Transcription | 转录 - aleveler.com

📚 Transcription | 转录

Transcription is the fundamental biological process through which genetic information encoded in DNA is copied into messenger RNA (mRNA). This step initiates gene expression by converting a specific segment of DNA into a portable RNA transcript that can be translated into a polypeptide. Understanding transcription is essential for grasping how cells regulate protein synthesis and respond to environmental signals.

转录是将 DNA 中编码的遗传信息复制到信使 RNA（mRNA）的基本生物学过程。这一步骤通过将特定的 DNA 片段转化为可携带的 RNA 转录本，从而启动基因表达，该转录本随后可被翻译为多肽。理解转录对于掌握细胞如何调控蛋白质合成及响应环境信号至关重要。

1. Overview of Transcription | 转录概述

Transcription synthesises an RNA molecule complementary to the template strand of DNA. The enzyme RNA polymerase reads the DNA sequence in the 3′ to 5′ direction and builds the RNA strand in the 5′ to 3′ direction. Only one of the two DNA strands, the template strand (antisense strand), is transcribed; the other strand is known as the coding strand (sense strand) because its sequence matches the RNA transcript (with thymine replaced by uracil).

转录合成一条与 DNA 模板链互补的 RNA 分子。RNA 聚合酶以 3′ 到 5′ 方向读取 DNA 序列，并以 5′ 到 3′ 方向合成 RNA 链。DNA 的两条链中只有一条，即模板链（反义链）被转录；另一条链称为编码链（有义链），因为其序列与 RNA 转录本一致（仅胸腺嘧啶被尿嘧啶取代）。

In prokaryotes, transcription occurs in the cytoplasm, while in eukaryotes it takes place inside the nucleus. Both domains share a common mechanism, but eukaryotes require extensive post-transcriptional modification of the primary transcript before it becomes functional mRNA.

在原核生物中，转录发生在细胞质中，而在真核生物中则发生在细胞核内。两类生物共享相似的转录机制，但真核生物需要对初级转录本进行大量转录后修饰，才能形成有功能的 mRNA。

2. Key Players: RNA Polymerase and Promoters | 关键要素：RNA 聚合酶与启动子

RNA polymerase is the core enzyme that catalyses the formation of phosphodiester bonds between ribonucleotides. Unlike DNA polymerase, RNA polymerase does not require a primer and lacks proofreading activity. In prokaryotes, a single type of RNA polymerase transcribes all genes, while eukaryotes have three main types: RNA polymerase I (rRNA), RNA polymerase II (mRNA and some snRNA), and RNA polymerase III (tRNA and other small RNAs).

RNA 聚合酶是催化核糖核苷酸之间形成磷酸二酯键的核心酶。与 DNA 聚合酶不同，RNA 聚合酶不需要引物，也没有校对功能。在原核生物中，单一类型的 RNA 聚合酶负责所有基因的转录，而真核生物有三种主要的 RNA 聚合酶：RNA 聚合酶 I（转录 rRNA）、RNA 聚合酶 II（转录 mRNA 和部分 snRNA）以及 RNA 聚合酶 III（转录 tRNA 和其他小 RNA）。

Transcription begins at specific DNA sequences called promoters. A promoter contains consensus sequences such as the Pribnow box (TATAAT at -10) and the -35 region in prokaryotes, or the TATA box, CAAT box, and GC box in eukaryotes. These sequences orient RNA polymerase and facilitate the melting of the DNA double helix.

转录起始于特定的 DNA 序列，即启动子。启动子包含共有序列，例如原核生物中的 Pribnow 框（位于 -10 处的 TATAAT）和 -35 区域，或真核生物中的 TATA 框、CAAT 框及 GC 框。这些序列定向 RNA 聚合酶并促进 DNA 双螺旋的解链。

3. Initiation of Transcription | 转录起始

Initiation begins when RNA polymerase, together with general transcription factors (in eukaryotes) or sigma factors (in prokaryotes), binds to the promoter. The DNA double helix is locally unwound, forming a transcription bubble of about 17 base pairs. The first two ribonucleoside triphosphates are aligned complementary to the template strand, and RNA polymerase catalyses the formation of the first phosphodiester bond. After the synthesis of the first few nucleotides, sigma factor dissociates in prokaryotes, or the initiation complex undergoes a conformational change in eukaryotes, allowing the elongation phase to proceed.

起始时，RNA 聚合酶与通用转录因子（真核生物）或 σ 因子（原核生物）共同结合至启动子。DNA 双螺旋局部解旋，形成一个约 17 个碱基对的转录泡。前两个核糖核苷三磷酸按碱基互补配对原则与模板链对齐，RNA 聚合酶催化第一个磷酸二酯键的形成。在原核生物中，合成了最初几个核苷酸后 σ 因子便脱离；在真核生物中，起始复合物发生构象变化，从而进入延伸阶段。

The region of DNA that is transcribed into a single RNA molecule is called a transcription unit. It extends from the promoter to the terminator sequence. In eukaryotes, additional regulatory elements such as enhancers and silencers can be located far from the promoter but still influence transcription initiation through DNA looping.

被转录成一条 RNA 分子的 DNA 区域称为一个转录单位，其范围从启动子延伸至终止子序列。在真核生物中，额外的调控元件（如增强子和沉默子）可能位于远离启动子的位置，但通过 DNA 环化仍可影响转录起始。

4. Elongation: Building the RNA Chain | 延伸：构建 RNA 链

During elongation, RNA polymerase moves along the template strand in the 3′ to 5′ direction while synthesising RNA in the 5′ to 3′ direction. The enzyme maintains a transcription bubble and catalyses the sequential addition of ribonucleotides that are complementary to the template. The energy for polymerisation comes from the hydrolysis of the high-energy phosphate bonds of incoming nucleoside triphosphates (ATP, UTP, GTP, CTP).

在延伸阶段，RNA 聚合酶沿模板链的 3′ 到 5′ 方向移动，同时以 5′ 到 3′ 方向合成 RNA。该酶维持转录泡的存在，并催化核糖核苷酸逐个按照模板链的互补顺序添加。聚合反应所需的能量来自进入的核苷三磷酸（ATP、UTP、GTP、CTP）中高能磷酸键的水解。

The newly synthesised RNA strand dissociates from the DNA template behind the polymerase, allowing the DNA double helix to re-form. In prokaryotes, multiple RNA polymerases can transcribe the same gene simultaneously, leading to polyribosome-like structures before transcription is complete. In eukaryotes, nucleosomes must be temporarily displaced to allow passage of RNA polymerase.

新合成的 RNA 链在聚合酶后方与 DNA 模板分离，使 DNA 双螺旋得以重新形成。在原核生物中，多个 RNA 聚合酶可以同时转录同一基因，在转录完成前即可形成类似多聚核糖体的结构。在真核生物中，核小体必须暂时移位以允许 RNA 聚合酶通过。

The error rate of transcription is approximately 1 in 10⁴ to 10⁵ nucleotides, which is higher than DNA replication but tolerated because many RNA copies are made and the RNA is transient.

转录的错误率约为每 10⁴ 到 10⁵ 个核苷酸中出现一个，高于 DNA 复制，但这可以被容忍，因为会生成大量 RNA 拷贝，且 RNA 是暂时性的。

5. Termination of Transcription | 转录终止

In prokaryotes, termination occurs by two main mechanisms: Rho-independent (intrinsic) termination and Rho-dependent termination. Intrinsic termination relies on a GC-rich hairpin loop in the RNA followed by a series of uracil residues, which causes RNA polymerase to pause and dissociate. Rho-dependent termination involves the Rho protein, an ATP-dependent helicase that binds to the RNA and translocates to the transcription complex, disrupting the RNA-DNA hybrid.

在原核生物中，终止主要通过两种机制实现：不依赖 Rho 因子（内在终止）和依赖 Rho 因子的终止。内在终止依赖于 RNA 中富含 GC 的发夹环结构及其后的一串尿嘧啶残基，这使 RNA 聚合酶暂停并脱离。依赖 Rho 因子的终止则需要 Rho 蛋白，这是一种 ATP 依赖性解旋酶，该蛋白结合到 RNA 上并移向转录复合物，破坏 RNA-DNA 杂合链。

In eukaryotes, termination of RNA polymerase II transcription is linked to a polyadenylation signal (AAUAAA). After RNA polymerase transcribes this signal, the nascent RNA is cleaved downstream by an endonuclease. The polymerase continues transcribing but eventually dissociates when a 5′-3′ exonuclease (Rat1 in yeast, Xrn2 in humans) degrades the trailing RNA and catches up with the polymerase (torpedo model).

在真核生物中，RNA 聚合酶 II 转录的终止与多聚腺苷酸化信号（AAUAAA）相关。RNA 聚合酶转录出这一信号后，新生的 RNA 在其下游被核酸内切酶切割。聚合酶虽然继续转录，但最终当一条 5′-3′ 外切核酸酶（酵母中为 Rat1，人类中为 Xrn2）降解尾随 RNA 并追上聚合酶时，聚合酶便脱离（鱼雷模型）。

6. Post-Transcriptional Modifications in Eukaryotes | 真核生物中的转录后修饰

Eukaryotic primary transcripts (pre-mRNA) undergo three major processing events before they can be translated: 5′ capping, 3′ polyadenylation, and splicing. These modifications increase mRNA stability, facilitate nuclear export, and enable efficient translation.

真核生物的初级转录本（前体 mRNA）在翻译前需经历三个主要的加工过程：5′ 加帽、3′ 多聚腺苷酸化和剪接。这些修饰可提高 mRNA 的稳定性、促进核输出，并确保高效翻译。

The 5′ cap is a 7-methylguanosine residue added to the first nucleotide of the transcript via a 5′-5′ triphosphate linkage. This cap protects the mRNA from 5′ exonucleases and is recognised by the cap-binding complex for translation initiation. The poly(A) tail, typically 150–250 adenosine residues, is added at the 3′ end after cleavage at the polyadenylation signal. It enhances stability and influences translation efficiency.

5′ 帽是一个 7-甲基鸟苷残基，通过 5′-5′ 三磷酸连接加在转录本的第一个核苷酸上。这顶帽子保护 mRNA 免受 5′ 外切核酸酶的降解，并被帽结合复合物识别以启动翻译。poly(A) 尾通常在切割多聚腺苷酸化信号后添加，由 150–250 个腺苷残基组成。它能增强稳定性并影响翻译效率。

7. RNA Splicing and the Spliceosome | RNA 剪接与剪接体

Most eukaryotic genes contain non-coding sequences (introns) interspersed among coding sequences (exons). During splicing, introns are precisely removed and exons are ligated to form a continuous coding sequence. The process is catalysed by a large ribonucleoprotein complex called the spliceosome, which consists of five small nuclear RNAs (U1, U2, U4, U5, U6) and numerous proteins.

大多数真核基因含有散布在编码序列（外显子）中的非编码序列（内含子）。在剪接过程中，内含子被精确切除，外显子连接起来形成连续的编码序列。该过程由一个称为剪接体的大型核糖核蛋白复合物催化，该复合物由五种小核 RNA（U1、U2、U4、U5、U6）及众多蛋白质组成。

Splicing relies on conserved splice-site sequences: GU at the 5′ end of the intron, AG at the 3′ end, and a branch point adenine near the 3′ end. The spliceosome assembles stepwise, catalysing two transesterification reactions that result in intron lariat formation and exon ligation. Alternative splicing allows a single gene to produce multiple protein isoforms by combining different exons, greatly expanding the proteome diversity.

剪接依赖于保守的剪接位点序列：内含子 5′ 端的 GU、3′ 端的 AG 以及靠近 3′ 端的分支点腺嘌呤。剪接体逐步装配，催化两步酯交换反应，导致内含子形成套索结构并被切除，外显子连接。可变剪接通过组合不同的外显子，使单个基因产生多种蛋白质异构体，大大扩展了蛋白质组的多样性。

8. Comparison of Prokaryotic and Eukaryotic Transcription | 原核与真核转录的比较

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA polymerase	One type	Three types (I, II, III)
Promoter recognition	Sigma factor binds -10 and -35	General transcription factors bind TATA, CAAT
Termination	Intrinsic (hairpin) or Rho-dependent	Linked to polyadenylation signal
Post-transcriptional processing	Rare; mRNA translated directly	5′ cap, poly(A) tail, splicing
Simultaneous transcription-translation	Yes	No (nuclear envelope separates processes)

该表总结了原核生物与真核生物在转录地点、RNA 聚合酶种类、启动子识别、终止机制、转录后加工以及转录翻译是否同步方面的关键差异。这些区别反映了真核细胞更复杂的基因调控和表达需求。

9. Regulation of Transcription | 转录调控

Transcriptional regulation determines which genes are expressed and at what levels. In prokaryotes, operons (e.g., the lac operon) allow coordinated control of genes with related functions. Repressor proteins bind operator sequences and block RNA polymerase, while activators enhance binding to weak promoters. The lac operon is regulated by both the lac repressor and catabolite activator protein (CAP), integrating signals from lactose and glucose availability.

转录调控决定了哪些基因表达及其表达水平。在原核生物中，操纵子（如乳糖操纵子）可协调控制功能相关的基因。阻遏蛋白结合操纵基因序列并阻碍 RNA 聚合酶，而激活蛋白则增强 RNA 聚合酶与较弱启动子的结合。乳糖操纵子同时受到乳糖阻遏蛋白和分解代谢物激活蛋白（CAP）的调控，整合来自乳糖和葡萄糖供应情况的信号。

In eukaryotes, gene regulation is more complex and involves chromatin remodelling, histone modifications (acetylation, methylation), DNA methylation, and a large repertoire of transcription factors that bind enhancers and silencers. Mediator complexes bridge activators to the basal transcription machinery. Epigenetic changes can modulate transcription without altering the DNA sequence.

在真核生物中，基因调控更为复杂，涉及染色质重塑、组蛋白修饰（乙酰化、甲基化）、DNA 甲基化以及大量结合增强子和沉默子的转录因子。中介复合物在激活因子与基础转录机器之间起桥梁作用。表观遗传变化可在不改变 DNA 序列的情况下调节转录。

10. Experimental Evidence and Key Techniques | 实验证据与关键技术

Understanding transcription has been advanced by techniques such as Northern blotting, reverse transcription PCR (RT-PCR), RNA sequencing, and chromatin immunoprecipitation (ChIP). Pulse-chase experiments demonstrated that RNA is synthesised in the nucleus and then exported to the cytoplasm. The discovery of split genes (introns and exons) came from electron microscopy of RNA-DNA hybrids (R-loops) where intron loops were visible.

Northern 印迹、反转录 PCR（RT-PCR）、RNA 测序和染色质免疫沉淀（ChIP）等技术推动了对转录的理解。脉冲追踪实验证明 RNA 在细胞核中合成，随后被输出到细胞质。断裂基因（内含子和外显子）的发现来自 RNA-DNA 杂交分子（R 环）的电镜观察，其中可见内含子形成的环状结构。

Run-on transcription assays and nuclear run-ons are used to measure transcription rates of specific genes. These approaches confirm that regulation can occur at the level of transcription initiation. Moreover, the use of α-amanitin, an inhibitor of RNA polymerase II, helped differentiate the functions of the three eukaryotic RNA polymerases.

连缀转录分析和细胞核连缀分析可用于测量特定基因的转录速率。这些方法证实调控可以发生在转录起始水平。此外，使用 RNA 聚合酶 II 抑制剂 α-鹅膏蕈碱帮助区分了三种真核 RNA 聚合酶的功能。

11. Clinical and Biotechnological Relevance | 临床与生物技术相关性

Defects in transcription and its processing are linked to numerous diseases. Mutations in splice sites can cause genetic disorders such as β-thalassemia. Dysregulation of transcription factors is a hallmark of many cancers; for example, the oncogene MYC is a transcription factor that promotes cell proliferation when overexpressed. Inhibitors of RNA polymerase are used as antibiotics (rifampicin targets bacterial RNA polymerase) and as chemotherapeutic agents.

转录及其加工过程的缺陷与多种疾病相关。剪接位点的突变可导致 β-地中海贫血等遗传病。转录因子失调是许多癌症的标志，例如癌基因 MYC 是一个转录因子，其过度表达会促进细胞增殖。RNA 聚合酶抑制剂被用作抗生素（利福平靶向细菌 RNA 聚合酶）和化疗药物。

In biotechnology, the ability to synthesise RNA in vitro using purified RNA polymerases underpins the production of mRNA vaccines and RNA interference technologies. CRISPR-based gene activation (CRISPRa) and interference (CRISPRi) systems directly manipulate transcription, enabling precise gene regulation studies and therapeutic applications.

在生物技术领域，利用纯化的 RNA 聚合酶在体外合成 RNA 的能力是 mRNA 疫苗和 RNA 干扰技术的基础。基于 CRISPR 的基因激活（CRISPRa）和干扰（CRISPRi）系统可直接操纵转录，使精确的基因调控研究和治疗应用成为可能。

12. Summary and Exam Tips | 总结与考试提示

Transcription is a tightly regulated, multi-step process that converts DNA into RNA. Key points for IB and AQA examination include: the directionality of synthesis (5’→3′ RNA, template read 3’→5′); the roles of promoters, RNA polymerase, and transcription factors; differences between prokaryotic and eukaryotic transcription; and the significance of post-transcriptional modifications such as capping, tailing, and splicing. Be prepared to interpret diagrams of transcription bubbles, identify coding versus template strands, and explain the consequences of mutations in regulatory regions or splice sites.

转录是一个受到精密调控的多步骤过程，将 DNA 转化为 RNA。IB 和 AQA 考试的关键点包括：合成的方向性（RNA 5’→3′ 合成，模板链 3’→5′ 读取）；启动子、RNA 聚合酶和转录因子的作用；原核与真核转录的区别；以及转录后修饰（加帽、加尾、剪接）的重要意义。准备好解读转录泡示意图，识别编码链与模板链，并解释调控区域或剪接位点突变的影响。

Common exam questions ask students to compare replication with transcription, outline the processing of pre-mRNA, or predict the effect of a mutation in a promoter element. Use precise terminology and always specify the organism (prokaryote vs. eukaryote) when answering.

常见考题包括比较复制与转录、概述前体 mRNA 的加工过程或预测启动子元件突变的影响。答题时请使用准确术语，并务必指明生物类型（原核或真核）。

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Transcription | 转录