📚 Transcription in IB and WJEC Biology | 转录：IB 与 WJEC 生物考点精讲
Transcription is the fundamental process by which genetic information encoded in DNA is copied into messenger RNA (mRNA). Understanding this mechanism is essential for both IB Biology and WJEC A-level Biology, as it bridges the gap between the stable storage of genetic instructions and their functional expression in the cell. This article breaks down every key concept, from initiation to termination, highlights differences between prokaryotes and eukaryotes, and addresses common exam pitfalls.
转录是将 DNA 中编码的遗传信息复制到信使 RNA (mRNA) 中的基本过程。理解这一机制对于 IB 生物和 WJEC A-level 生物都至关重要，因为它是连接遗传信息的稳定储存与其在细胞中功能性表达的桥梁。本文分解了从起始到终止的每一个核心概念，突出了原核生物与真核生物的区别，并针对常见考试误区进行了剖析。
1. Overview of Transcription | 转录概述
Transcription is the synthesis of a single-stranded RNA molecule from a DNA template. It is catalysed by the enzyme RNA polymerase, which reads the template DNA strand in the 3′ to 5′ direction and builds a complementary RNA strand in the 5′ to 3′ direction. The RNA produced is identical in sequence to the coding DNA strand (with uracil replacing thymine). This process is the first step in gene expression and occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes.
转录是以 DNA 为模板合成单链 RNA 分子的过程。该过程由 RNA 聚合酶催化，该酶沿 3′→5′ 方向读取模板 DNA 链，并以 5′→3′ 方向合成互补的 RNA 链。生成的 RNA 序列与编码 DNA 链相同（除尿嘧啶替代胸腺嘧啶外）。这是基因表达的第一步，在真核生物的细胞核和原核生物的细胞质中进行。
The main types of RNA produced are mRNA (carries the code for protein synthesis), tRNA (transfers amino acids), and rRNA (structural component of ribosomes). In both syllabi, focus is primarily on mRNA transcription.
产生的主要 RNA 类型有 mRNA（携带蛋白质合成密码）、tRNA（转运氨基酸）和 rRNA（核糖体的结构组分）。在两个教学大纲中，重点都主要放在 mRNA 转录上。
2. Key Components: DNA Template and RNA Polymerase | 关键组分：DNA 模板与 RNA 聚合酶
The DNA double helix must unwind to expose the template strand. Only one of the two strands acts as the template for transcription; this is the antisense strand. The other strand is the sense or coding strand. RNA polymerase binds to a promoter region and separates the strands locally, forming a transcription bubble. It does not require a primer — unlike DNA polymerase, it can start synthesis de novo.
DNA 双螺旋必须解旋以暴露模板链。两条链中只有一条作为转录模板，即反义链。另一条链为有义链或编码链。RNA 聚合酶与启动子区域结合，并在局部将两条链分开，形成转录泡。它不需要引物——与 DNA 聚合酶不同，它可以从头开始合成。
In prokaryotes, there is a single type of RNA polymerase that synthesises all RNA. In eukaryotes, three main types exist: RNA polymerase I (for rRNA), RNA polymerase II (for mRNA and some small RNAs), and RNA polymerase III (for tRNA and other small RNAs). RNA polymerase II is the key enzyme for mRNA synthesis and is the focus of most exam questions.
在原核生物中，仅有一种 RNA 聚合酶负责所有 RNA 的合成。真核生物中存在三种主要类型：RNA 聚合酶 I（合成 rRNA）、RNA 聚合酶 II（合成 mRNA 和部分小 RNA）以及 RNA 聚合酶 III（合成 tRNA 和其他小 RNA）。RNA 聚合酶 II 是合成 mRNA 的关键酶，也是多数考题的核心。
3. Initiation: Promoters and Sigma Factors | 起始：启动子与 σ 因子
Initiation begins when RNA polymerase recognises a specific DNA sequence called the promoter. In prokaryotes, the promoter contains two conserved regions at -10 (TATAAT) and -35 relative to the transcription start site. A sigma factor (σ) associated with the core polymerase enables this specific binding. Once bound, the DNA unwinds and the first few nucleotides are joined.
起始阶段从 RNA 聚合酶识别称为启动子的特定 DNA 序列开始。在原核生物中，启动子含有相对于转录起始位点的 -10 区（TATAAT）和 -35 区两个保守区域。与核心聚合酶结合的 σ 因子使其能够特异性结合。一旦结合，DNA 解旋，最初几个核苷酸得以连接。
In eukaryotes, the process is more complex. RNA polymerase II is directed to the promoter by general transcription factors such as TFIID, which binds to the TATA box. This forms a transcription initiation complex. The mediator complex and activator proteins further regulate the rate of transcription, making eukaryotic initiation a highly controlled event.
在真核生物中，过程更为复杂。RNA 聚合酶 II 在通用转录因子（如 TFIID）的引导下结合至启动子，TFIID 与 TATA 框结合，形成转录起始复合体。中介复合体和激活蛋白进一步调控转录速率，使真核生物的转录起始成为一个高度受控的事件。
4. Elongation: Building the RNA Strand | 延伸：RNA 链的合成
After initiation, the sigma factor is released in prokaryotes, and the core RNA polymerase moves along the template DNA. The enzyme adds ribonucleotides complementary to the DNA template: adenine pairs with uracil (in RNA), cytosine with guanine, and so on. The incoming nucleotides are in the form of nucleoside triphosphates (ATP, UTP, GTP, CTP); cleavage of two phosphate groups provides the energy for bond formation.
起始之后，原核生物中的 σ 因子被释放，核心 RNA 聚合酶沿模板 DNA 移动。该酶添加与 DNA 模板互补的核糖核苷酸：腺嘌呤与尿嘧啶配对（在 RNA 中），胞嘧啶与鸟嘌呤配对，等等。掺入的核苷酸以核苷三磷酸（ATP、UTP、GTP、CTP）形式存在；两个磷酸基团的断裂为磷酸二酯键的形成提供能量。
Elongation continues at about 40–80 nucleotides per second in prokaryotes. The transcription bubble moves along the DNA, and the newly synthesised RNA peels away from the template, allowing the DNA double helix to reform. Proofreading mechanisms exist, though less robust than those of DNA replication.
原核生物中延伸以每秒约 40–80 个核苷酸的速度进行。转录泡沿 DNA 移动，新合成的 RNA 从模板上剥离，DNA 双螺旋重新形成。虽然存在校正机制，但不如 DNA 复制的校正那么强大。
5. Termination: Rho-dependent and Independent | 终止：依赖 ρ 因子与不依赖 ρ 因子的终止
In prokaryotes, termination occurs by two main mechanisms. Rho-independent termination relies on a GC-rich palindrome followed by a string of uracils in the RNA. The palindrome forms a hairpin loop, which destabilises the RNA-DNA hybrid, causing RNA polymerase to pause and release the transcript. The weak A-U base pairing further aids detachment.
原核生物中终止通过两种主要机制实现。不依赖 ρ 因子的终止依赖于 RNA 中一串富含 GC 的回文序列和随后的一串尿嘧啶。回文序列形成发夹环，破坏 RNA-DNA 杂交分子的稳定性，导致 RNA 聚合酶暂停并释放转录产物。脆弱的 A-U 碱基配对进一步促进脱离。
Rho-dependent termination requires the Rho protein, which is an ATP-dependent helicase. Rho binds to a rut site on the RNA, translocates along it, and catches up with the paused polymerase, unwinding the RNA-DNA duplex and causing termination. In eukaryotes, termination mechanisms are more intricate and often involve cleavage of the pre-mRNA downstream of the polyadenylation signal.
依赖 ρ 因子的终止需要 Rho 蛋白，它是一种依赖 ATP 的解旋酶。Rho 与 RNA 上的 rut 位点结合，沿 RNA 移位，追赶暂停的聚合酶，解开 RNA-DNA 双链并导致终止。真核生物的终止机制更为复杂，通常涉及在多聚腺苷酸化信号下游切割前体 mRNA。
6. Eukaryotic Transcription: RNA Polymerases I, II, III | 真核转录：RNA 聚合酶 I、II、III
Eukaryotic cells compartmentalise transcription inside the nucleus, and each RNA polymerase is dedicated to specific RNA classes. RNA polymerase I synthesises the 45S precursor rRNA, which is processed into 18S, 5.8S, and 28S rRNAs. RNA polymerase II produces all protein-coding mRNA and many non-coding RNAs like snRNA. RNA polymerase III makes tRNA, 5S rRNA, and other small RNAs.
真核细胞将转录区室化在细胞核内，每种 RNA 聚合酶专司特定的 RNA 类别。RNA 聚合酶 I 合成 45S 前体 rRNA，该前体被加工成 18S、5.8S 和 28S rRNA。RNA 聚合酶 II 产生所有编码蛋白质的 mRNA 以及许多非编码 RNA，如 snRNA。RNA 聚合酶 III 合成 tRNA、5S rRNA 和其他小 RNA。
For IB and WJEC exams, the central role of RNA polymerase II in synthesising pre-mRNA and the subsequent processing steps are most significant. The other polymerases are typically mentioned in the context of ribosomal and transfer RNA production.
对 IB 和 WJEC 考试而言，RNA 聚合酶 II 在合成前体 mRNA 中的核心作用及其后续加工步骤最为重要。其他聚合酶通常是在核糖体 RNA 和转运 RNA 产生的背景下被提及。
7. Post-transcriptional Modifications | 转录后修饰
In eukaryotes, the primary transcript (pre-mRNA) undergoes extensive processing before it can be translated. A 5′ cap (7-methylguanosine) is added shortly after initiation; it protects the mRNA from degradation and aids ribosomal binding. A poly-A tail (approximately 200 adenine residues) is added to the 3′ end, which also enhances stability and facilitates export from the nucleus.
在真核生物中，初级转录产物（前体 mRNA）在翻译前需经过广泛加工。起始后不久即添加一个 5′ 帽子（7-甲基鸟苷），它可保护 mRNA 免遭降解并协助核糖体结合。在 3′ 端添加 poly-A 尾（约 200 个腺苷酸残基），这同样能增强稳定性并促进向核外输出。
Splicing is the removal of non-coding introns and the joining of exons. This step is carried out by the spliceosome, a large complex of snRNPs (small nuclear ribonucleoproteins). Alternative splicing allows a single gene to produce multiple protein variants, a key topic emphasised in IB higher level and relevant for WJEC depth of understanding.
剪接是指去除非编码内含子并连接外显子的过程。该步骤由剪接体——一个由 snRNP（小核核糖核蛋白）组成的大型复合体执行。可变剪接使单个基因能够产生多种蛋白质变体，这是 IB 高级课程中的重点话题，也与 WJEC 的深入理解要求相关。
8. Differences between Prokaryotic and Eukaryotic Transcription | 原核与真核转录的差异
A comparison table is useful for exam revision:
一个对比表格有助于考前复习：
| Feature 特征 | Prokaryotes 原核生物 | Eukaryotes 真核生物 |
| Location 位置 | Cytoplasm 细胞质 | Nucleus 细胞核 |
| RNA polymerase(s) RNA 聚合酶 | One type 一种类型 | Three main types (I, II, III) 三种主要类型 |
| Promoter recognition 启动子识别 | Sigma factor σ 因子 | General transcription factors (e.g. TFIID) 通用转录因子 |
| mRNA processing mRNA 加工 | Rare; mRNA often polycistronic 罕见；mRNA 常为多顺反子 | 5′ capping, splicing, 3′ polyadenylation; monocistronic 5′加帽、剪接、3′加尾；单顺反子 |
| Coupling with translation 与翻译偶联 | Simultaneous 同时进行 | Separated by nuclear membrane 被核膜隔开 |
9. Directionality and Strands: Template vs Coding | 方向性与链：模板链与编码链
Understanding strand nomenclature is critical. The template strand is read in the 3′ to 5′ direction, while the RNA strand grows in the 5′ to 3′ direction. The coding strand has the same sequence as the RNA (with T instead of U) and is often shown in textbook diagrams. Examiners expect you to identify which strand is used for transcription by looking at promoter orientation.
理解链的命名至关重要。模板链沿 3′→5′ 方向被读取，而 RNA 链沿 5′→3′ 方向延伸。编码链与 RNA 具有相同的序列（T 代替 U），常见于教科书图表中。考官期望你能够通过观察启动子方向来确定哪条链用于转录。
An easy rule: the RNA polymerase moves along the template strand from 3′ to 5′; thus, the template’s 3′ end corresponds to the upstream promoter region. In diagrams, the coding strand is often labelled ‘sense’, and the template ‘antisense’. Being able to deduce the RNA sequence when given a template segment is a standard exam skill.
一个简易规则：RNA 聚合酶沿模板链从 3′ 端向 5′ 端移动；因此模板链的 3′ 端对应上游启动子区域。在示意图中，编码链常标注为“有义”，模板链为“反义”。能够根据给定的模板片段推断 RNA 序列是一项标准的考试技能。
10. Regulation and the Role of Transcription Factors | 调控与转录因子的作用
Gene expression is primarily controlled at the level of transcription. Specific transcription factors bind to enhancer or silencer DNA sequences and either activate or repress transcription. In eukaryotes, the promoter-proximal elements and distant enhancers interact through DNA looping, bringing activator proteins into contact with the transcription initiation complex.
基因表达主要在转录水平受到调控。特异性转录因子与增强子或沉默子 DNA 序列结合，从而激活或抑制转录。在真核生物中，启动子近端元件和远端增强子通过 DNA 成环相互作用，使激活蛋白与转录起始复合体接触。
Operons, such as the lac operon in bacteria, illustrate how repressors and inducers control transcription in prokaryotes. While operons are predominantly a prokaryotic feature, the principle of regulatory proteins is universal. For IB students, the environment-gene interaction in E. coli is a classic example; WJEC students also study the lac operon as a model of gene regulation.
操纵子，如细菌中的乳糖操纵子，展示了阻遏物和诱导物如何控制原核转录。虽然操纵子主要是原核生物的特征，但调控蛋白的原理是普遍存在的。对 IB 学生而言，大肠杆菌中环境与基因的相互作用是经典示例；WJEC 学生同样将乳糖操纵子作为基因调控的模型进行学习。
11. Common Misconceptions and Exam Tips | 常见误区与考试技巧
One frequent mistake is confusing transcription with DNA replication. Remember: replication produces a DNA copy; transcription uses a DNA template to make RNA. Another common error is reversing the direction of synthesis — RNA polymerase always adds nucleotides to the 3′ end of the growing RNA chain. Also, students often forget that uracil replaces thymine in RNA.
一个常见错误是将转录与 DNA 复制混淆。记住：复制产生的是 DNA 副本；而转录是利用 DNA 模板制备 RNA。另一个常见错误是颠倒了合成方向——RNA 聚合酶始终在新生 RNA 链的 3′ 端添加核苷酸。此外，学生们经常忘记 RNA 中尿嘧啶替代了胸腺嘧啶。
When labelling diagrams, clearly mark the template and coding strands, the direction of movement of RNA polymerase, and the 5′ and 3′ ends of all nucleic acids. In extended response questions, always discuss the role of transcription factors and the significance of post-transcriptional modifications for eukaryotes. Using precise terminology such as ‘antisense strand’, ‘promoter’, ‘phosphodiester bond’, and ‘hairpin loop’ earns high marks.
在标注图表时，要清晰标出模板链和编码链、RNA 聚合酶的移动方向以及所有核酸的 5′ 端和 3′ 端。在论述题中，一定要讨论转录因子的作用以及真核生物转录后修饰的意义。使用精确的术语，如“反义链”、“启动子”、“磷酸二酯键”、“发夹环”，将赢得高分。
12. Connections to Translation and Disease | 与翻译和疾病的关联
Transcription is intimately linked to translation; in prokaryotes they occur simultaneously, while in eukaryotes the processed mRNA must be exported to the cytoplasm. Errors in transcription or its regulation can lead to diseases such as cancer. For instance, mutations in tumour suppressor genes (like p53) often involve defective transcriptional control. Certain antibiotics, like rifampicin, inhibit prokaryotic RNA polymerase, making them clinically relevant examples.
转录与翻译紧密相连；在原核生物中它们可同时进行，而在真核生物中经过加工的 mRNA 须输出到细胞质。转录或其调控中的错误可导致癌症等疾病。例如，抑癌基因（如 p53）的突变往往涉及转录控制缺陷。某些抗生素如利福平可抑制原核 RNA 聚合酶，这使其成为临床相关实例。
Both IB and WJEC specifications favour integrative questions that link transcription to protein synthesis, mutations, and biotechnology (e.g., reverse transcription PCR). Understanding transcription is thus not an isolated topic but a cornerstone of molecular biology.
IB 和 WJEC 的考试大纲都偏爱将转录与蛋白质合成、突变和生物技术（如逆转录 PCR）联系起来的综合题。因此，理解转录并非孤立的主题，而是分子生物学的基石。
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