Transcription | 转录考点精讲

📚 Transcription | 转录考点精讲

Transcription is the first step of gene expression, where a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase. This process is essential for conveying genetic information from DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis. In AQA A-level Biology, you need to understand the structure of a transcription unit, the roles of different strands, the step-by-step mechanism, and the post-transcriptional modifications that produce mature mRNA in eukaryotes.

转录是基因表达的第一步，在此过程中，特定的DNA片段由RNA聚合酶拷贝成RNA（特别是mRNA）。这一过程对于将遗传信息从细胞核内的DNA传递到细胞质中的核糖体以进行蛋白质合成至关重要。在AQA A-level生物学中，你需要理解转录单位的结构、不同链的作用、逐步机制以及真核生物中产生成熟mRNA的转录后修饰过程。

1. Overview of Transcription | 转录概述

During transcription, the DNA double helix unwinds locally, and one strand serves as a template to synthesise a complementary RNA molecule. The RNA produced is a single-stranded copy of the gene’s coding sequence, except that thymine (T) is replaced by uracil (U). In eukaryotes, transcription occurs inside the nucleus and is followed by RNA processing before the mRNA is exported to the cytoplasm.

在转录过程中，DNA双螺旋局部解旋，其中一条链充当模板，合成互补的RNA分子。生成的RNA是基因编码序列的单链拷贝，不同之处在于胸腺嘧啶（T）被尿嘧啶（U）替代。在真核生物中，转录发生在细胞核内，随后进行RNA加工，然后mRNA被运输到细胞质。

2. Gene Structure: Promoter, Coding Region, Terminator | 基因结构：启动子、编码区、终止子

A typical gene that undergoes transcription contains three key regions: the promoter, the coding region, and the terminator. The promoter is a DNA sequence located upstream of the coding region that signals the start point for transcription and acts as a binding site for RNA polymerase and transcription factors. The coding region contains the actual sequence that will be transcribed into pre-mRNA, including both exons and introns in eukaryotes. The terminator is a sequence that signals the end of transcription, causing RNA polymerase to detach.

一个典型的进行转录的基因包含三个关键区域：启动子、编码区和终止子。启动子是位于编码区上游的DNA序列，它提示转录的起始点，并作为RNA聚合酶和转录因子的结合位点。编码区包含将被转录成前体mRNA的实际序列，在真核生物中包括外显子和内含子。终止子是一个提示转录结束的序列，导致RNA聚合酶脱落。

3. The Role of RNA Polymerase | RNA聚合酶的作用

RNA polymerase is the central enzyme responsible for catalysing the synthesis of RNA from a DNA template. Unlike DNA polymerase, it does not require a primer, it uses ribonucleoside triphosphates (ATP, UTP, GTP, CTP) as substrates, and it lacks proofreading activity. In eukaryotes, there are different RNA polymerases; RNA polymerase II is responsible for synthesising pre-mRNA.

RNA聚合酶是负责催化以DNA为模板合成RNA的核心酶。与DNA聚合酶不同，它不需要引物，使用核糖核苷三磷酸（ATP、UTP、GTP、CTP）作为底物，并且缺乏校对活性。在真核生物中，存在不同的RNA聚合酶；RNA聚合酶II负责合成前体mRNA。

4. Template and Coding Strands | 模板链与编码链

Of the two DNA strands, the one that is read by RNA polymerase is called the template strand (also antisense strand), because its sequence is complementary to the RNA transcript. The other strand, which has the same sequence as the RNA (with T instead of U), is called the coding strand (or sense strand). It is important to remember that RNA polymerase moves along the template strand in the 3′→5′ direction while synthesising RNA in the 5′→3′ direction.

两条DNA链中，被RNA聚合酶解读的那条链称为模板链（也称反义链），因为它的序列与RNA转录本互补。另一条链的序列与RNA相同（只是T代替U），称为编码链（或有义链）。重要的是要记住，RNA聚合酶以3′→5′方向沿着模板链移动，同时以5′→3′方向合成RNA。

5. Initiation of Transcription | 转录起始

Transcription begins when RNA polymerase binds to the promoter region with the help of transcription factors. In eukaryotes, this forms a transcription initiation complex. The DNA double helix unwinds to expose the template strand, and RNA polymerase starts adding ribonucleotides complementary to the DNA template. The first ribonucleotide is placed at the +1 site, and no primer is required.

当RNA聚合酶在转录因子的帮助下与启动子区域结合时，转录开始。在真核生物中，这会形成一个转录起始复合物。DNA双螺旋解旋，暴露出模板链，RNA聚合酶开始添加与DNA模板互补的核糖核苷酸。第一个核糖核苷酸置于+1位点，此过程不需要引物。

6. Elongation: Building the pre-mRNA | 延伸：构建前体mRNA

Once initiation is complete, RNA polymerase moves along the template strand, unwinding the DNA ahead and rewinding it behind. It catalyses the formation of phosphodiester bonds between ribonucleotides, using the template’s base sequence to add complementary bases: A pairs with U, T pairs with A, C pairs with G, and G pairs with C. The growing pre-mRNA strand is synthesised in the 5′→3′ direction.

一旦起始完成，RNA聚合酶沿着模板链移动，前方解旋DNA后方重新螺旋。它催化核糖核苷酸之间形成磷酸二酯键，利用模板的碱基序列添加互补碱基：A与U配对，T与A配对，C与G配对，G与C配对。不断增长的前体mRNA链以5′→3′方向合成。

7. Termination of Transcription | 转录终止

In eukaryotes, termination occurs when RNA polymerase encounters a specific terminator sequence and is released from the DNA. The newly synthesised RNA molecule, called the primary transcript or pre-mRNA, then undergoes a series of modifications before it can function in translation. The exact mechanism of termination can involve cleavage of the RNA downstream of the polyadenylation signal.

在真核生物中，当RNA聚合酶遇到特定的终止子序列并从DNA上释放时，转录终止。新合成的RNA分子，称为初级转录本或前体mRNA，随后需要经历一系列修饰才能在翻译中发挥作用。终止的确切机制可能涉及在poly-A信号下游切割RNA。

8. Post-Transcriptional Modification: 5′ Capping | 转录后修饰：5′加帽

Shortly after transcription begins, the 5′ end of the pre-mRNA is modified by the addition of a 7-methylguanosine cap. This modified guanine nucleotide is linked via a 5′-5′ triphosphate bridge and is essential for protecting the mRNA from degradation by exonucleases. The cap also helps the ribosome recognise and bind to the mRNA during translation initiation.

转录开始后不久，前体mRNA的5′端通过添加一个7-甲基鸟苷帽而被修饰。这个修饰的鸟嘌呤核苷酸通过5′-5′三磷酸桥连接，对于保护mRNA免受核酸外切酶降解至关重要。该帽还帮助核糖体在翻译起始时识别并结合mRNA。

9. Polyadenylation: Adding a poly-A Tail | 多聚腺苷酸化：添加poly-A尾

At the 3′ end of the pre-mRNA, an enzyme adds a tail consisting of about 50–250 adenine nucleotides, known as the poly-A tail. This modification is not directly encoded in the DNA; instead, the pre-mRNA is cleaved at a specific site downstream of the AAUAAA signal, and the adenine residues are added by poly-A polymerase. The poly-A tail increases the stability of the mRNA and aids in its export from the nucleus.

在前体mRNA的3′端，一种酶添加一个由约50–250个腺嘌呤核苷酸组成的尾巴，称为poly-A尾。这一修饰不是由DNA直接编码的；相反，前体mRNA在AAUAAA信号下游的特定位点被切割，然后由poly-A聚合酶添加腺嘌呤残基。poly-A尾增加了mRNA的稳定性，并帮助其从细胞核输出。

10. RNA Splicing: Removing Introns | RNA剪接：切除内含子

Eukaryotic genes often contain non-coding sequences called introns interspersed among coding sequences called exons. Before the mRNA can be used for translation, these introns must be removed through a process called splicing, carried out by a complex of snRNPs and proteins known as the spliceosome. The spliceosome recognises specific sequences at the intron–exon boundaries, cuts out the introns, and joins the exons together to form a continuous coding sequence.

真核基因通常包含称为内含子的非编码序列，它们散布在称为外显子的编码序列之间。在mRNA用于翻译之前，这些内含子必须通过称为剪接的过程去除，该过程由被称为剪接体的snRNPs和蛋白质复合物执行。剪接体识别内含子-外显子边界处的特定序列，切除内含子，并将外显子连接起来形成连续的编码序列。

11. From pre-mRNA to Mature mRNA | 从前体mRNA到成熟mRNA

After capping, polyadenylation, and splicing, the pre-mRNA is transformed into a mature mRNA molecule. This mature mRNA is now ready to be transported through the nuclear pore complex into the cytoplasm, where it will be translated into a polypeptide chain by ribosomes. The processing steps are not only protective but also enable the generation of multiple mRNA variants from a single gene through alternative splicing, increasing protein diversity.

经过加帽、多聚腺苷酸化和剪接后，前体mRNA转变为成熟的mRNA分子。这种成熟mRNA现在可以通过核孔复合体运输到细胞质，在那里由核糖体翻译成多肽链。这些加工步骤不仅具有保护作用，还能通过可变剪接从一个基因产生多种mRNA变体，增加了蛋白质的多样性。

12. Significance of Post-Transcriptional Modifications | 转录后修饰的重要性

Post-transcriptional modifications are crucial for eukaryotic gene expression. The 5′ cap and poly-A tail protect mRNA from degradation, facilitate nuclear export, and enhance translation efficiency. Splicing removes non-coding regions, ensuring that the correct amino acid sequence is produced. Furthermore, alternative splicing allows a single gene to code for multiple proteins, which is fundamental to the complexity of higher organisms. Without these modifications, mRNA would be rapidly degraded and translation would be inaccurate or impossible.

转录后修饰对真核基因表达至关重要。5′帽和poly-A尾保护mRNA不被降解，促进核输出，并提高翻译效率。剪接去除非编码区，确保产生正确的氨基酸序列。此外，可变剪接使得一个基因能够编码多种蛋白质，这是高等生物复杂性的基础。如果没有这些修饰，mRNA会迅速降解，翻译将不准确或无法进行。

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Transcription | 转录 考点精讲