📚 Transcription for IGCSE WJEC Biology | IGCSE WJEC 生物：转录 考点精讲
Transcription is the fundamental biological process that converts the genetic information stored in DNA into a portable messenger RNA (mRNA) copy. For IGCSE WJEC Biology students, mastering transcription is essential because it bridges the gap between the static genetic code and the dynamic synthesis of proteins. This article covers every exam-critical aspect, from the role of RNA polymerase to the precise base pairing rules, ensuring you can tackle any related question with confidence.
转录是将储存在 DNA 中的遗传信息转化为可携带的信使 RNA (mRNA) 副本的基本生物学过程。对于 IGCSE WJEC 生物学的学生来说，掌握转录至关重要，因为它连接了静态的遗传密码和动态的蛋白质合成。本文涵盖了从 RNA 聚合酶的作用到精确的碱基配对规则等所有考试重点，确保你能够自信地解答任何相关问题。
1. What is Transcription? | 什么是转录？
Transcription is the process by which a specific segment of DNA is used as a template to synthesise a complementary single-stranded RNA molecule. In eukaryotic cells, this occurs inside the nucleus, while in prokaryotes it takes place in the cytoplasm. The resulting molecule is messenger RNA (mRNA), which carries the genetic instructions from the DNA to the ribosomes for protein production. Unlike DNA replication, transcription only copies a single gene or a small set of genes, making it a highly selective and regulated process.
转录是指以特定 DNA 片段为模板，合成互补单链 RNA 分子的过程。在真核细胞中，这一过程发生在细胞核内，而在原核生物中则在细胞质中进行。产生的分子是信使 RNA (mRNA)，它将遗传指令从 DNA 携带到核糖体以供蛋白质生产。与 DNA 复制不同，转录只复制单个基因或一小组基因，因此是一个高度选择性和受调控的过程。
The significance of transcription lies in its role as the first step of gene expression. Without transcription, the hereditary information would remain locked inside the DNA double helix, unable to direct the activities of the cell. The mRNA transcript serves as a temporary and mobile copy that can be decoded by ribosomes during translation.
转录的重要性在于它是基因表达的第一步。没有转录，遗传信息就会被锁在 DNA 双螺旋内部，无法指导细胞的活动。mRNA 转录本作为一种临时且可移动的副本，在翻译过程中可被核糖体解码。
2. The Key Enzyme: RNA Polymerase | 关键酶：RNA 聚合酶
The central enzyme responsible for transcription is RNA polymerase. This remarkable enzyme unwinds the DNA double helix over a short region, separates the two strands, and then catalyses the formation of phosphodiester bonds between incoming ribonucleotides to build the mRNA chain. Unlike DNA polymerase, RNA polymerase does not require a primer to begin synthesis; it can initiate nucleotide addition de novo.
负责转录的核心酶是 RNA 聚合酶。这种非凡的酶能在短区域内解开 DNA 双螺旋，分离两条链，然后催化进入的核糖核苷酸之间形成磷酸二酯键，以构建 mRNA 链。与 DNA 聚合酶不同，RNA 聚合酶不需要引物即可开始合成；它可以从头起始核苷酸的添加。
RNA polymerase moves along the template strand of DNA in the 3′ to 5′ direction, reading the nucleotide sequence and incorporating complementary RNA nucleotides in the growing mRNA strand that elongates in the 5′ to 3′ direction. The energy for this polymerisation comes from the breaking of high-energy phosphate bonds in the nucleoside triphosphates (ATP, UTP, CTP, GTP).
RNA 聚合酶沿着 DNA 模板链的 3′ 到 5′ 方向移动，读取核苷酸序列，并将互补的核糖核苷酸掺入正在延伸的 mRNA 链中，该链沿 5′ 到 3′ 方向延长。此聚合反应的能量来源于核苷三磷酸 (ATP、UTP、CTP、GTP) 中高能磷酸键的断裂。
3. Promoters and Initiation | 启动子与转录起始
Transcription does not begin randomly; it requires a specific DNA sequence called the promoter. The promoter is located just upstream of the gene to be transcribed and acts as a binding site for RNA polymerase. In many genes, the promoter contains a conserved sequence such as the TATA box (rich in thymine and adenine), which helps position the enzyme correctly.
转录并非随机开始；它需要一个称为启动子的特定 DNA 序列。启动子位于待转录基因的上游，充当 RNA 聚合酶的结合位点。在许多基因中，启动子含有一段保守序列，例如 TATA 盒（富含胸腺嘧啶和腺嘌呤），有助于正确安置酶的位置。
During initiation, RNA polymerase recognises and binds to the promoter, causing the DNA double helix to unwind locally. This exposes the template strand, and the first few ribonucleotides are paired with the DNA. The transcription initiation complex stabilises the open complex, and synthesis begins at the +1 site—the first nucleotide to be transcribed into mRNA.
在起始阶段，RNA 聚合酶识别并结合到启动子上，导致 DNA 双螺旋局部解旋。这暴露出模板链，前几个核糖核苷酸与 DNA 配对。转录起始复合物稳定开放复合体，合成从 +1 位点——被转录成 mRNA 的第一个核苷酸——开始。
4. Template Strand versus Coding Strand | 模板链与编码链
Only one of the two DNA strands serves as the template for transcription. This strand is known as the template strand or antisense strand, and it is read in the 3′ to 5′ direction. The other strand, which is not transcribed, is called the coding strand or sense strand because its nucleotide sequence (with thymine replaced by uracil) is identical to the mRNA sequence produced.
DNA 的两条链中只有一条充当转录的模板。这条链被称为模板链或反义链，它按 3′ 到 5′ 方向被读取。另一条不被转录的链称为编码链或有义链，因为其核苷酸序列（胸腺嘧啶替换为尿嘧啶）与所产生的 mRNA 序列一致。
This distinction is crucial for predicting mRNA sequences in exam questions. If you are given a DNA sequence, you must identify which strand is the template. Typically, the template strand runs antiparallel to the new mRNA, and the mRNA will be complementary to the template and a near-copy of the coding strand (with U replacing T).
这一区别对于考试题目中预测 mRNA 序列至关重要。如果给你一段 DNA 序列，你必须识别哪条链是模板链。通常，模板链与新 mRNA 反向平行，而 mRNA 将与模板链互补，并与编码链几乎完全相同（只是 U 替代了 T）。
5. Elongation: Building the mRNA Chain | 延伸：构建 mRNA 链
Once initiation is complete, RNA polymerase advances along the template strand, continuously unwinding the DNA ahead and rewinding it behind. The enzyme maintains a transcription bubble of approximately 15–20 base pairs where the DNA is temporarily single-stranded. Inside this bubble, complementary RNA nucleotides are added to the growing 3′ end of the mRNA.
一旦起始完成，RNA 聚合酶就沿着模板链推进，在前方不断解开 DNA，并在后方重新缠绕。该酶维持一个大约 15–20 个碱基对的转录泡，在此区域内 DNA 暂时为单链。在这个转录泡内，互补的核糖核苷酸被添加到正在生长的 mRNA 的 3′ 端。
Each incoming ribonucleotide forms hydrogen bonds with its complementary DNA base (A–U, T–A, C–G, G–C) before RNA polymerase catalyses the formation of a phosphodiester bond between the 3′ hydroxyl group of the existing chain and the 5′ phosphate of the new nucleotide. This process repeats rapidly, and the mRNA chain grows stepwise in the 5′→3′ direction.
每个进入的核糖核苷酸在与互补的 DNA 碱基形成氢键（A–U、T–A、C–G、G–C）之后，RNA 聚合酶催化现有链的 3′ 羟基与新核苷酸的 5′ 磷酸之间形成磷酸二酯键。该过程快速重复，mRNA 链沿 5′→3′ 方向逐步增长。
6. Base Pairing Rules in Transcription | 转录中的碱基配对规则
When RNA polymerase reads the template strand, it follows strict complementary base pairing rules. The most important difference from DNA replication is that uracil (U) replaces thymine (T) in RNA. This means that an adenine (A) on the DNA template pairs with uracil in the growing mRNA, while a thymine (T) on the template still pairs with adenine (A) in the mRNA.
当 RNA 聚合酶读取模板链时，它遵循严格的互补碱基配对规则。与 DNA 复制最重要的区别在于，在 RNA 中尿嘧啶 (U) 代替了胸腺嘧啶 (T)。这意味着 DNA 模板上的腺嘌呤 (A) 与正在生长的 mRNA 中的尿嘧啶配对，而模板上的胸腺嘧啶 (T) 仍与 mRNA 中的腺嘌呤 (A) 配对。
| DNA Template Base (模板碱基) | mRNA Base (mRNA 碱基) |
|---|---|
| Adenine (A) | Uracil (U) |
| Thymine (T) | Adenine (A) |
| Cytosine (C) | Guanine (G) |
| Guanine (G) | Cytosine (C) |
For example, if the template strand has the sequence 3′-TACG-5′, the mRNA produced will be 5′-AUGC-3′. Remember that the mRNA sequence is always written in the 5′ to 3′ direction, which is the direction of synthesis.
例如，如果模板链序列为 3′-TACG-5′，则生成的 mRNA 将为 5′-AUGC-3′。请记住，mRNA 序列始终按 5′ 到 3′ 的方向书写，这也是合成的方向。
7. Termination of Transcription | 转录的终止
Transcription does not continue indefinitely; it ends when RNA polymerase reaches a termination signal in the DNA sequence. In prokaryotes, specific terminator sequences form a hairpin loop in the RNA that destabilises the transcription complex, causing the mRNA and RNA polymerase to dissociate from the DNA template.
转录不会无限进行；当 RNA 聚合酶到达 DNA 序列中的终止信号时，转录就会结束。在原核生物中，特定的终止子序列会在 RNA 中形成一个发夹环，此结构使转录复合物不稳定，导致 mRNA 和 RNA 聚合酶从 DNA 模板上脱落。
In eukaryotic cells, termination is coupled with the addition of a poly-A tail; a specific sequence signals the enzyme to cleave the pre-mRNA, after which the mRNA is released and processed further. Regardless of the organism, the result is a freed mRNA molecule that is ready to be used in translation, while RNA polymerase becomes available for a new round of transcription.
在真核细胞中，终止与多聚腺苷酸尾的添加相耦合；一段特定序列提示酶切割前体 mRNA，之后 mRNA 被释放并进一步加工。不论哪种生物，最终结果都是释放出一个 mRNA 分子，准备用于翻译，而 RNA 聚合酶可再次用于新一轮的转录。
8. From mRNA to Translation – The Next Step | 从 mRNA 到翻译——下一步
Once transcription is complete in eukaryotes, the mRNA molecule typically undergoes processing—including the addition of a 5′ cap and a poly-A tail, and sometimes splicing to remove introns—before it exits the nucleus. However, for the IGCSE WJEC examination, the core requirement is to understand that the mRNA now carries a sequence of codons, each consisting of three nucleotides, which are read by ribosomes during translation to assemble amino acids into a polypeptide.
在真核细胞中，转录完成后，mRNA 分子通常会经历加工——包括添加 5′ 帽子和多聚腺苷酸尾，有时还会剪接以去除内含子——之后才离开细胞核。不过，对于 IGCSE WJEC 考试，核心要求是理解 mRNA 现在携带一系列密码子，每个密码子由三个核苷酸组成，在翻译过程中被核糖体读取，从而将氨基酸组装成多肽。
The continuity from gene to protein can be summarised as: DNA → mRNA → polypeptide. Transcription is therefore not an isolated event; it is the indispensable link that converts the stable genetic archive into a functional RNA messenger, setting the stage for protein synthesis. This central dogma of molecular biology is a favourite topic in WJEC papers.
从基因到蛋白质的连续性可以概括为：DNA → mRNA → 多肽。因此，转录不是一个孤立的事件；它是一个不可或缺的环节，将稳定的遗传档案转化为可用的 RNA 信使，为蛋白质合成奠定基础。分子生物学的这个中心法则是 WJEC 试卷的热门考点。
9. Transcription vs DNA Replication – Key Differences | 转录与 DNA 复制的关键区别
Students often confuse transcription with DNA replication. The table below highlights the critical differences that examiners frequently test:
学生经常将转录与 DNA 复制混淆。下表突出了考官经常测试的关键区别：
| Feature (特征) | DNA Replication (DNA 复制) | Transcription (转录) |
|---|---|---|
| Purpose (目的) | To duplicate the entire genome for cell division | To copy a specific gene or genes into mRNA |
| Enzyme (酶) | DNA polymerase | RNA polymerase |
| Product (产物) | Double-stranded DNA | Single-stranded mRNA |
| Base pairing (碱基配对) | A–T, T–A, C–G, G–C | A–U, T–A, C–G, G–C |
| Primer needed? (需要引物?) | Yes (RNA primer) | No |
| Template (模板) | Both strands of DNA | Only one strand (template strand) |
| Location in eukaryotes (真核细胞中的位置) | Nucleus | Nucleus |
This comparison reinforces the idea that transcription is far more selective and produces a transient RNA copy rather than a permanent DNA duplicate. In the exam, you may be asked to explain why uracil is used instead of thymine, which is linked to RNA’s shorter lifespan and its role as a temporary message.
这一对比强化了转录的选择性远高于复制，且产生的是瞬时 RNA 副本而非永久的 DNA 副本的观念。考试中可能要求你解释为什么使用尿嘧啶而不是胸腺嘧啶，这与 RNA 较短的寿命及其作为临时信使的功能有关。
10. Common Exam Questions and Tips for WJEC IGCSE | WJEC IGCSE 常见考题与技巧
1. Describe the sequence of events during transcription, from initiation to termination. Make sure you include the roles of the promoter, RNA polymerase, complementary base pairing, and termination sequences. Marks are awarded for correct terminology and stepwise order.
1. 描述转录过程中从起始到终止的事件顺序。确保包括启动子、RNA 聚合酶、互补碱基配对和终止序列的作用。评分会奖励正确的术语和分步顺序。
2. Given a DNA coding strand sequence, deduce the mRNA sequence. Many candidates lose marks by not replacing T with U or by forgetting that the mRNA is complementary to the template, not the coding strand. Always determine the template strand first: if the coding strand is 5′-ATGC-3′, the template is 3′-TACG-5′, so the mRNA is 5′-AUGC-3′.
2. 给定一段 DNA 编码链序列，推断出 mRNA 序列。许多考生因忘记将 T 替换为 U，或者忘记 mRNA 与模板链互补而非与编码链互补而失分。务必先确定模板链：如果编码链是 5′-AT
Published by TutorHao | IGCSE Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导