, and the mRNA is read in successive groups of three bases.
遗传密码是不重叠的，并以三联体形式读取：起始密码子设定阅读框，mRNA 以连续的三个碱基为一组进行读取。
8. Mutations and Their Effects on Gene Expression | 突变及其对基因表达的影响
A gene mutation is a random change in the nucleotide sequence of DNA. Mutations can occur spontaneously during DNA replication or be induced by mutagens such as radiation, certain chemicals, or viruses. They affect gene expression by altering the mRNA sequence, which may change the amino acid sequence of the protein produced.
基因突变是 DNA 核苷酸序列的随机改变。突变可在 DNA 复制过程中自发发生，或由诱变剂（如辐射、某些化学物质或病毒）诱导产生。它们通过改变 mRNA 序列影响基因表达，进而可能改变所产生蛋白质的氨基酸序列。
Point mutations involve a change in a single nucleotide. A substitution replaces one base with another. This may lead to a silent mutation (no change in amino acid due to degeneracy), a missense mutation (a different amino acid is incorporated), or a nonsense mutation (a premature stop codon is created).
点突变涉及单个核苷酸的改变。替换是指一个碱基被另一个取代。这可能导致沉默突变（由于密码简并性氨基酸不改变）、错义突变（掺入不同的氨基酸）或无义突变（产生提前终止密码子）。
Insertions and deletions involve the addition or removal of one or more nucleotides. If the number of inserted or deleted bases is not a multiple of three, a frameshift mutation occurs. This shifts the reading frame, altering every codon downstream and usually resulting in a completely non-functional protein.
插入和缺失涉及一个或多个核苷酸的添加或删除。如果插入或缺失的碱基数不是三的倍数，就会发生移码突变。这会改变阅读框，使下游的每个密码子都发生改变，通常导致蛋白质完全丧失功能。
9. Examples of Mutations in IGCSE Context | IGCSE 背景下的突变举例
Sickle cell anaemia is a classic example of a missense mutation. A substitution of thymine for adenine in the gene for the β-globin chain of haemoglobin changes the codon GAG to GTG, leading to the replacement of glutamic acid with valine. This single amino acid change causes haemoglobin molecules to stick together, distorting red blood cells into a sickle shape and impairing oxygen transport.
镰状细胞贫血是错义突变的经典例子。编码血红蛋白 β 珠蛋白链的基因中胸腺嘧啶替换了腺嘌呤，使密码子 GAG 变为 GTG，导致谷氨酸被缬氨酸取代。这单个氨基酸的改变使血红蛋白分子相互粘连，红细胞扭曲成镰刀形，损害氧气运输。
Cystic fibrosis can be caused by a deletion of three nucleotides in the CFTR gene, removing the amino acid phenylalanine from the protein. This deletion does not cause a frameshift, but it still results in a misfolded protein that fails to transport chloride ions correctly, leading to thick mucus production.
囊性纤维化可由 CFTR 基因中三个核苷酸的缺失引起，使蛋白质中缺失苯丙氨酸。虽然这一缺失没有引起移码，但仍导致蛋白质错误折叠，无法正常运输氯离子，进而产生浓稠的黏液。
10. Summary of Protein Synthesis | 蛋白质合成总结
The whole journey of gene expression can be summarised in a few steps: A gene is transcribed in the nucleus to produce pre-mRNA; introns are removed and cap and tail added to form mature mRNA; mRNA exits the nucleus; ribosomes bind to mRNA; tRNA molecules deliver amino acids according to codon-anticodon pairing; peptide bonds form a polypeptide; the polypeptide folds into its functional shape, sometimes joining with other polypeptides or cofactors to form the final protein.
整个基因表达的过程可概括为几个步骤：基因在细胞核中转录产生前体 mRNA；移除内含子并加上帽和尾形成成熟 mRNA；mRNA 离开细胞核；核糖体结合到 mRNA 上；tRNA 分子根据密码子-反密码子配对运送氨基酸；肽键形成多肽；多肽折叠成功能形状，有时与其他多肽或辅因子结合形成最终蛋白质。
Remember that the entire protein synthesis process consumes energy, mainly from ATP. The ribosome, tRNA, mRNA, and amino acids are all recycled within the cell.
请记住，整个蛋白质合成过程消耗能量，主要来自 ATP。核糖体、tRNA、mRNA 和氨基酸都在细胞内循环再利用。
11. Exam Tips for Gene Expression | 基因表达的考试技巧
In IGCSE CIE Biology exams, you may be asked to describe the process of protein synthesis step by step. Use precise scientific language: ‘RNA polymerase binds to the promoter’, ‘complementary base pairing ensures accurate transcription’, ‘peptide bonds form between amino acids’.
在 IGCSE CIE 生物考试中，你可能会被要求逐步描述蛋白质合成过程。要使用精确的科学用语：“RNA 聚合酶结合到启动子”，“互补碱基配对确保准确转录”，“氨基酸之间形成肽键”。
Be able to distinguish between transcription and translation, identifying where each occurs and the molecules involved. Practice interpreting diagrams of ribosomes, tRNA, and mRNA sequences. When answering mutation questions, always mention the type of mutation, the change in the DNA sequence, the effect on the mRNA codon, and the outcome for the protein.
能够区分转录和翻译，指出各自发生的场所和涉及的分子。练习解读核糖体、tRNA 和 mRNA 序列图。在回答突变问题时，始终提及突变类型、DNA 序列的变化、对 mRNA 密码子的影响以及对蛋白质的最终结果。
Common exam pitfalls include confusing uracil with thymine, forgetting that transcription occurs in the nucleus, or thinking that the entire DNA unwinds — only the gene being transcribed is unzipped.
常见的考试错误包括混淆尿嘧啶和胸腺嘧啶，忘记转录发生在细胞核内，或认为整个 DNA 都会解旋——实际上只有被转录的基因区域才会解链。
Revising with annotated diagrams and being able to explain the central dogma in your own words will greatly strengthen your answers.
使用标注清晰的图表进行复习，并能够用自己的语言解释中心法则，将极大增强你的答案。
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