Transcription | 转录 - aleveler.com

📚 Transcription | 转录

Transcription is the first stage of protein synthesis, where a specific section of DNA is used as a template to produce a messenger RNA (mRNA) molecule. This process is essential because DNA is too large to leave the nucleus, so a smaller, portable copy of the genetic code must be made to direct protein building at the ribosomes. In AQA GCSE Biology, you need to understand the steps of transcription, the role of RNA polymerase, the base pairing rules, and how this fits into the overall flow of genetic information from DNA to protein.

转录是蛋白质合成的第一阶段，在这一过程中，DNA 的特定片段被用作模板来合成信使 RNA (mRNA) 分子。这一过程至关重要，因为 DNA 分子太大，无法离开细胞核，因此需要制作一个更小、可携带的遗传密码副本，来指导核糖体处的蛋白质组装。在 AQA GCSE 生物学中，你需要理解转录的步骤、RNA 聚合酶的作用、碱基配对规则，以及它如何融入从 DNA 到蛋白质的整个遗传信息流。

1. What Is Transcription? | 什么是转录？

Transcription is the synthesis of a single-stranded mRNA molecule using the base sequence of a gene on one strand of DNA as a template. The resulting mRNA carries a complementary copy of the genetic information, but instead of thymine (T), it contains uracil (U). This process allows the genetic instructions to be transported out of the nucleus while the original DNA remains safely stored inside.

转录是利用 DNA 一条链上的基因碱基序列作为模板，合成单链 mRNA 分子的过程。生成的 mRNA 携带着互补的遗传信息副本，但与 DNA 不同，其中不含胸腺嘧啶 (T)，而是含有尿嘧啶 (U)。这一过程使得遗传指令能够被运输出细胞核，而原始的 DNA 仍安全地储存在核内。

2. Where Transcription Happens | 转录发生的场所

In eukaryotic cells, transcription takes place entirely inside the nucleus. This is where the cell’s DNA is housed in the form of chromatin. By keeping transcription within the nucleus, the DNA is protected from damage, and only the specific mRNA copies are exported to the cytoplasm for the next step — translation.

在真核细胞中，转录完全在细胞核内进行。这是细胞 DNA 以染色质形式存储的地方。通过将转录限制在细胞核内，DNA 得到了保护，免受损伤，只有特定的 mRNA 副本被输出到细胞质中进行下一步——翻译。

3. Unwinding the DNA Double Helix | DNA 双螺旋的解旋

Before the gene can be read, the DNA double helix must unwind and the two strands must separate locally. In transcription, this is carried out when the enzyme RNA polymerase binds to a promoter region near the start of the gene. The hydrogen bonds between complementary bases are broken, creating a small open ‘bubble’ of single-stranded DNA that exposes the template strand.

在读取基因之前，DNA 双螺旋必须解开，两条链必须局部分离。在转录过程中，当 RNA 聚合酶结合到基因起始附近的启动子区域时，这一步骤便完成了。互补碱基之间的氢键被断开，形成了一个小小的单链 DNA ‘开口泡’，暴露出模板链。

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

RNA polymerase is the central enzyme that drives transcription. It travels along the DNA template strand in the 3′ to 5′ direction, reading the nucleotide sequence. Simultaneously, it catalyses the formation of phosphodiester bonds between incoming RNA nucleotides, assembling them into a growing mRNA strand in the 5′ to 3′ direction. This enzyme also handles the unwinding and rewinding of the DNA ahead of and behind the transcription bubble.

RNA 聚合酶是驱动转录的核心酶。它沿着 DNA 模板链以 3′ 到 5′ 的方向移动，读取核苷酸序列。同时，它催化进入的 RNA 核苷酸之间形成磷酸二酯键，将它们组装成沿 5′ 到 3′ 方向不断延长的 mRNA 链。这种酶还负责转录泡前后 DNA 的解旋和重新缠绕。

5. The Template Strand | 模板链

Only one of the two DNA strands is transcribed into mRNA — this is called the template strand (also known as the antisense strand). The opposite strand, which is not used by RNA polymerase, is called the coding strand (or sense strand). Interestingly, the coding strand has a sequence identical to that of the new mRNA, except that thymine (T) appears in DNA where uracil (U) appears in RNA.

DNA 的两条链中只有一条被转录为 mRNA——这条链称为模板链（也称反义链）。另一条不被 RNA 聚合酶使用的链称为编码链（或有义链）。有趣的是，编码链的序列与新生成的 mRNA 序列相同，只是 DNA 中的胸腺嘧啶 (T) 对应于 RNA 中的尿嘧啶 (U)。

6. Complementary Base Pairing in Transcription | 转录中的互补碱基配对

As RNA polymerase moves along the template strand, free-floating RNA nucleotides in the nucleus pair with their complementary DNA bases. The base-pairing rules ensure the mRNA is an accurate copy of the gene:

随着 RNA 聚合酶沿着模板链移动，细胞核中游离的 RNA 核苷酸与它们互补的 DNA 碱基进行配对。碱基配对规则确保了 mRNA 是基因的精准副本：

DNA adenine (A) always pairs with RNA uracil (U)
DNA thymine (T) pairs with RNA adenine (A)
DNA cytosine (C) pairs with RNA guanine (G)
DNA guanine (G) pairs with RNA cytosine (C)

DNA 腺嘌呤 (A) 总是与 RNA 尿嘧啶 (U) 配对；DNA 胸腺嘧啶 (T) 与 RNA 腺嘌呤 (A) 配对；DNA 胞嘧啶 (C) 与 RNA 鸟嘌呤 (G) 配对；DNA 鸟嘌呤 (G) 与 RNA 胞嘧啶 (C) 配对。

DNA Base on Template Strand 模板链上的DNA碱基	Complementary RNA Base 互补的RNA碱基
Adenine (A) 腺嘌呤	Uracil (U) 尿嘧啶
Thymine (T) 胸腺嘧啶	Adenine (A) 腺嘌呤
Cytosine (C) 胞嘧啶	Guanine (G) 鸟嘌呤
Guanine (G) 鸟嘌呤	Cytosine (C) 胞嘧啶

7. Building the Messenger RNA | 信使 RNA 的合成

RNA polymerase joins the correctly paired RNA nucleotides together by forming phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next. The growing mRNA strand peels away from the template, and the DNA double helix reseals behind the enzyme. The result is a single-stranded mRNA molecule that is complementary to the template strand of the gene.

RNA 聚合酶通过在相邻核苷酸的糖与磷酸之间形成磷酸二酯键，将正确配对的 RNA 核苷酸连接在一起。不断延长的 mRNA 链从模板上剥离下来，DNA 双螺旋则在酶经过后重新闭合。最终产物是一条与基因模板链互补的单链 mRNA 分子。

8. Termination of Transcription | 转录的终止

Transcription does not continue indefinitely. When RNA polymerase reaches a specific termination sequence at the end of the gene, it detaches from the DNA. The newly formed pre-mRNA (or primary transcript) is then released, and the DNA rewinds completely into its stable double-helix form. At GCSE level, you simply need to know that the process stops and the mRNA is dispatched.

转录并不会一直持续下去。当 RNA 聚合酶到达基因末端特定的终止序列时，它便会与 DNA 分离。此时新形成的前体 mRNA（初级转录本）被释放，DNA 完全重新缠绕成稳定的双螺旋结构。在 GCSE 阶段，你只需知道转录停止并且 mRNA 被送出即可。

9. mRNA Exits the Nucleus | mRNA 离开细胞核

Once formed, the mRNA molecule passes through one of the numerous nuclear pores that perforate the nuclear envelope. These tiny channels allow the selective transport of RNA out of the nucleus whilst preventing most large molecules from entering. In the cytoplasm, the mRNA will locate a ribosome and begin the process of translation, in which its sequence of codons is decoded into a chain of amino acids.

一旦形成，mRNA 分子便穿过核膜上众多核孔中的一个。这些微小的通道能让 RNA 被选择性地运输出细胞核，同时阻止大多数大分子进入。在细胞质中，mRNA 将找到核糖体，并开始翻译过程，将其密码子序列解码为氨基酸链。

10. Transcription vs Replication | 转录与复制的比较

Transcription and DNA replication are often confused, but they serve very different purposes. Replication copies the entire genome before cell division, whereas transcription reads only one gene at a time to make a protein-coding mRNA. The table below summarises the key differences:

转录和 DNA 复制常被混淆，但它们的目的截然不同。复制是在细胞分裂前拷贝整个基因组，而转录每次只读取一个基因，生成编码蛋白质的 mRNA。下表总结了主要区别：

Feature 特征	Transcription 转录	DNA Replication DNA 复制
Product 产物	mRNA (single-stranded) 单链 mRNA	Two identical DNA molecules 两个相同的DNA分子
Enzyme 酶	RNA polymerase RNA 聚合酶	DNA helicase, DNA polymerase 解旋酶、DNA 聚合酶
Template used 使用的模板	One strand of one gene 一个基因的一条链	Both strands of the entire DNA 整个DNA的两条链
Base pairing 碱基配对	A-U, T-A, C-G, G-C	A-T, T-A, C-G, G-C
When it occurs 何时发生	Whenever the cell needs a particular protein 当细胞需要特定蛋白质时	During the S phase of the cell cycle, before division 细胞周期S期，分裂前

11. Key Points for Your AQA Exam | AQA 考试关键点

For the AQA GCSE Biology exam, you should be able to describe transcription as the production of mRNA from a DNA template. You must recall that the enzyme involved is RNA polymerase and that it uses free RNA nucleotides in the nucleus. Be sure to state the base-pairing rule that uracil replaces thymine (U pairs with A), and that the mRNA then leaves the nucleus to attach to a ribosome. It is also useful to know that transcription only produces an mRNA copy of a small section of DNA — a single gene — not the whole chromosome.

在 AQA GCSE 生物学考试中，你需要能够将转录描述为利用 DNA 模板产生 mRNA 的过程。你必须记住所涉及的酶是 RNA 聚合酶，并且它使用细胞核中游离的 RNA 核苷酸。务必说明碱基配对规则中尿嘧啶取代胸腺嘧啶（U 与 A 配对），以及 mRNA 随后离开细胞核附着到核糖体上。同时，了解转录只产生一小段 DNA——即单个基因——的 mRNA 副本，而不是整条染色体，这对考试也很有帮助。

12. Summary: From DNA to mRNA | 总结：从 DNA 到 mRNA

Transcription is a highly regulated process that converts a gene’s instruction into a portable mRNA molecule. Starting in the nucleus, RNA polymerase unwinds the DNA, reads the

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Transcription | 转录