📚 Transcription in A-Level Biology: Exam-Focused Guide | A-Level 生物：转录考点精讲
Transcription is the first step of gene expression, involving the transfer of genetic information from DNA to messenger RNA (mRNA). Understanding the intricate mechanisms of transcription—from initiation to termination and post-transcriptional modification—is essential for A-Level Biology. This revision guide breaks down every key concept, ensures clarity on commonly examined points, and provides exam-focused insights.
转录是基因表达的第一步，涉及遗传信息从DNA传递至信使RNA（mRNA）。理解转录的复杂机制——从起始、延伸、终止到转录后修饰——是A-Level生物考试的关键。本考点精讲分解每个核心概念，厘清高频考点，并提供应试要点。
1. Overview of Transcription | 转录概述
In transcription, an enzyme called RNA polymerase synthesises a single-stranded RNA molecule complementary to one of the two DNA strands. This RNA may be mRNA, tRNA, or rRNA, but our focus for protein-coding genes is on mRNA. The process occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotes. The newly made RNA carries the genetic code from the gene to the ribosomes for translation.
在转录中，RNA聚合酶以DNA双链中的一条为模板合成互补的单链RNA。这些RNA可以是mRNA、tRNA或rRNA，但编码蛋白质的基因主要产生mRNA。真核细胞的转录发生在细胞核内，原核生物则发生在细胞质中。新合成的RNA将基因的遗传密码携带至核糖体以进行翻译。
The basic steps include initiation (RNA polymerase binds to the promoter), elongation (the RNA chain grows), and termination (synthesis stops). In eukaryotes, the primary transcript then undergoes extensive processing: capping, polyadenylation, and splicing. Each step is tightly controlled to ensure accurate gene expression.
基本步骤包括起始（RNA聚合酶与启动子结合）、延伸（RNA链延长）和终止（合成停止）。在真核生物中，初级转录本随后还要经历复杂的加工：加帽、加尾和剪接。每个步骤都受到严格调控，以确保基因表达的准确性。
2. DNA Strands: Template vs. Coding | DNA 链：模板链与编码链
Transcription uses only one of the two DNA strands as a template. The template strand (antisense strand) is read by RNA polymerase in the 3′ to 5′ direction, allowing RNA to be synthesised in the 5′ to 3′ direction. The other strand is known as the coding strand (sense strand) and carries the same nucleotide sequence as the RNA transcript, except that thymine (T) is replaced by uracil (U) in RNA.
转录只以DNA双链中的一条作为模板。模板链（反义链）被RNA聚合酶以3’至5’方向读取，从而使RNA以5’至3’方向合成。另一条链称为编码链（有义链），其核苷酸序列与RNA转录本相同，只是RNA中用尿嘧啶（U）替代了胸腺嘧啶（T）。
Exam questions often ask you to identify the template strand from a given double-stranded DNA sequence and then predict the mRNA sequence. Remember: the mRNA is complementary to the template strand and identical to the coding strand (U substituted for T). A common pitfall is to confuse the two strands—check the direction of transcription carefully.
考题经常要求从给定的双链DNA序列中确认模板链，并推导出mRNA序列。请牢记：mRNA与模板链互补，并且与编码链相同（只是U代替T）。一个常见的误区是混淆这两条链——仔细核对转录方向。
3. RNA Polymerase and Promoters | RNA 聚合酶与启动子
RNA polymerase is the core enzyme responsible for RNA synthesis. Unlike DNA polymerase, it does not require a primer and can initiate de novo synthesis. In eukaryotes, RNA polymerase II transcribes protein-coding genes to produce pre-mRNA. In prokaryotes, a single type of RNA polymerase synthesises all RNA molecules.
RNA聚合酶是负责合成RNA的核心酶。与DNA聚合酶不同，它不需要引物，可以直接从头开始合成。在真核生物中，RNA聚合酶II负责将编码蛋白质的基因转录为前体mRNA。原核生物中仅有一种RNA聚合酶合成所有类型的RNA分子。
The enzyme binds to a specific DNA sequence called the promoter, located upstream of the transcription start site. Eukaryotic promoters often contain a TATA box (consensus sequence TATAAA) around position -25. Prokaryotic promoters have conserved -10 (TATAAT) and -35 (TTGACA) regions. General transcription factors (GTFs) assist RNA polymerase in recognising and binding to the promoter, forming the preinitiation complex.
RNA聚合酶结合到转录起始位点上游一段称为启动子的特定DNA序列。真核启动子通常包含位于-25位附近的TATA盒（一致序列为TATAAA）。原核启动子具有保守的-10区（TATAAT）和-35区（TTGACA）。通用转录因子帮助RNA聚合酶识别并稳定结合启动子，形成前起始复合体。
4. Initiation: The Beginning of Transcription | 转录的起始
Initiation begins when RNA polymerase and general transcription factors assemble at the promoter to form a closed complex. The DNA double helix then unwinds over approximately 14 base pairs, creating an open complex (transcription bubble). The first ribonucleoside triphosphate is placed at the +1 start site, and the enzyme catalyses the formation of phosphodiester bonds between complementary ribonucleotides.
起始阶段，RNA聚合酶与通用转录因子在启动子处组装形成闭合复合体。随后DNA双螺旋解开约14个碱基对，形成开放复合体（转录泡）。第一个核糖核苷三磷酸被放置在+1起始位点，酶催化互补核糖核苷酸之间形成磷酸二酯键。
In eukaryotes, TFIID recognises the TATA box via its TATA-binding protein (TBP) subunit. Other transcription factors and RNA polymerase II then join. Once the open complex is formed, RNA polymerase begins RNA synthesis. The 5′ triphosphate of the first nucleotide is retained, which later serves as the site for the 5′ cap addition.
在真核生物中，TFIID通过它的TATA结合蛋白（TBP）亚基识别TATA盒，随后其他转录因子和RNA聚合酶II加入。开放复合体一旦形成，RNA聚合酶便开始合成RNA。第一个核苷酸的5’三磷酸基团被保留下来，随后成为5’帽的添加位点。
5. Elongation: Building the RNA Chain | 延伸：构建RNA链
During elongation, RNA polymerase moves along the DNA template strand in the 3′ to 5′ direction, adding ribonucleotides to the growing 3′ end of the RNA molecule. The enzyme maintains a transcription bubble of about 12-14 base pairs. As it advances, the DNA rewinds behind the polymerase, displacing the nascent RNA.
在延伸过程中，RNA聚合酶沿DNA模板链以3’至5’方向移动，将核糖核苷酸添加到RNA链伸长的3’端。酶维持着一个约12-14碱基对的转录泡。随着RNA聚合酶向前移动，DNA在其后重新螺旋化，将新生的RNA排挤出去。
The sugar-phosphate backbone is formed via phosphodiester linkages, and base pairing obeys the rules A-U and C-G. The energy for polymerisation comes from the hydrolysis of the high-energy phosphate bonds of incoming nucleoside triphosphates. Elongation rates are roughly 20-50 nucleotides per second in eukaryotes, and even faster in prokaryotes.
糖-磷酸骨架通过磷酸二酯键连接，碱基遵循A-U和C-G的配对规则。聚合所需的能量来源于进入的核苷三磷酸高能磷酸键的水解。真核生物中的延伸速度约为每秒20-50个核苷酸，原核生物则更快。
6. Termination of Transcription | 转录的终止
Termination signals the end of RNA synthesis. In prokaryotes, termination can be rho-dependent (requiring the Rho protein, which follows RNA polymerase and causes release of the transcript) or rho-independent, where a GC-rich palindrome forms a hairpin loop in the RNA, followed by a poly-U sequence, destabilising the complex.
终止标志着RNA合成的结束。原核生物的终止可以是依赖Rho蛋白的终止（Rho蛋白跟随RNA聚合酶导致转录本释放），也可以是不依赖Rho的内在终止——一段富含GC的回文序列在RNA中形成发夹结构，后接poly-U序列，使复合体解离。
In eukaryotes, RNA polymerase II continues transcription beyond the end of the gene. The primary transcript is cleaved at a specific polyadenylation signal (AAUAAA in the RNA) downstream of the stop codon. After cleavage, the polymerase eventually dissociates from the DNA, and the downstream RNA is rapidly degraded. This cleavage is coupled with the addition of the poly(A) tail.
在真核生物中，RNA聚合酶II会继续转录直到基因末端之后。初级转录本在终止密码子下游的特定多腺苷酸化信号（RNA中的AAUAAA）处被切割。切割后，聚合酶最终从DNA上脱离，下游的RNA被迅速降解。这一切割过程与poly(A)尾的添加紧密偶联。
7. Post-Transcriptional Modifications in Eukaryotes | 真核生物的转录后修饰
Primary transcripts (pre-mRNA) in eukaryotes undergo three major modifications to become functional mRNA: 5′ capping, addition of a 3′ poly(A) tail, and splicing. The 5′ cap (7-methylguanosine linked via a 5′-5′ triphosphate bridge) is added soon after initiation. It protects the mRNA from exonucleases, facilitates ribosome binding, and promotes nuclear export.
真核生物的初级转录本（前体mRNA）需经历三种主要修饰才能成为成熟的mRNA：5’加帽、3’端添加poly(A)尾以及剪接。5’帽（7-甲基鸟苷通过5′-5’三磷酸桥连接）在起始后不久加入，保护mRNA免受外切核酸酶降解，促进核糖体结合及核输出。
The 3′ poly(A) tail, a stretch of about 200 adenine nucleotides, is added by poly(A) polymerase after cleavage at the polyadenylation signal. This tail enhances mRNA stability and translation efficiency. The length of the tail gradually shortens in the cytoplasm, influencing mRNA half-life.
3′ poly(A)尾由约200个腺苷酸组成，在多腺苷酸化信号处切割后由poly(A)聚合酶添加。该尾巴增强mRNA的稳定性和翻译效率。尾巴长度在细胞质中会逐渐缩短，影响mRNA的半衰期。
8. RNA Splicing and the Spliceosome | RNA 剪接与剪接体
Splicing removes introns (non-coding intervening sequences) from the pre-mRNA and joins exons (coding sequences) together. It is catalysed by the spliceosome, a large ribonucleoprotein complex composed of small nuclear RNAs (snRNAs) and numerous proteins. The spliceosome recognises conserved splice site sequences at the intron-exon boundaries (GU at the 5′ end and AG at the 3′ end of the intron).
剪接从前体mRNA中去除内含子（非编码间隔序列）并将外显子（编码序列）连接起来。这一过程由剪接体催化，剪接体是由小核RNA（snRNA）和大量蛋白质组成的大分子核糖核蛋白复合体。剪接体识别内含子-外显子交界处的保守剪接位点序列（内含子5’端为GU，3’端为AG）。
The intron is excised as a lariat-shaped structure, and the exons are ligated together. Alternative splicing allows a single gene to produce multiple protein isoforms by selectively including or excluding particular exons. It is estimated that over 95% of human multi-exon genes undergo alternative splicing, dramatically increasing proteomic diversity.
内含子以套索状结构被切除，外显子随之连接。可变剪接使得同一个基因通过选择性地包含或排除特定外显子来产生多种蛋白质同工型。据估计，超过95%的人类多外显子基因存在可变剪接，极大地增加了蛋白质组的多样性。
9. Prokaryotic vs. Eukaryotic Transcription | 原核与真核转录对比
Transcription in prokaryotes is generally simpler. It takes place in the cytoplasm, often produces polycistronic mRNA (one mRNA encoding several proteins), and lacks post-transcriptional modifications such as splicing. Translation can begin on the nascent RNA even before transcription is complete. Prokaryotic genes rarely contain introns.
原核生物的转录通常较为简单：发生在细胞质中，常产生多顺反子mRNA（一条mRNA编码多个蛋白质），并且缺乏剪接等转录后修饰。在转录尚未
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