Transcription | 转录 - aleveler.com

📚 Transcription | 转录

Transcription is the first step of gene expression, where a specific segment of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase. This process is fundamental to all living organisms, as it bridges the genetic information stored in DNA with the production of proteins. In eukaryotes, transcription occurs inside the nucleus, and the primary RNA transcript undergoes several modifications before it becomes a mature mRNA molecule ready for translation.

转录是基因表达的第一步，指特定的DNA片段被RNA聚合酶复制为信使RNA（mRNA）的过程。这一过程对所有生物体都至关重要，因为它连接了储存在DNA中的遗传信息和蛋白质的制造。在真核生物中，转录发生在细胞核内，且初级RNA转录本需要经历一系列修饰，才能成为可供翻译的成熟mRNA分子。

1. Overview of Transcription | 转录概述

Transcription is the synthesis of a single‑stranded RNA molecule from a double‑stranded DNA template. The goal is to produce an mRNA copy that carries the genetic code from the nucleus to the ribosomes in the cytoplasm. Unlike DNA replication, only one strand of DNA serves as the template, and the product is RNA, which contains uracil instead of thymine.

转录是以双链DNA为模板合成单链RNA分子的过程。其目标是生成一个携带遗传密码的mRNA副本，将信息从细胞核传递到细胞质中的核糖体。与DNA复制不同，转录只以一条DNA链为模板，且产物是RNA，其中用尿嘧啶（U）代替了胸腺嘧啶（T）。

The enzyme responsible for transcription is DNA‑dependent RNA polymerase. In eukaryotes, RNA polymerase II synthesises pre‑mRNA. The process can be divided into three main stages: initiation, elongation, and termination. In eukaryotes, the primary transcript then undergoes post‑transcriptional processing before functioning as mature mRNA.

负责转录的酶是DNA依赖性RNA聚合酶。在真核生物中，RNA聚合酶II合成前体mRNA。整个过程可分为三个主要阶段：起始、延伸和终止。真核生物中，初级转录本还需经过转录后加工，才能作为成熟的mRNA行使功能。

2. Key Enzymes and Molecules | 关键酶与分子

RNA polymerase is the central enzyme that catalyses the formation of phosphodiester bonds between ribonucleotides, building the RNA chain in the 5′ → 3′ direction. It does not require a primer and can initiate de novo synthesis. In eukaryotes, RNA polymerase II requires a set of general transcription factors, such as TFIID and TFIIB, to recognise the promoter and begin transcription.

RNA聚合酶是核心酶，催化核糖核苷酸之间形成磷酸二酯键，沿5’→3’方向构建RNA链。它不需要引物，能够从新开始合成。在真核生物中，RNA聚合酶II需要一组通用转录因子（如TFIID和TFIIB）来识别启动子并启动转录。

Complementary base pairing governs the incorporation of nucleotides: adenine (A) in the DNA template pairs with uracil (U) in the RNA, thymine (T) pairs with adenine (A), cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C). Free ribonucleoside triphosphates (ATP, UTP, GTP, CTP) provide both the monomers and the energy for polymerisation.

核苷酸的掺入遵循互补碱基配对原则：DNA模板中的腺嘌呤（A）与RNA中的尿嘧啶（U）配对，胸腺嘧啶（T）与腺嘌呤（A）配对，胞嘧啶（C）与鸟嘌呤（G）配对，鸟嘌呤（G）与胞嘧啶（C）配对。游离的核糖核苷三磷酸（ATP、UTP、GTP、CTP）既提供单体，也提供聚合所需的能量。

3. Template vs Coding Strand | 模板链与编码链

During transcription, only one of the two DNA strands is used as a template for RNA synthesis; this is called the template strand or antisense strand. The other DNA strand is the coding strand or sense strand, which has the same sequence as the newly made RNA (with T instead of U). By convention, gene sequences are often written as the coding strand, but RNA polymerase reads the template strand in the 3′ → 5′ direction while synthesising RNA in the 5′ → 3′ direction.

在转录过程中，DNA双链中只有一条被用作RNA合成的模板，这条链称为模板链或反义链。另一条DNA链是编码链或有义链，其序列与新合成的RNA相同（只是T代替U）。按照惯例，基因序列通常按编码链书写，但RNA聚合酶以3’→5’方向阅读模板链，同时沿5’→3’方向合成RNA。

This arrangement ensures that the RNA transcript carries the correct genetic code for translation. The coding strand is so named because its sequence corresponds to the codons that will be translated into amino acids, except that the mature mRNA will have U in place of T.

这种安排确保RNA转录本携带有适合翻译的遗传密码。编码链之所以如此命名，是因为它的序列与将被翻译成氨基酸的密码子一致，只是成熟mRNA中用U替换了T。

4. Initiation of Transcription | 转录起始

Initiation begins when RNA polymerase, guided by transcription factors, binds to a specific DNA region called the promoter. In eukaryotes, promoters often contain a TATA box (consensus sequence TATAAA) located about 25‑35 base pairs upstream of the transcription start site. General transcription factors, particularly TFIID, recognise the TATA box and recruit RNA polymerase II to form the pre‑initiation complex.

起始阶段始于RNA聚合酶在转录因子的引导下结合到特定的DNA区域——启动子。在真核生物中，启动子常包含一个TATA框（共有序列TATAAA），位于转录起始位点上游约25‑35个碱基对处。通用转录因子，特别是TFIID，识别TATA框并招募RNA聚合酶II，形成前起始复合物。

Once assembled, RNA polymerase unwinds a short stretch of the DNA double helix, creating a transcription bubble. This exposes the template strand, and the first few ribonucleotides are positioned by complementary base pairing. The initial phosphodiester bond is formed without a primer, and the enzyme releases most of the transcription factors before proceeding into elongation.

一旦组装完成，RNA聚合酶解开一小段DNA双螺旋，形成转录泡。这使模板链暴露出来，最初的几个核糖核苷酸通过互补碱基配对定位。第一个磷酸二酯键在没有引物的情况下形成，酶随即释放大多数转录因子，之后进入延伸阶段。

5. Elongation | 链延长

During elongation, RNA polymerase moves along the template strand, unwinding the DNA ahead and rewinding it behind. The enzyme maintains a transcription bubble of approximately 17 base pairs. Ribonucleotides complementary to the template are added one by one to the growing RNA chain, with the 3’‑OH group of the nascent RNA attacking the α‑phosphate of the incoming nucleoside triphosphate, releasing pyrophosphate.

在延伸过程中，RNA聚合酶沿模板链移动，在前方解开DNA螺旋并在后方重新缠绕。该酶维持约17个碱基对的转录泡。与模板互补的核糖核苷酸逐个添加到增长的RNA链上，新生RNA的3’‑OH基团攻击进入的核苷三磷酸的α‑磷酸，释放出焦磷酸。

The rate of elongation in eukaryotes is around 20–40 nucleotides per second. The enzyme has proofreading capabilities, though less efficient than DNA polymerases. The RNA transcript peels away from the DNA template as the polymerase advances, allowing the DNA double helix to reform.

真核生物中的延伸速度大约为每秒20–40个核苷酸。该酶具有一定的校正能力，但比DNA聚合酶要低。随着聚合酶向前推进，RNA转录本从DNA模板上剥离，使DNA双螺旋得以重新形成。

6. Termination | 转录终止

Transcription termination in eukaryotes differs significantly from that in prokaryotes. For protein‑coding genes transcribed by RNA polymerase II, termination is coupled with cleavage and polyadenylation. The polymerase continues beyond the coding region until it transcribes a polyadenylation signal sequence, typically AAUAAA in the RNA.

真核生物的转录终止与原核生物显著不同。对于RNA聚合酶II转录的蛋白质编码基因，终止与切割及多聚腺苷酸化相偶联。聚合酶会继续转录，越过编码区，直到转录出多聚腺苷酸化信号序列，在RNA中通常为AAUAAA。

Proteins associated with the RNA recognise this signal, cut the nascent RNA downstream of the signal, and add a poly(A) tail. The RNA polymerase, now transcribing uncapped and unprocessed RNA, becomes unstable and eventually dissociates from the DNA template, completing termination.

与RNA结合的蛋白质识别这一信号，在信号下游切割新生RNA，并添加poly(A)尾巴。此时RNA聚合酶正在转录无帽且未加工的RNA，变得不稳定，最终从DNA模板上脱落，完成终止。

7. mRNA Processing: Capping and Tailing | mRNA加工：加帽与加尾

In eukaryotes, the primary RNA transcript (pre‑mRNA) is extensively modified before it can be translated. Soon after transcription begins, the 5′ end receives a 7‑methylguanosine cap. This 5′ cap protects the mRNA from degradation by exonucleases, facilitates export from the nucleus, and assists in ribosome binding during translation initiation.

在真核生物中，初级RNA转录本（前体mRNA）在翻译前需要进行广泛的修饰。转录开始后不久，5’端会接收一个7‑甲基鸟苷帽子（5′ cap）。这个帽子保护mRNA免受核酸外切酶降解，促进其从细胞核输出，并在翻译起始时帮助核糖体结合。

At the 3′ end, after cleavage at the polyadenylation signal, a poly(A) tail consisting of about 200 adenine residues is added by poly(A) polymerase. This tail also protects the mRNA, enhances its stability, and plays a role in the regulation of translation. The length of the poly(A) tail influences the lifespan of the mRNA in the cytoplasm.

在3’端，于多聚腺苷酸化信号处切割后，poly(A)聚合酶会添加一条约含200个腺嘌呤残基的poly(A)尾巴。这条尾巴同样保护mRNA，增强其稳定性，并在翻译调控中发挥作用。poly(A)尾的长度会影响mRNA在细胞质中的寿命。

8. RNA Splicing: Introns and Exons | RNA剪接：内含子与外显子

Eukaryotic genes contain coding regions called exons, interrupted by non‑coding sequences called introns. After capping and tailing, the pre‑mRNA undergoes splicing, a process that removes introns and joins exons together. Splicing is catalysed by a large ribonucleoprotein complex called the spliceosome, which is composed of small nuclear RNAs (snRNAs) and proteins.

真核基因含有编码区段（外显子），其间被非编码序列（内含子）隔开。在加帽和加尾之后，前体mRNA会进行剪接，即切除内含子并将外显子连接起来的过程。剪接由一种称为剪接体的大型核糖核蛋白复合物催化，剪接体由小核RNA（snRNA）和蛋白质组成。

The spliceosome recognises conserved sequences at the exon‑intron boundaries: the 5′ splice site (GU), the branch point adenine, and the 3′ splice site (AG). Through two sequential transesterification reactions, the intron is released as a lariat structure and the exons are ligated. Alternative splicing allows a single gene to produce multiple protein isoforms by combining different sets of exons.

剪接体识别外显子‑内含子交界处的保守序列：5’剪接位点（GU）、分支点腺嘌呤和3’剪接位点（AG）。通过两步连续的酯交换反应，内含子以套索结构释放，外显子连接起来。可变剪接使得单个基因可以通过组合不同的外显子集产生多种蛋白质异构体。

9. Differences Between RNA and DNA | RNA与DNA的差异

The product of transcription is RNA, which differs from DNA in several key aspects relevant to the A‑Level syllabus. RNA contains the sugar ribose, whereas DNA contains deoxyribose (lacking a hydroxyl group at the 2′ position). RNA uses the base uracil (U) in place of thymine (T). RNA is generally single‑stranded, although it can fold into complex secondary structures.

转录的产物是RNA，它在几个与A‑Level考纲相关的关键方面与DNA不同。RNA含有核糖，而DNA含有脱氧核糖（在2’位缺少一个羟基）。RNA使用尿嘧啶（U）代替胸腺嘧啶（T）。RNA通常是单链的，但也能折叠形成复杂的二级结构。

Structurally, the presence of the 2’‑OH group in ribose makes RNA more chemically reactive and less stable than DNA. This lability is actually exploited in cells, as mRNA molecules are designed to be transient, allowing rapid regulation of gene expression. DNA, being more stable, is better suited for long‑term storage of genetic information.

从结构上看，核糖中2’‑OH基团的存在使RNA比DNA更具化学反应活性，稳定性也较低。细胞实际上利用这种不稳定性，因为mRNA分子被设计为短暂存在，从而能够快速调控基因表达。DNA更为稳定，更适合长期储存遗传信息。

10. Importance of Transcription | 转录的重要性

Transcription is a critical control point in gene expression. By regulating which genes are transcribed and at what rate, cells can respond to internal and external signals, differentiate into specialised cell types, and maintain homeostasis. In multicellular organisms, differential gene expression through transcriptional control underlies development and tissue‑specific functions.

转录是基因表达中一个关键的控制节点。通过调控哪些基因被转录以及转录的速率，细胞能够响应内部和外部信号，分化为特化细胞类型，并维持稳态。在多细胞生物中，通过转录控制实现的差异基因表达，构成了发育和组织特异性功能的基础。

Errors in transcription or mutations affecting processing signals can lead to non‑functional proteins and disease. For example, mutations at splice sites can cause inherited disorders such as β‑thalassaemia. Understanding transcription is also fundamental for biotechnological applications, including recombinant protein production and gene therapy.

转录错误或影响加工信号的突变可能导致无功能蛋白质和疾病。例如，剪接位点的突变可引起遗传性疾病，如β‑地中海贫血。理解转录对于生物技术应用也至关重要，包括重组蛋白生产和基因治疗。

11. Comparison with Prokaryotic Transcription | 与原核生物转录的比较

Although A‑Level OCR focuses primarily on eukaryotic transcription, it is useful to note key differences in prokaryotes. Prokaryotes have a single type of RNA polymerase (core enzyme plus sigma factor) that transcribes all RNA types. Promoters have conserved –35 and –10 regions. There is no processing of mRNA: transcription and translation are coupled, occurring simultaneously in the cytoplasm.

尽管OCR A‑Level主要关注真核生物的转录，但了解原核生物的关键差异依然有所裨益。原核生物只有一种RNA聚合酶（核心酶加σ因子），负责转录所有类型的RNA。启动子具有保守的–35区和–10区。mRNA不经加工：转录与翻译偶联，在细胞质中同时进行。

Termination in prokaryotes can occur via rho‑dependent or rho‑independent (intrinsic) mechanisms, often relying on hairpin structures in the RNA. Unlike eukaryotes, prokaryotic genes are often organised into operons, allowing coordinated transcription of functionally related genes.

原核生物的终止可通过ρ依赖或非依赖（固有）机制进行，常依赖RNA中发卡结构的形成。与真核生物不同，原核基因通常组织成操纵子，允许功能相关基因的协同转录。

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Transcription | 转录