📚 CCEA A-Level Biology: Transcription | 转录考点精讲
Transcription is the fundamental process by which genetic information encoded in DNA is copied into a complementary messenger RNA (mRNA) molecule. It is the first essential step in gene expression, allowing the information stored in the nucleus to be translated into proteins. For CCEA A‑Level Biology, you must understand the detailed molecular mechanisms, the differences between prokaryotic and eukaryotic systems, and the critical post‑transcriptional modifications that occur in eukaryotes.
转录是将DNA中编码的遗传信息复制到互补信使RNA（mRNA）分子中的基本过程。它是基因表达的第一个关键步骤，使得储存在细胞核中的信息能够被翻译成蛋白质。对于CCEA A‑Level生物学，你必须理解详细的分子机制、原核与真核系统之间的差异，以及真核生物中发生的关键的转录后修饰。
1. The Central Dogma and Transcription Overview | 中心法则与转录总览
The central dogma of molecular biology states that DNA makes RNA, and RNA makes protein. Transcription is the DNA‑directed synthesis of RNA. Only one strand of the DNA duplex serves as the template for RNA synthesis; this strand is read in the 3′ to 5′ direction, and the newly synthesised RNA chain grows in the 5′ to 3′ direction. The enzyme responsible for catalysing transcription is DNA‑dependent RNA polymerase.
分子生物学的中心法则指出DNA制造RNA，RNA制造蛋白质。转录是以DNA为模板指导RNA合成的过程。DNA双链中只有一条链作为RNA合成的模板；该模板链以3′到5′方向被读取，新合成的RNA链沿5′到3′方向延伸。负责催化转录的酶是依赖于DNA的RNA聚合酶。
In prokaryotes, transcription occurs in the cytoplasm and can be coupled directly with translation. In eukaryotes, transcription takes place inside the nucleus, and the primary transcript must undergo several processing steps before it becomes mature mRNA capable of being exported and translated.
在原核生物中，转录发生在细胞质中，并可直接与翻译偶联。在真核生物中，转录在细胞核内进行，初级转录物必须经过若干加工步骤，才能成为能够输出并翻译的成熟mRNA。
2. Template Strand and Coding Strand | 模板链与编码链
Of the two DNA strands, the one that is transcribed into RNA is called the template strand or antisense strand. Its sequence is complementary to the RNA transcript. The opposite strand is the coding strand or sense strand; its sequence is identical to the RNA sequence (with thymine replaced by uracil) and is often presented when describing a gene’s sequence.
在两条DNA链中，被转录为RNA的那条链称为模板链或反义链。其序列与RNA转录物互补。相对的链是编码链或有义链；其序列与RNA序列完全相同（胸腺嘧啶被尿嘧啶取代），在描述基因序列时通常呈现的就是这条链。
Example:
DNA coding strand: 5′‑ATGCGT‑3′
DNA template strand: 3′‑TACGCA‑5′
mRNA transcript: 5′‑AUGCGU‑3′
During transcription, RNA polymerase reads the template strand from 3′ to 5′, and polymerises ribonucleotides to produce a complementary RNA molecule in a 5′→3′ direction.
在转录过程中，RNA聚合酶从3′到5′方向读取模板链，并以5′→3′方向聚合核糖核苷酸，生成互补的RNA分子。
3. RNA Polymerase: Structure and Function | RNA聚合酶：结构与功能
DNA‑dependent RNA polymerase catalyses the formation of phosphodiester bonds between ribonucleoside triphosphates (NTPs: ATP, GTP, CTP, UTP). The reaction requires Mg²⁺ ions and releases pyrophosphate (PPi) with each nucleotide addition. Unlike DNA polymerase, RNA polymerase does not require a primer and can initiate synthesis de novo. It also possesses limited proof‑reading activity.
依赖于DNA的RNA聚合酶催化核糖核苷三磷酸（NTPs：ATP、GTP、CTP、UTP）之间形成磷酸二酯键。该反应需要Mg²⁺离子，每添加一个核苷酸就释放一分子焦磷酸（PPi）。与DNA聚合酶不同，RNA聚合酶不需要引物，能够从头起始合成。它也具有有限的校对活性。
In E. coli, a single type of RNA polymerase synthesises all RNA classes. The core enzyme consists of five subunits (α₂ββ′ω), but it requires a sigma factor (σ) to bind specifically to promoter sequences. The holoenzyme is α₂ββ′ωσ. Eukaryotes possess three nuclear RNA polymerases: RNA polymerase I (rRNA), RNA polymerase II (mRNA and some snRNA) and RNA polymerase III (tRNA, 5S rRNA). CCEA candidates must be able to associate Pol II with mRNA synthesis.
在大肠杆菌中，单一类型的RNA聚合酶合成所有种类的RNA。核心酶由五个亚基组成（α₂ββ′ω），但它需要σ因子才能特异性结合启动子序列。全酶的构成是α₂ββ′ωσ。真核生物拥有三种细胞核RNA聚合酶：RNA聚合酶I（合成rRNA）、RNA聚合酶II（合成mRNA和一些snRNA）以及RNA聚合酶III（合成tRNA和5S rRNA）。CCEA考生必须能够将Pol II与mRNA合成关联起来。
4. Prokaryotic Promoters and Initiation | 原核生物的启动子与转录起始
A promoter is a DNA sequence located upstream of a gene that provides a binding site for RNA polymerase. In prokaryotes, two conserved hexameric sequences are critical: the –10 region (Pribnow box, consensus TATAAT) and the –35 region (consensus TTGACA). The sigma factor recognises and binds to the –35 and –10 elements, positioning the RNA polymerase holoenzyme to form a closed complex. Subsequently, the DNA around the –10 region unwinds over approximately 14 bases, creating the open complex. The first few ribonucleotides are joined, and once a short RNA chain (about 10 nucleotides) has been synthesised, the sigma factor typically dissociates, marking the transition to the elongation phase.
启动子是位于基因上游、为RNA聚合酶提供结合位点的DNA序列。在原核生物中，两个保守的六碱基序列至关重要：–10区（Pribnow框，共有序列TATAAT）和–35区（共有序列TTGACA）。σ因子识别并结合至–35与–10元件，将RNA聚合酶全酶定位以形成闭合复合物。随后，–10区附近的DNA解旋大约14个碱基，形成开放复合物。最初几个核糖核苷酸被连接，一旦合成了短的RNA链（约10个核苷酸），σ因子通常会脱落，标志着进入延伸阶段。
5. Eukaryotic Promoters and Transcription Factors | 真核启动子与转录因子
Eukaryotic promoters are more complex. Many protein‑coding genes contain a TATA box (consensus TATAAAA) about 25–35 base pairs upstream of the transcription start site, a CAAT box and GC‑rich elements. Assembly of the transcription initiation complex requires general transcription factors (GTFs). The TATA‑binding protein (TBP), a subunit of TFIID, binds to the TATA box and distorts the DNA. TFIIB then helps recruit RNA polymerase II, and other factors (TFIIE, TFIIF, TFIIH) join the complex. TFIIH possesses helicase activity that unwinds the DNA and a kinase that phosphorylates the C‑terminal domain (CTD) of Pol II, triggering the transition to elongation. Enhancer and silencer sequences, which can be located far from the promoter, bind activator and repressor proteins to fine‑tune transcription rates.
真核启动子更为复杂。许多蛋白质编码基因在转录起始位点上游约25–35个碱基对处含有一个TATA框（共有序列TATAAAA），此外还有CAAT框和富含GC的元件。转录起始复合物的组装需要通用转录因子（GTFs）。TATA结合蛋白（TBP）是TFIID的一个亚基，与TATA框结合并使DNA变形。TFIIB随后协助招募RNA聚合酶II，其他因子（TFIIE、TFIIF、TFIIH）再加入复合物。TFIIH具有解旋酶活性，可解开DNA双链，同时还具有激酶活性，能磷酸化Pol II的C末端结构域（CTD），从而启动向延伸阶段的转换。增强子和沉默子序列可以位于远离启动子的位置，分别结合激活蛋白和阻遏蛋白，以微调转录速率。
6. Elongation of the RNA Chain | RNA链的延伸
During elongation, RNA polymerase moves along the template strand, unwinding the DNA ahead and rewinding it behind. A transcription bubble of approximately 17 base pairs is maintained, with an RNA–DNA hybrid of about 8 nucleotides. Ribonucleoside triphosphates enter through a channel and are added to the 3′‑OH end of the growing RNA chain, forming new phosphodiester bonds and releasing pyrophosphate. The rate of elongation in E. coli is about 40–50 nucleotides per second. The polymerase pauses at certain sequences and can proofread by reversing and cleaving misincorporated nucleotides, a process stimulated by Gre factors in bacteria and TFIIS in eukaryotes.
在延伸过程中，RNA聚合酶沿着模板链移动，在前方解开双链，后方重新卷绕。维持一个大约17个碱基对的转录泡，其中RNA–DNA杂交体大约8个核苷酸。核糖核苷三磷酸通过通道进入，被添加到生长中RNA链的3′‑OH端，形成新的磷酸二酯键，并释放焦磷酸。大肠杆菌中延伸速率约为每秒40–50个核苷酸。聚合酶在某些序列处会暂停，并可通过反向移动并切除错误掺入的核苷酸进行校对；该过程在细菌中由Gre因子刺激，在真核生物中由TFIIS刺激。
7. Termination of Transcription in Prokaryotes | 原核生物转录的终止
Prokaryotes employ two principal mechanisms of termination. Rho‑independent (intrinsic) termination relies on a terminator sequence that is transcribed into an RNA hairpin immediately followed by a stretch of 6–8 uridines. The hairpin causes RNA polymerase to pause, and the weak A‑U base pairs between the U‑rich RNA and the template DNA allow the transcript to dissociate. Rho‑dependent termination requires the Rho protein, an ATP‑dependent helicase that binds to a C‑rich, G‑poor rut site on the nascent RNA, translocates along the RNA, and catches up with the paused polymerase, unwinding the RNA–DNA hybrid and releasing the transcript.
原核生物采用两种主要的终止机制。不依赖ρ（内在）终止依赖于一个终止子序列，该序列转录出的RNA形成发夹结构，紧接着是一段6–8个尿苷。发夹结构使RNA聚合酶暂停，而富含U的RNA与模板DNA链之间较弱的A–U碱基配对促使转录物释放。依赖ρ的终止需要ρ蛋白，它是一种ATP依赖性解旋酶，结合到新生RNA上富含C、贫G的rut位点，沿RNA移动，追上暂停的聚合酶，解开RNA–DNA杂交体并释放转录物。
8. Termination in Eukaryotes | 真核生物的转录终止
Termination for RNA polymerase II is coupled with RNA processing. After the enzyme transcribes past the polyadenylation signal (AAUAAA), an endonuclease cleaves the nascent RNA downstream of this signal. The 5′ piece receives a poly‑A tail, while the polymerase continues transcribing and soon terminates. Two models explain the final disengagement: the allosteric model, in which passage through the poly‑A signal induces a conformational change in the polymerase, and the torpedo model, in which a 5′‑exonuclease degrades the newly exposed downstream RNA and catches up with the polymerase to destabilise it. In contrast, RNA polymerase I and III use specific termination factors but follow principles more akin to prokaryotic termination.
RNA聚合酶II的终止与RNA加工相偶联。当该酶转录通过聚腺苷酸化信号（AAUAAA）之后，一种核酸内切酶在该信号下游切割新生RNA。5′端片段被加上了poly‑A尾，而聚合酶继续转录并很快终止。有两种模型解释最终的脱离：变构模型认为经过poly‑A信号引起聚合酶构象变化，而鱼雷模型认为一种5′‑核酸外切酶降解新暴露出的下游RNA并追上聚合酶，使其失去稳定性。相比之下，RNA聚合酶I和III使用特异的终止因子，但遵循与更接近原核终止的原理。
9. Post‑transcriptional Modifications of Eukaryotic mRNA | 真核mRNA的转录后修饰
The primary transcript (pre‑mRNA) in eukaryotes is not yet functional. It must undergo three major modifications inside the nucleus: 5′ capping, 3′ polyadenylation, and RNA splicing.
真核生物中的初级转录物（前体mRNA）尚不具备功能。它必须在细胞核内经历三种主要的修饰：5′加帽、3′聚腺苷酸化和RNA剪接。
5′ capping: Early in transcription, a 7‑methylguanosine cap is added to the 5′ end of the RNA via a 5′‑5′ triphosphate linkage. This cap protects the mRNA from exonucleases, assists in export from the nucleus, and promotes ribosome binding during translation.
5′加帽：在转录早期，通过一个5′‑5′三磷酸键将一个7‑甲基鸟苷帽添加到RNA的5′端。该帽保护mRNA免受核酸外切酶降解、协助从细胞核输出，并在翻译时促进核糖体结合。
3′ polyadenylation: After cleavage at the poly‑A signal, poly(A) polymerase adds approximately 200 adenine nucleotides to the 3′ end, forming the poly‑A tail. This tail increases mRNA stability and facilitates translation initiation.
3′聚腺苷酸化：在poly‑A信号处切割之后，poly(A)聚合酶在3′端添加约200个腺嘌呤核苷酸，形成poly‑A尾。这一尾部增强mRNA稳定性并促进翻译起始。
RNA splicing: Eukaryotic genes often contain introns (non‑coding sequences) that must be removed and exons (coding sequences) that are ligated together. The process is catalysed by the spliceosome, a large complex comprising small nuclear ribonucleoproteins (snRNPs: U1, U2, U4, U5, U6). Key conserved sequences at the intron boundaries are the GU at the 5′ splice site, an internal branch point A, and the AG at the 3′ splice site. Through two trans‑esterification reactions, the intron is excised as a lariat and the exons are joined. Alternative splicing allows a single gene to produce multiple protein isoforms by including or excluding different exons – a key concept for CCEA candidates.
RNA剪接：真核基因通常含有内含子（非编码序列），这些内含子需要被去除，而外显子（编码序列）则连接在一起。该过程由剪接体催化，剪接体是一个由小核核糖核蛋白（snRNPs：U1、U2、U4、U5、U6）组成的大型复合物。内含子边界的关键保守序列是5′剪接位点的GU、内部的分支点A以及3′剪接位点的AG。通过两次转酯反应，内含子以套索形式被切除，外显子被连接。可变剪接使得一个基因通过包含或排除不同外显子产生多种蛋白质亚型——这是CCEA考生需要掌握的关键概念。
10. Comparing Prokaryotic and Eukaryotic Transcription | 原核与真核转录的比较
The following table summarises the major differences that CCEA exam questions often target.
下表总结了CCEA考试中常考的主要差异。
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Location | Cytoplasm; coupling with translation | Nucleus; transcription and translation are separated |
| RNA polymerase | Single type (α₂ββ′ωσ) | Three types: Pol I, Pol II (mRNA), Pol III |
| Promoter recognition | Sigma factor binds –35 and –10 boxes | General transcription factors (TFIID, TFIIB etc.) bind TATA box and recruit Pol II |
| Termination | Rho‑independent (hairpin + U‑stretch) or Rho‑dependent | Pol II: coupled to poly‑A signal cleavage; torpedo/allosteric models |
| Post‑transcriptional processing | Very rare; mRNA used directly | 5′ capping, 3′ poly‑A tail, intron splicing, alternative splicing |
| Operon organisation | Polycistronic mRNA common | Monocistronic mRNA typical |
In addition, inhibitors such as rifampicin (which binds bacterial RNA polymerase) and α‑amanitin (which blocks Pol II) can be used to demonstrate the specificity of transcription mechanisms in different organisms. Understanding these differences is essential for answering extended‑response questions on transcription control and gene expression.
此外，诸如利福平（结合细菌RNA聚合酶）和α‑鹅膏蕈碱（阻断Pol II）等抑制剂可用来表明不同生物转录机制的特异性。理解这些差异对于回答有关转录调控和基因表达的拓展性题目至关重要。
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