📚 Transcription in IB & CIE Biology | IB CIE 生物：转录 考点精讲
Transcription is the fundamental process by which genetic information stored in DNA is copied into messenger RNA (mRNA). For both IB and CIE Biology students, a detailed understanding of transcription—from the structural role of RNA polymerase to the intricacies of eukaryotic post‑transcriptional modifications—is essential for exam success. In this comprehensive revision guide, we break down each stage, compare prokaryotic and eukaryotic systems, and highlight the most common pitfalls in assessment questions.
转录是将储存在DNA中的遗传信息复制为信使RNA (mRNA) 的基本过程。对IB和CIE生物学学生来说，透彻理解转录——从RNA聚合酶的结构功能到真核生物转录后修饰的微妙细节——是考试成功的关键。在这份全面的复习指南中，我们将分解每一个阶段，比较原核与真核系统，并突出评估题目中最常见的错误。
1. Overview of Transcription | 转录概述
Transcription is the first step of gene expression, during which a particular segment of DNA is used as a template to synthesise a complementary RNA strand.
转录是基因表达的第一步，在此过程中，一段特定的DNA序列被用作模板来合成互补的RNA链。
The enzyme responsible for this process is DNA‑dependent RNA polymerase, which reads the template strand in the 3′ to 5′ direction and builds the RNA transcript in the 5′ to 3′ direction.
负责此过程的酶是依赖DNA的RNA聚合酶，它沿3′ → 5′方向读取模板链，并以5′ → 3′方向合成RNA转录本。
Unlike DNA replication, transcription does not require a primer and only a short region of the DNA double helix is unwound at any one time, forming a transcription bubble.
与DNA复制不同，转录不需要引物，而且每次只解开DNA双螺旋的一小段区域，形成转录泡。
2. The Central Dogma and Gene Expression | 中心法则与基因表达
iption converts the DNA code into an mRNA intermediate, which is then translated into a polypeptide.
分子生物学的中心法则描述了遗传信息的流动方向：DNA → RNA → 蛋白质。转录将DNA密码转换为mRNA中间体，随后翻译为多肽。
In eukaryotes, the primary RNA transcript (pre‑mRNA) often requires extensive processing before it can be exported to the cytoplasm for translation. Prokaryotes lack a nucleus, so transcription and translation can occur simultaneously.
在真核生物中，初级RNA转录本 (前mRNA) 通常需要经过大量加工才能被输出到细胞质进行翻译。原核生物没有细胞核，因此转录和翻译可以同时进行。
Understanding this framework is vital because exam questions frequently ask you to predict the consequences of mutations on both transcription and the final protein product.
理解这一框架至关重要，因为考题经常会要求你预测突变对转录以及最终蛋白质产物的影响。
3. Key Players: DNA Template and RNA Polymerase | 关键参与者：DNA模板与RNA聚合酶
Only one of the two DNA strands, the template strand (also called the antisense strand), is transcribed. The other strand, the coding strand (sense strand), has the same sequence as the mRNA (with thymine replaced by uracil).
两条DNA链中只有模板链 (又称反义链) 被转录。另一条编码链 (有义链) 具有与mRNA相同的序列 (只是胸腺嘧啶被尿嘧啶取代)。
RNA polymerase recognises specific promoter sequences upstream of the gene. In prokaryotes, the sigma factor helps the core enzyme bind to the −10 (TATAAT) and −35 consensus sequences. In eukaryotes, a complex of general transcription factors assembles at the TATA box, recruiting RNA polymerase II for mRNA synthesis.
RNA聚合酶识别基因上游特定的启动子序列。在原核生物中，σ因子帮助核心酶结合到−10区 (TATAAT) 和−35区共有序列。真核生物中，一组通用转录因子在TATA盒上组装，招募RNA聚合酶II来合成mRNA。
RNA polymerase does not proofread as efficiently as DNA polymerase, leading to a higher error rate; however, the transient nature of RNA means these errors are less catastrophic.
RNA聚合酶的校对能力不如DNA聚合酶，导致错误率较高；但由于RNA寿命较短，这些错误造成的后果要轻得多。
4. The Genetic Code: Codons and Anticodons | 遗传密码：密码子与反密码子
During transcription, the genetic code is copied in the form of nucleotide triplets called codons. Each codon corresponds to a specific amino acid or a stop signal.
在转录过程中，遗传密码以称为密码子的核苷酸三联体形式被复制。每个密码子对应一种特定的氨基酸或终止信号。
The relationship between codons and amino acids is degenerate: multiple codons can code for the same amino acid. This redundancy often protects against point mutations.
密码子与氨基酸之间的关系具有简并性：多个密码子可以编码同一种氨基酸。这种冗余性通常能缓冲点突变的影响。
While codons reside on the mRNA, anticodons are found on transfer RNA (tRNA) molecules. Students often confuse the two; remember that anticodons base‑pair with codons during translation, not transcription.
密码子存在于mRNA上，而反密码子则位于转运RNA (tRNA) 分子上。学生经常混淆两者；请记住，反密码子是在翻译过程中与密码子进行碱基配对的，而非转录过程。
5. Initiation of Transcription | 转录的起始
Transcription begins when RNA polymerase binds to the promoter region. In E. coli, the σ⁷⁰ subunit directs the holoenzyme to the −10 and −35 elements, after which the DNA duplex is melted at the −10 sequence (Pribnow box).
当RNA聚合酶结合到启动子区域时，转录便开始了。在大肠杆菌中，σ⁷⁰亚基引导全酶结合−10和−35元件，随后DNA双链在−10序列 (普里布诺框) 处解链。
In eukaryotes, the TATA‑binding protein (TBP), a subunit of TFIID, binds to the TATA box, bending the DNA and enabling the assembly of the pre‑initiation complex. RNA polymerase II is then phosphorylated at its C‑terminal domain (CTD) to escape the promoter and begin elongation.
在真核生物中，TFIID的一个亚基——TATA结合蛋白 (TBP) 结合到TATA盒上，使DNA弯曲，并促成起始前复合体的组装。随后RNA聚合酶II的C端结构域 (CTD) 发生磷酸化，从而逃离启动子，开始延伸。
Exam tip: clearly state that the initial binding does not require ATP hydrolysis; however, promoter unwinding and escape are energy‑dependent steps.
考试技巧：要明确指出初始结合不需要ATP水解；然而，启动子解链和逃离是依赖能量的步骤。
6. Elongation: Building the mRNA Strand | 延伸：合成mRNA链
Once the transcription bubble is formed, RNA polymerase catalyses the formation of phosphodiester bonds between incoming ribonucleoside triphosphates (ATP, GTP, CTP, UTP) based on complementary base pairing with the template strand. The energy for polymerisation comes from the cleavage of high‑energy phosphate bonds.
一旦转录泡形成，RNA聚合酶便催化进入的核糖核苷三磷酸 (ATP, GTP, CTP, UTP) 根据与模板链的互补配对形成磷酸二酯键。聚合所需的能量来源于高能磷酸键的断裂。
The enzyme maintains a transcription bubble of about 17 base pairs in prokaryotes, rewinding the DNA behind the bubble as it advances. The RNA:DNA hybrid within the bubble is kept short (~8 bp) to ensure processivity.
在原核生物中，该酶维持约17个碱基对的转录泡，在前进的同时将泡后方的DNA重新盘旋。泡内的RNA:DNA杂交体保持很短 (约8 bp)，以确保持续合成能力。
In eukaryotes, elongation factors such as FACT facilitate the disassembly and reassembly of nucleosomes ahead of and behind the polymerase, allowing transcription through chromatin.
在真核生物中，延伸因子如FACT协助聚合酶前方和后方的核小体解聚与重新组装，从而使转录能够穿越染色质进行。
7. Termination of Transcription | 转录的终止
In bacteria, termination can be either intrinsic (Rho‑independent) or Rho‑dependent. Intrinsic termination relies on a GC‑rich hairpin loop followed by a poly‑U sequence, which destabilises the RNA:DNA hybrid and causes the polymerase to dissociate.
在细菌中，终止可以是内在终止 (不依赖Rho) 或依赖Rho的终止。内在终止依赖于一段富含GC的发夹结构及其后的多聚U序列，该序列破坏RNA:DNA杂交体的稳定性，导致聚合酶脱离。
Rho‑dependent termination requires the hexameric helicase Rho, which binds to a rut site on the nascent RNA and translocates towards the polymerase, unwinding the hybrid when it reaches the enzyme.
依赖Rho的终止需要六聚体解旋酶Rho蛋白，它结合新生RNA上的rut位点并向聚合酶方向移位，当抵达酶的位置时将杂交体解开。
Eukaryotic mRNA termination by RNA polymerase II is coupled to cleavage and polyadenylation. After a poly‑A signal (AAUAAA) is transcribed, the pre‑mRNA is cleaved downstream, and the polymerase eventually dissociates, aided by an exonuclease that degrades the remaining RNA behind it (the torpedo model).
真核生物RNA聚合酶II对mRNA的终止与切割及多聚腺苷酸化相偶联。当转录出多聚A信号 (AAUAAA) 后，前mRNA在下游被切割，聚合酶最终在一种核酸外切酶的帮助下脱离，该外切酶降解其后的残余RNA (鱼雷模型)。
8. Post‑transcriptional Modifications (Eukaryotes) | 转录后修饰（真核生物）
Eukaryotic pre‑mRNA undergoes three major modifications: 5′ capping, splicing, and 3′ polyadenylation. These steps are essential for mRNA stability, export, and efficient translation.
真核前mRNA经历三种主要修饰：5′加帽、剪接和3′多聚腺苷酸化。这些步骤对于mRNA的稳定性、核输出以及高效翻译至关重要。
These modifications occur co‑transcriptionally, meaning they begin while the RNA strand is still being synthesised. The C‑terminal domain (CTD) of RNA polymerase II coordinates the recruitment of capping, splicing, and polyadenylation factors.
这些修饰是共转录发生的，也就是在RNA链还在合成时就已经开始。RNA聚合酶II的C端结构域 (CTD) 协调招募加帽、剪接和多聚腺苷酸化因子。
Students frequently lose marks by suggesting that these modifications occur in prokaryotes; remember that prokaryotic mRNA is generally not processed in the same way and is often polycistronic.
学生常常因为声称这些修饰发生在原核生物中而丢分；请记住，原核mRNA通常不进行此类加工，而且往往是多顺反子。
9. mRNA Processing: 5′ Capping, Splicing, and 3′ Poly‑A Tail | mRNA加工：5′端加帽、剪接与3′端Poly‑A尾
The 5′ cap (7‑methylguanosine) is added to the triphosphate group at the 5′ end via a 5′‑to‑5′ triphosphate bridge. This cap protects the mRNA from exonucleases and is required for ribosome binding during translation.
5′帽 (7‑甲基鸟苷) 通过5′‑5′三磷酸桥加到5′端的三磷酸基团上。这顶帽子保护mRNA免受核酸外切酶降解，并且是翻译过程中核糖体结合所必需的。
Splicing removes non‑coding introns and joins coding exons together. The spliceosome, a large ribonucleoprotein complex, recognises splice sites at the intron‑exon boundaries and catalyses two transesterification reactions. Alternative splicing allows a single gene to produce multiple protein variants, a key concept in IB and CIE syllabi.
剪接去除非编码内含子，将编码外显子连接起来。剪接体是一个巨大的核糖核蛋白复合物，它识别内含子-外显子边界的剪接位点，并催化两次转酯反应。可变剪接允许一个基因产生多种蛋白质变体，这是IB和CIE教学大纲中的一个关键概念。
The 3′ poly‑A tail consists of around 200 adenine nucleotides added after cleavage of the pre‑mRNA. It further enhances stability and facilitates nuclear export. The length of the poly‑A tail influences the lifespan of the mRNA in the cytoplasm.
3′ Poly‑A尾由约200个腺嘌呤核苷酸组成，在前mRNA切割后添加。它进一步增强稳定性，促进核输出。Poly‑A尾的长度影响mRNA在细胞质中的寿命。
10. Comparison of Prokaryotic and Eukaryotic Transcription | 原核与真核转录的比较
Prokaryotic transcription occurs in the cytoplasm, is coupled with translation, and typically involves a single type of RNA polymerase. Genes are often organised into operons, and mRNA is polycistronic.
原核转录发生在细胞质中，与翻译相偶联，通常只涉及一种RNA聚合酶。基因常常组织成操纵子，mRNA为多顺反子。
Eukaryotic transcription takes place in the nucleus, is separated from translation, and uses three distinct RNA polymerases (I, II, III). Chromatin remodelling is necessary to give the polymerase access to the DNA.
真核转录发生在细胞核内，与翻译分隔开，并使用三种不同的RNA聚合酶 (I、II、III)。染色质重塑是聚合酶接近DNA的先决条件。
| 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 Polymerases / RNA聚合酶 | One / 一种 | Three (I, II, III) / 三种 |
| Promoter recognition / 启动子识别 | Sigma factor / σ因子 | General transcription factors / 通用转录因子 |
| Post‑transcriptional processing / 转录后加工 | Rare / 罕见 | Capping, splicing, poly‑A tail / 加帽, 剪接, Poly‑A尾 |
| Coupling with translation / 与翻译偶联 | Simultaneous / 同时 | Separated / 分隔 |
11. Transcriptional Control and Gene Regulation | 转录调控与基因表达调节
The regulation of transcription is a central mechanism controlling gene expression. In prokaryotes, the lac operon is the classic example: a repressor protein binds to the operator in the absence of lactose, blocking transcription. Allolactose acts as an inducer by inactivating the repressor.
转录调控是控制基因表达的核心机制。在原核生物中，乳糖操纵子是经典实例：没有乳糖时，阻遏蛋白结合到操纵基因上，阻挡转录。异乳糖作为诱导剂使阻遏蛋白失活。
Enhancers and silencers are DNA sequences that can be bound by activator or repressor proteins to increase or decrease the rate of transcription. These elements can act over large distances and are brought into proximity with the promoter through DNA looping.
增强子和沉默子是能被激活蛋白或阻遏蛋白结合的DNA序列，它们可以提高或降低转录速率。这些元件可以在远距离起作用，并通过DNA环化与启动子靠近。
Epigenetic modifications, such as DNA methylation and histone acetylation, also modulate the accessibility of DNA to the transcriptional machinery. Deacetylation of histones generally leads to chromatin condensation and gene silencing.
表观遗传修饰，如DNA甲基化和组蛋白乙酰化，也能调节DNA对转录机器的可接近性。组蛋白去乙酰化通常导致染色质凝聚和基因沉默。
In eukaryotes, the Mediator complex acts as a bridge between transcription factors bound to distal regulatory elements and the basal transcription apparatus at the promoter. Disruption of mediator subunits is linked to various diseases.
在真核生物中，中介体复合物充当结合于远端调控元件的转录因子与启动子处基础转录装置之间的桥梁。中介体亚基的破坏与多种疾病有关。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
is read 3′ → 5′. Confusing the coding strand with the template strand is a frequent error.
一定要说明转录的方向性 (5′ → 3′) 并指出模板链是按3′ → 5′方向读取的。将编码链与模板链混淆是一个常见错误。
When discussing eukaryotic gene expression, clearly distinguish between transcription and post‑transcriptional modifications. Many marks are lost by using the term “RNA editing” when the intended process is splicing or capping.
在讨论真核基因表达时，要清楚区分转录与转录后修饰。许多学生因为想表达的是剪接或加帽，却使用了“RNA编辑”一词而失分。
Be precise about the location of processes: transcription of a eukaryotic protein‑coding gene occurs in the nucleus, while translation occurs in the cytoplasm. A classic misconception is that ribosomes travel into the nucleus to translate mRNA.
务必准确说明过程发生的位置：真核蛋白编码基因的转录发生在细胞核，而翻译发生在细胞质。一个典型的误区是认为核糖体会进入细胞核去翻译mRNA。
Use appropriate terminology: introns are removed, exons are expressed. Alternative splicing can produce multiple mRNAs from the same primary transcript, greatly increasing proteomic diversity.
要使用恰当术语：内含子被切除，外显子被表达。可变剪接能从同一个初级转录本产生多种mRNA，极大地增加了蛋白质组多样性。
Finally, practice drawing and labelling a transcription bubble, including the template and coding strands, RNA polymerase, RNA transcript, and the direction of synthesis. Diagrams are often required and can secure easy marks.
最后，练习绘制并标注转录泡，包括模板链和编码链、RNA聚合酶、RNA转录本以及合成方向。图表往往是要求的作答形式，可以确保轻松得分。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图