Transcription | IGCSE AQA Biology Exam-Focused Revision

📚 Transcription | IGCSE AQA Biology Exam-Focused Revision | 转录考点精讲

Transcription is the first step of protein synthesis where a specific segment of DNA is copied into messenger RNA (mRNA). For IGCSE AQA Biology, you need to understand the process, the enzymes involved, the base‑pairing rules, and why this step is essential for gene expression. This article breaks down every key point you must know, with paired English–Chinese explanations to help you grasp the concepts and ace your exam.

转录是蛋白质合成的第一步，在这一过程中，特定的 DNA 片段被拷贝成信使 RNA（mRNA）。在 IGCSE AQA 生物学考试中，你需要理解转录的过程、参与的酶、碱基配对规则，以及为什么这一步对基因表达至关重要。本文分解了所有你必须掌握的关键知识点，并提供中英对照解释，帮助你掌握概念并在考试中取得高分。

1. What Is Transcription? | 什么是转录？

Transcription is the process by which the genetic code in a gene (a section of DNA) is used to produce a complementary molecule of messenger RNA (mRNA). It is the first stage of protein synthesis, taking the information stored in DNA and converting it into a form that can be read by ribosomes to build proteins.

转录是利用基因（DNA 上的一个片段）中的遗传密码来产生互补的信使 RNA（mRNA）分子的过程。它是蛋白质合成的第一个阶段，将储存在 DNA 中的信息转换成一种能被核糖体读取以构建蛋白质的形式。

Remember that not all DNA is transcribed at once – only the gene that codes for the required protein is ‘switched on’ and transcribed. The rest of the DNA remains tightly packed and inactive.

请记住，并非所有 DNA 都会同时被转录——只有编码所需蛋白质的那个基因才会被“开启”并转录。其余 DNA 保持紧密折叠的非活性状态。

2. Where Does Transcription Occur? | 转录发生在哪里？

In eukaryotic cells (such as animal and plant cells), transcription takes place inside the nucleus. The DNA is too large to leave the nucleus, so a smaller, single‑stranded mRNA copy is made to carry the genetic message out into the cytoplasm.

在真核细胞（如动物和植物细胞）中，转录发生在细胞核内。DNA 太大，无法离开细胞核，因此会制造一个较小的单链 mRNA 副本，将遗传信息携带到细胞质中。

After transcription, the mRNA molecule travels through a nuclear pore to the cytoplasm, where it attaches to a ribosome for the next stage – translation. For your IGCSE exam, always state that transcription happens in the nucleus.

转录完成后，mRNA 分子通过核孔进入细胞质，并在那里附着到核糖体上进行下一个阶段——翻译。在你参加 IGCSE 考试时，一定要写明转录发生在细胞核内。

3. Key Molecules Involved | 参与的关键分子

Several molecules are essential for transcription. The DNA double helix acts as the template. An enzyme called RNA polymerase binds to the DNA and separates the two strands. Free RNA nucleotides (ATP, UTP, GTP, CTP) present in the nucleoplasm are assembled into the mRNA strand. Unlike DNA nucleotides, RNA nucleotides contain the sugar ribose and the base uracil (U) instead of thymine (T).

转录需要几种关键分子。DNA 双螺旋作为模板。一种叫做RNA 聚合酶的酶与 DNA 结合，并将两条链分开。核质中游离的RNA 核苷酸（ATP、UTP、GTP、CTP）被组装成 mRNA 链。与 DNA 核苷酸不同，RNA 核苷酸含有核糖和碱基尿嘧啶（U）而非胸腺嘧啶（T）。

You should be able to name the enzyme and describe its function clearly: RNA polymerase unwinds the DNA double helix, breaks the hydrogen bonds between complementary bases, and catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides.

你应该能够清晰地命名这种酶并描述其功能：RNA 聚合酶解开 DNA 双螺旋，断裂互补碱基之间的氢键，并催化相邻 RNA 核苷酸之间形成磷酸二酯键。

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

During transcription, only one of the two DNA strands is copied. This strand is called the template strand (or antisense strand). The RNA polymerase reads this strand in the 3′ to 5′ direction to synthesise mRNA in the 5′ to 3′ direction. The other DNA strand is the coding strand (or sense strand) – it has the same base sequence as the newly made mRNA, except that thymine (T) is replaced by uracil (U).

在转录过程中，两条 DNA 链中只有一条被拷贝。这条链称为模板链（或反义链）。RNA 聚合酶从 3′ 到 5′ 方向读取这条链，以 5′ 到 3′ 方向合成 mRNA。另一条 DNA 链是编码链（或有义链）——它的碱基序列与新合成的 mRNA 相同，只不过胸腺嘧啶（T）被尿嘧啶（U）取代。

Many students confuse the two strands. A simple rule: the template strand is the one that is transcribed; the coding strand is not used directly but its sequence matches the mRNA (with T→U).

许多学生会混淆这两条链。一条简单的规则：模板链是被转录的那条链；编码链不直接使用，但它的序列与 mRNA 匹配（T 变为 U）。

5. Step‑by‑Step: Initiation | 分步详解：起始

Transcription begins when RNA polymerase binds to a specific region on the DNA called the promoter. The promoter is located just before the start of the gene. Once bound, RNA polymerase unwinds a short section of the DNA double helix, breaking the hydrogen bonds between bases. This exposes the template strand for base pairing.

当 RNA 聚合酶与 DNA 上一个称为启动子的特定区域结合时，转录就开始了。启动子位于基因起始点的前方。结合后，RNA 聚合酶解开一小段 DNA 双螺旋，断裂碱基之间的氢键。这使得模板链暴露出来，以便进行碱基配对。

In IGCSE AQA questions, you may be asked to label a diagram showing RNA polymerase at the promoter. Make sure you know that the promoter is not transcribed – it simply marks where transcription should start.

在 IGCSE AQA 试题中，你可能会被要求在图中标出启动子处的 RNA 聚合酶。请确保你知道启动子本身并不被转录——它仅仅标记转录应从哪里开始。

6. Step‑by‑Step: Elongation | 分步详解：延伸

Once the DNA is unwound, free RNA nucleotides in the nucleus align along the exposed template strand by complementary base pairing. RNA polymerase moves along the template strand in the 3’→5′ direction, adding RNA nucleotides one by one to the growing mRNA chain. The enzyme catalyses the formation of phosphodiester bonds between the nucleotides, creating the sugar‑phosphate backbone of mRNA.

DNA 解开后，细胞核中游离的 RNA 核苷酸通过互补碱基配对沿暴露的模板链排列。RNA 聚合酶沿模板链的 3’→5′ 方向移动，逐个将 RNA 核苷酸添加到正在延长的 mRNA 链上。该酶催化核苷酸之间形成磷酸二酯键，从而构建 mRNA 的糖‑磷酸骨架。

As RNA polymerase moves forward, the DNA behind it rewinds back into a double helix. This means only a small region of the DNA is single‑stranded at any one time – sometimes called the transcription bubble.

随着 RNA 聚合酶向前移动，它身后的 DNA 重新缠绕成双螺旋。这意味着在任何时刻只有一小段 DNA 是单链状态——有时被称为转录泡。

7. Step‑by‑Step: Termination | 分步详解：终止

Transcription continues until RNA polymerase reaches a terminator sequence on the DNA. This sequence signals the end of the gene. At this point, RNA polymerase detaches from the DNA, and the newly formed mRNA molecule is released. The DNA completely rewinds into its double‑stranded form.

转录一直持续，直到 RNA 聚合酶到达 DNA 上的终止子序列。这段序列标志着基因的结束。此时 RNA 聚合酶从 DNA 上脱离，新形成的 mRNA 分子被释放出来。DNA 完全重新缠绕成双链形式。

In prokaryotes, the mRNA can be used immediately for translation. In eukaryotes, the mRNA undergoes further processing before it leaves the nucleus – a detail you may need to mention in higher‑tier IGCSE questions.

在原核生物中，mRNA 可立即用于翻译。在真核生物中，mRNA 在离开细胞核之前还需要经过进一步加工——你可能需要在 IGCSE 的高阶试题中提到这一细节。

8. Base Pairing Rules in Transcription | 转录中的碱基配对规则

The pairing rules during transcription are almost the same as in DNA replication, but with one critical difference: adenine (A) on the DNA template strand pairs with uracil (U) on the RNA strand, not thymine. The complete rules are:

转录过程中的碱基配对规则与 DNA 复制几乎相同，但有一个关键区别：DNA 模板链上的腺嘌呤（A）与 RNA 链上的尿嘧啶（U）配对，而不是胸腺嘧啶。完整的规则如下：

DNA Template Base	mRNA Base
Adenine (A)	Uracil (U)
Thymine (T)	Adenine (A)
Cytosine (C)	Guanine (G)
Guanine (G)	Cytosine (C)

Because U replaces T, any question asking you to transcribe a DNA sequence into mRNA should use U in place of every T from the coding strand, or follow the complementary rule from the template strand.

由于 U 替代了 T，任何要求你将 DNA 序列转录为 mRNA 的题目，都应从编码链出发，用 U 替代每一个 T，或根据模板链遵守互补规则。

Example: If the DNA coding strand is 3’‑TAC GGA TCA‑5′, the mRNA will be 5’‑AUG CCU AGU‑3′. Check carefully which strand you are given in the exam.

例如：如果 DNA 编码链是 3’‑TAC GGA TCA‑5’，那么 mRNA 将是 5’‑AUG CCU AGU‑3’。考试时务必仔细看清给了你哪条链。

9. The Product: Messenger RNA (mRNA) | 产物：信使 RNA（mRNA）

The mRNA molecule produced is a single‑stranded copy of the gene. It is much shorter than the entire DNA molecule and contains only the code for one specific polypeptide. mRNA carries the genetic message from the nucleus to a ribosome in the cytoplasm. Each set of three bases on the mRNA is called a codon, and each codon codes for one amino acid.

产生的 mRNA 分子是基因的单链副本。它比整个 DNA 分子短得多，只包含一个特定多肽的编码。mRNA 将遗传信息从细胞核携带到细胞质中的核糖体。mRNA 上每三个碱基为一组，称为一个密码子，每个密码子编码一种氨基酸。

In eukaryotic cells, the initial mRNA transcript (pre‑mRNA) is modified before it becomes functional mRNA: a 5′ cap and a poly‑A tail are added, and non‑coding regions (introns) are removed through splicing. These details may appear in exam questions testing your understanding of gene expression.

在真核细胞中，初始的 mRNA 转录本（前体 mRNA）在成为功能性 mRNA 之前会被修饰：添加 5′ 帽和 poly‑A 尾，并通过剪接去除非编码区（内含子）。这些细节可能出现在考查基因表达理解的试题中。

10. Why Is Transcription Important? | 转录为何重要？

Transcription is vital because it allows the genetic information stored in DNA to be accessed without risking damage to the DNA molecule itself. By making an mRNA copy of a gene, the cell can produce many copies of a protein from a single gene, amplifying the signal. Moreover, different genes can be transcribed in different cell types, allowing cell specialisation.

转录至关重要，因为它使得储存在 DNA 中的遗传信息能够被读取，同时避免 DNA 分子本身受损的风险。通过制作基因的 mRNA 副本，细胞可以从一个基因出发，制造出许多蛋白质分子，放大信号。此外，不同类型的细胞可以转录不同的基因，从而实现细胞特化。

Transcription is also a point of control: by switching genes on or off at the promoter, a cell can regulate which proteins are made and when. This is crucial for development, response to stimuli, and maintaining homeostasis.

转录也是一个控制节点：通过在启动子处开启或关闭基因，细胞可以调节制造哪些蛋白质以及何时制造。这对于发育、应对外界刺激和维持稳态都至关重要。

11. Common Exam Mistakes and How to Avoid Them | 常见考试错误及避免方法

Confusing transcription and replication: Transcription produces mRNA, not a DNA copy. Replication makes DNA. Always check the context.
混淆转录和复制：转录产生 mRNA，而非 DNA 副本。复制产生 DNA。务必根据上下文区分。
Forgetting that RNA uses uracil: Never write thymine in an RNA sequence or when answering questions about mRNA.
忘记 RNA 使用尿嘧啶：在书写 RNA 序列或回答有关 mRNA 的问题时，千万不要写胸腺嘧啶。
Misidentifying strands: If the question gives you a DNA sequence, clarify whether it is the template strand or the coding strand before transcribing.
弄错链的类别：如果题目给出了 DNA 序列，在转录之前先弄清楚它是模板链还是编码链。
Omitting the enzyme: Always name RNA polymerase and describe its role, especially in longer answer questions.
遗漏酶的名称：一定要写出 RNA 聚合酶并描述其作用，尤其是在较长的问答题中。

Practice transcribing sequences in both directions, and label diagrams of transcription clearly showing the promoter, RNA polymerase, template strand, and mRNA product.

练习双向转录序列，并在转录示意图上清晰地标出启动子、RNA 聚合酶、模板链和 mRNA 产物。

12. Summary: Transcription in a Nutshell | 总结：转录一句话

Transcription is the DNA‑directed synthesis of mRNA, catalysed by RNA polymerase, occurring in the nucleus according to the base‑pairing rules (A‑U, T‑A, C‑G, G‑C), and producing a molecule that carries the genetic code to the ribosome for protein synthesis.

转录是由 DNA 指导的 mRNA 合成，由 RNA 聚合酶催化，在细胞核内按照碱基配对规则（A‑U, T‑A, C‑G, G‑C）进行，产生一个将遗传密码携带至核糖体以进行蛋白质合成的分子。

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Transcription | IGCSE AQA Biology Exam-Focused Revision | 转录 考点精讲