📚 Gene Expression Revision for WJEC A-Level Biology | A-Level WJEC 生物：基因表达 考点精讲
Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a functional gene product, typically a protein. In the WJEC A-Level Biology specification, this topic covers transcription, translation, the genetic code, and the mechanisms that control when, where, and to what extent genes are expressed. Understanding these processes is fundamental to explaining how cells differentiate, respond to their environment, and how errors can lead to disease. This revision guide walks you through every key concept you need to master for the exam.
基因表达是指基因中编码的信息被用来指导合成功能性基因产物（通常是蛋白质）的过程。在WJEC A-Level生物学考试大纲中，该主题涵盖转录、翻译、遗传密码，以及控制基因在何时、何处和表达程度的调控机制。理解这些过程是解释细胞分化、对环境做出反应以及错误如何导致疾病的基础。本复习指南将带你逐一攻克考试所需的每一个核心概念。
1. Overview of Gene Expression | 基因表达概述
The ‘central dogma’ of molecular biology describes the flow of genetic information: DNA is transcribed into messenger RNA (mRNA), which is then translated into a polypeptide chain. Not all genes are expressed in every cell; the pattern of gene expression determines a cell’s structure and function. In both prokaryotes and eukaryotes, gene expression can be regulated at multiple points – from chromatin remodelling to post‑translational modification.
分子生物学的’中心法则’描述了遗传信息的流动：DNA 转录成信使 RNA（mRNA），后者再翻译成多肽链。并非所有基因都在每个细胞中表达；基因表达的模式决定了细胞的结构和功能。无论是在原核生物还是真核生物中，基因表达都可以在多个节点进行调控——从染色质重塑到翻译后修饰。
2. Transcription: DNA to mRNA | 转录：从DNA到mRNA
Transcription is the synthesis of a complementary mRNA strand from a DNA template. The enzyme RNA polymerase binds to the promoter region upstream of the gene, unwinds the DNA double helix, and begins adding ribonucleotides in the 5′ to 3′ direction. The base‑pairing rules are the same as in DNA replication, except that uracil (U) replaces thymine (T). In prokaryotes, a single RNA polymerase carries out all transcription, whereas eukaryotes have distinct RNA polymerases (RNA polymerase II being responsible for mRNA synthesis).
转录是以DNA为模板合成互补mRNA链的过程。RNA聚合酶与基因上游的启动子区域结合，解开DNA双螺旋，并从5’向3’方向添加核糖核苷酸。碱基配对规则与DNA复制相同，但尿嘧啶（U）取代了胸腺嘧啶（T）。在原核生物中，单一的RNA聚合酶负责所有转录，而真核生物具有不同类型的RNA聚合酶（RNA聚合酶II负责mRNA的合成）。
The transcription bubble moves along the DNA, and as soon as the polymerase has moved past, the DNA rewinds behind it. Termination occurs when the polymerase encounters a terminator sequence; in prokaryotes, this often involves a hairpin loop structure in the RNA, while in eukaryotes the process is more complex and coupled with RNA processing.
转录泡沿着DNA移动，聚合酶一旦经过，其后的DNA立即重新缠绕。当聚合酶遇到终止子序列时转录终止；在原核生物中，这通常涉及RNA中的发卡环结构，而在真核生物中过程更为复杂，并与RNA加工相偶联。
3. RNA Processing in Eukaryotes | 真核生物中的RNA加工
The primary transcript (pre‑mRNA) in eukaryotes must be modified before it leaves the nucleus. A 5′ cap (a modified guanine nucleotide) is added for protection and ribosome recognition. A poly‑A tail – a string of adenine nucleotides – is attached to the 3′ end to increase stability and facilitate export. Most importantly, the non‑coding regions (introns) are removed and the coding regions (exons) are spliced together by a spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). Alternative splicing allows a single gene to produce multiple different mRNA molecules and therefore different proteins.
真核生物的初始转录本（前体mRNA）在离开细胞核之前必须经过修饰。5’端添加一个帽结构（修饰的鸟嘌呤核苷酸）用于保护和核糖体识别；3’端附着多聚腺苷酸尾（poly‑A尾）以增加稳定性并促进输出。最重要的是，非编码区（内含子）被切除，编码区（外显子）由剪接体（由小核核糖核蛋白snRNPs组成的复合物）连接在一起。可变剪接使单个基因能够产生多种不同的mRNA分子，从而形成不同的蛋白质。
4. Translation: mRNA to Polypeptide | 翻译：从mRNA到多肽
Translation occurs on ribosomes, which are composed of ribosomal RNA (rRNA) and proteins. In the cytoplasm, the small ribosomal subunit binds to the mRNA near the 5′ cap (in eukaryotes) or the Shine‑Dalgarno sequence (in prokaryotes). Transfer RNA (tRNA) molecules act as adaptors: each tRNA carries a specific amino acid at its 3′ end and has an anticodon that base‑pairs with a complementary codon on the mRNA.
翻译在核糖体上进行，核糖体由核糖体RNA（rRNA）和蛋白质组成。在细胞质中，核糖体小亚基与mRNA的5’帽附近（真核生物）或Shine‑Dalgarno序列（原核生物）结合。转运RNA（tRNA）分子充当适配器：每种tRNA的3’端携带特定的氨基酸，并具有与mRNA上互补密码子配对的反密码子。
The ribosome moves along the mRNA in the 5′ to 3′ direction, and peptide bonds form between adjacent amino acids. The process occurs in three stages: initiation, elongation, and termination. A stop codon (UAA, UAG, or UGA) signals the end of translation, and release factors cause the polypeptide to be released. Multiple ribosomes often translate the same mRNA simultaneously, forming a polysome.
核糖体沿mRNA以5’向3’方向移动，相邻氨基酸之间形成肽键。该过程分为三个阶段：起始、延伸和终止。终止密码子（UAA、UAG或UGA）发出翻译终止信号，释放因子导致多肽释放。多个核糖体常常同时翻译同一条mRNA，形成多核糖体。
5. The Genetic Code | 遗传密码
The genetic code is a set of rules by which the sequence of nucleotides in mRNA is translated into the sequence of amino acids. It is a triplet code – three nucleotides (a codon) specify one amino acid. The code is degenerate (most amino acids are encoded by more than one codon), non‑overlapping, and nearly universal across all organisms. The start codon AUG codes for methionine and signals the beginning of translation.
遗传密码是一套将mRNA中的核苷酸序列翻译为氨基酸序列的规则。它是一个三联体密码——三个核苷酸（一个密码子）决定一个氨基酸。密码子具有简并性（大多数氨基酸由不止一种密码子编码）、非重叠性，且几乎在所有生物中通用。起始密码子AUG编码甲硫氨酸，并指示翻译的起点。
| Codon | Amino Acid | 密码子 | 氨基酸 |
|---|---|---|---|
| AUG | Methionine (Start) | AUG | 甲硫氨酸（起始） |
| UUU, UUC | Phenylalanine | UUU, UUC | 苯丙氨酸 |
| UAA, UAG, UGA | Stop | UAA, UAG, UGA | 终止 |
6. Regulation in Prokaryotes: The lac Operon | 原核生物调控：乳糖操纵子
In prokaryotes, genes with related functions are often clustered into operons under the control of a single promoter. The lac operon in E. coli is a classic example, containing three structural genes (lacZ, lacY, lacA) required for lactose metabolism. Upstream lies the operator site and the promoter. A regulatory gene (lacI) located elsewhere produces the lac repressor protein.
在原核生物中，功能相关的基因通常成簇组成操纵子，受单一启动子控制。大肠杆菌的乳糖操纵子是一个经典例子，包含三个代谢乳糖所需的结构基因（lacZ、lacY、lacA）。其上游是操纵基因位点和启动子。位于别处的调节基因（lacI）产生乳糖阻遏蛋白。
When lactose is absent, the repressor binds tightly to the operator, blocking RNA polymerase from transcribing the structural genes. When lactose is present, it is converted to allolactose, which acts as an inducer by binding to the repressor and causing a conformational change that prevents it from binding to the operator. Transcription proceeds, and the enzymes for lactose breakdown are produced. This ensures that energy is not wasted on making unneeded enzymes.
当缺乏乳糖时，阻遏蛋白与操纵基因紧密结合，阻止RNA聚合酶转录结构基因。当乳糖存在时，它被转化成异乳糖，后者作为诱导物与阻遏蛋白结合，引起构象变化使其无法与操纵基因结合。转录得以进行，分解乳糖的酶被合成。这确保了能量不会被浪费在制造不需要的酶上。
7. Regulation in Eukaryotes: Transcription Factors | 真核生物调控：转录因子
Eukaryotic gene regulation is more intricate than the prokaryotic operon model. Upstream of many eukaryotic genes are promoter elements such as the TATA box, where general transcription factors and RNA polymerase II assemble to form the basal transcription complex. However, high levels of transcription usually require additional activator proteins (specific transcription factors) that bind to enhancer regions, sometimes located thousands of base pairs away. Repressor proteins can bind to silencer elements or interfere with activators.
真核生物的基因调控比原核生物的操纵子模型更为复杂。许多真核基因的上游存在启动子元件，如TATA框，通用转录因子和RNA聚合酶II在此组装形成基础转录复合体。然而，高水平转录通常还需要额外的激活蛋白（特异性转录因子）与增强子区域结合，这些增强子有时位于数千碱基对之外。阻遏蛋白可以与沉默子元件结合或干扰激活蛋白的功能。
DNA looping allows enhancer‑bound activators to interact with the initiation complex directly or via coactivators. The combination of transcription factors present in a cell determines which genes are transcribed, allowing the differentiation of cells with identical genomes into diverse cell types.
DNA环化使结合在增强子上的激活蛋白能够直接或通过共激活因子与起始复合物相互作用。细胞中存在的转录因子组合决定了哪些基因被转录，从而使具有相同基因组的细胞分化成不同的细胞类型。
8. Epigenetics: DNA Methylation and Histone Modification | 表观遗传学：DNA甲基化与组蛋白修饰
Epigenetic changes are heritable alterations in gene expression that do not involve changes to the DNA nucleotide sequence. Two major mechanisms are DNA methylation and histone modification. Methylation of cytosine bases in CpG islands of promoter regions typically represses transcription by preventing transcription factors from binding or by recruiting proteins that compact chromatin. Histone acetylation, on the other hand, tends to activate gene expression: adding acetyl groups to lysine residues on histone tails reduces their positive charge, loosening the interaction with negatively charged DNA and making the chromatin more accessible to the transcription machinery.
表观遗传变化是可遗传的基因表达改变，但不涉及DNA核苷酸序列的变化。两种主要机制是DNA甲基化和组蛋白修饰。启动子区域CpG岛上胞嘧啶碱基的甲基化通常通过阻止转录因子结合或招募使染色质凝集的蛋白质来抑制转录。而组蛋白乙酰化则倾向于激活基因表达：在组蛋白尾部的赖氨酸残基上添加乙酰基会减少其正电荷，松弛与带负电DNA的相互作用，使染色质更易被转录机器接触。
These modifications can be influenced by environmental factors and are crucial in processes such as X‑inactivation and genomic imprinting. Unlike genetic mutations, epigenetic marks are reversible, making them attractive targets for therapeutic interventions.
这些修饰可能受环境因素影响，并在X染色体失活和基因组印记等过程中至关重要。与基因突变不同，表观遗传标记是可逆的，这使其成为治疗干预的诱人靶点。
9. RNA Interference and siRNA | RNA干扰与小干扰RNA
RNA interference (RNAi) is a mechanism by which the expression of specific genes is silenced post‑transcriptionally. Double‑stranded RNA (dsRNA) is cleaved by the enzyme Dicer into small interfering RNA (siRNA) molecules, typically 21–23 nucleotides long. One strand of the siRNA is incorporated into the RNA‑induced silencing complex (RISC). The RISC‑siRNA complex base‑pairs with a complementary mRNA sequence, leading to cleavage and degradation of that mRNA, thereby preventing translation.
RNA干扰（RNAi）是一种在转录后水平沉默特定基因表达的机制。双链RNA（dsRNA）被Dicer酶切割成小干扰RNA（siRNA）分子，通常长度为21–23个核苷酸。siRNA的一条链被整合到RNA诱导的沉默复合物（RISC）中。RISC‑siRNA复合物与互补的mRNA序列碱基配对，导致该mRNA被切割和降解，从而阻止翻译。
This pathway is not only a natural regulatory mechanism but also a powerful research tool for knocking down gene expression. In addition, plant and animal cells also use microRNAs (miRNAs) derived from hairpin precursors to fine‑tune gene expression, often by imperfect base‑pairing that blocks translation without immediate mRNA destruction.
该通路不仅是一种天然的调控机制，也是一种敲低基因表达的强效研究工具。此外，植物和动物细胞还利用来自于发卡前体的微小RNA（miRNA）来微调基因表达，通常通过不完全碱基配对来阻断翻译，而不立即降解mRNA。
10. Gene Expression and Cancer | 基因表达与癌症
Cancer results from uncontrolled cell division, often caused by mutations that affect the expression of genes regulating the cell cycle. Proto‑oncogenes code for proteins that promote cell division; a gain‑of‑function mutation can convert a proto‑oncogene into an oncogene, causing it to be permanently active. Tumour suppressor genes, such as p53, normally inhibit cell division or promote apoptosis; loss‑of‑function mutations in these genes remove the normal ‘brakes’ on the cell cycle.
癌症源于不受控制的细胞分裂，通常由影响细胞周期调控基因表达的突变引起。原癌基因编码促进细胞分裂的蛋白质；功能获得性突变可将原癌基因转变为癌基因，使其永久活化。抑癌基因（如p53）通常抑制细胞分裂或促进细胞凋亡；这些基因的功能丧失性突变则移除了细胞周期的正常’刹车’。
Epigenetic alterations, such as hypermethylation of tumour suppressor promoters or global DNA hypomethylation, also play critical roles in cancer development. Understanding the links between gene expression dysregulation and cancer is a vital part of the WJEC specification and provides context for many exam questions on gene regulation.
表观遗传改变，如抑癌基因启动子的高度甲基化或全基因组DNA低甲基化，也在癌症发展中起关键作用。理解基因表达失调与癌症之间的联系是WJEC考纲的重要组成部分，并为许多关于基因调控的考题提供了背景。
11. Summary of Key Concepts | 核心概念总结
Mastering gene expression for the WJEC A‑Level exam requires a secure grasp of the sequence: transcription → RNA processing → translation. You must be able to compare regulation in prokaryotes (lac operon) and eukaryotes (transcription factors, epigenetics, RNAi). Key terminology – promoter, operator, enhancer, siRNA, RISC, methylation, acetylation – must be used accurately. Diagrams of the lac operon in the presence and absence of lactose, or the steps of translation, are frequent exam requirements, so practise drawing and labelling them from memory.
要在WJEC A‑Level考试中掌握基因表达，需要牢牢掌握转录→RNA加工→翻译这一流程。你必须能够比较原核生物（乳糖操纵子）和真核生物（转录因子、表观遗传学、RNAi）的调控机制。关键术语——启动子、操纵基因、增强子、siRNA、RISC、甲基化、乙酰化——必须准确使用。乳糖存在与缺失时的lac操纵子示意图或翻译步骤图是常见的考试要求，因此要练习凭记忆画出并标注它们。
Revise how mutations in regulatory proteins or epigenetic marks can lead to disease, and be ready to apply your knowledge to unfamiliar scenarios. With a clear understanding of the mechanisms and plenty of practice, you will be well equipped to tackle any gene expression question on the paper.
复习调控蛋白突变或表观遗传标记如何导致疾病，并准备好将知识应用于陌生情境。通过清晰地理解机制和大量练习，你将能够从容应对试卷中任何关于基因表达的题目。
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