📚 Transcription: IGCSE Biology Revision | IGCSE 生物：转录 考点精讲
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase. This process converts the genetic information stored in DNA into a form that can be read by ribosomes to build proteins.
转录是基因表达的第一步，指由 RNA 聚合酶将 DNA 的特定片段拷贝成信使 RNA (mRNA) 的过程。它把储存在 DNA 中的遗传信息转变为核糖体能够读取的形式，用于合成蛋白质。
1. What Is Transcription? | 转录的定义
Transcription is the synthesis of an RNA molecule from a DNA template. Only one of the two DNA strands serves as the template, and the RNA produced is complementary to this template strand, carrying the same base sequence as the coding strand (with uracil replacing thymine).
转录是指以 DNA 为模板合成 RNA 分子的过程。两条 DNA 链中仅有一条充当模板，合成的 RNA 与模板链互补，其碱基序列与编码链相同（只是胸腺嘧啶被尿嘧啶取代）。
The newly formed RNA molecule is single-stranded and can be a messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), or other regulatory RNAs. In IGCSE, the focus is on mRNA synthesis for protein coding.
新合成的 RNA 分子是单链的，可以是信使 RNA (mRNA)、转运 RNA (tRNA)、核糖体 RNA (rRNA) 或其他调控 RNA。IGCSE 的重点在于编码蛋白质的 mRNA 合成过程。
2. Where Does Transcription Occur? | 转录的发生位置
In eukaryotic cells, transcription takes place inside the nucleus because DNA is confined there. The mRNA must later travel out through nuclear pores into the cytoplasm for translation.
在真核细胞中，转录在细胞核内进行，因为 DNA 局限在核内。mRNA 之后必须通过核孔进入细胞质才能进行翻译。
In prokaryotic cells (bacteria), there is no nucleus, so transcription occurs directly in the cytoplasm. Translation can even begin while transcription is still in progress, because the mRNA is immediately available to ribosomes.
在原核细胞（细菌）中没有细胞核，因此转录直接在细胞质内进行。翻译甚至能在转录尚未完成时就开始，因为 mRNA 立刻就能被核糖体捕获。
3. Machinery Required for Transcription | 转录所需的分子机器
The central enzyme is RNA polymerase. It binds to a specific region called the promoter, unwinds the DNA double helix, and adds RNA nucleotides complementary to the DNA template strand. RNA polymerase does not need a primer to begin synthesis, unlike DNA polymerase.
核心酶是 RNA 聚合酶。它与 DNA 上称为启动子的特定区域结合，解开双螺旋，并根据模板链添加互补的 RNA 核苷酸。与 DNA 聚合酶不同，RNA 聚合酶不需要引物就能启动合成。
Other essential ingredients include free ribonucleoside triphosphates (ATP, UTP, GTP, CTP) as building blocks, transcription factors (in eukaryotes) that help RNA polymerase recognise the promoter, and the DNA double helix itself which provides the template.
其他必需的原料包括作为构建单元的游离核糖核苷三磷酸（ATP、UTP、GTP、CTP），以及在真核生物中帮助 RNA 聚合酶识别启动子的转录因子，当然还有提供模板的 DNA 双螺旋。
4. The Promoter and the Initiation Phase | 启动子与起始阶段
Transcription begins when RNA polymerase and associated transcription factors bind to the promoter, a specific nucleotide sequence located just before the start of a gene. In many genes, the promoter contains a TATA box (rich in thymine and adenine).
当 RNA 聚合酶和相关的转录因子与启动子结合时，转录便开始了。启动子是紧邻基因起始位点的一段特定核苷酸序列。许多基因的启动子都含有一段 TATA 盒（富含胸腺嘧啶和腺嘌呤）。
Binding causes the DNA double helix to unwind locally, exposing about 10-20 base pairs of the template strand. This forms the transcription bubble. RNA polymerase then starts linking the first two RNA nucleotides without a primer, always in the 5′ → 3′ direction.
结合导致 DNA 双螺旋局部解开，暴露约 10–20 个碱基对的模板链，形成转录泡。随后 RNA 聚合酶在无引物的情况下连接最初的两个 RNA 核苷酸，合成方向始终是 5′ → 3’。
5. Elongation: Building the RNA Chain | 延伸：RNA 链的延伸
During elongation, RNA polymerase moves along the DNA template strand in the 3′ → 5′ direction, reading the bases and adding complementary RNA nucleotides. The base pairing rules are:
在延伸阶段，RNA 聚合酶沿 DNA 模板链 3′ → 5′ 方向移动，读取碱基并添加互补的 RNA 核苷酸。碱基配对规则如下：
- Adenine (A) on the DNA template pairs with uracil (U) in RNA. | DNA 模板上的腺嘌呤 (A) 与 RNA 中的尿嘧啶 (U) 配对。
- Thymine (T) pairs with adenine (A). | 胸腺嘧啶 (T) 与腺嘌呤 (A) 配对。
- Cytosine (C) pairs with guanine (G). | 胞嘧啶 (C) 与鸟嘌呤 (G) 配对。
- Guanine (G) pairs with cytosine (C). | 鸟嘌呤 (G) 与胞嘧啶 (C) 配对。
As RNA polymerase advances, the newly formed RNA strand peels away from the template and the DNA double helix rewinds behind the enzyme. Multiple RNA polymerase molecules can transcribe the same gene simultaneously, producing many mRNA copies in a short time.
随着 RNA 聚合酶前移，新合成的 RNA 链不断脱离模板，DNA 双螺旋在酶的后方重新缠绕。同一个基因上可同时有多个 RNA 聚合酶分子工作，短时间内产生大量 mRNA 拷贝。
6. Termination: Stopping at the Right Place | 终止：在正确的位置停下
Elongation continues until RNA polymerase encounters a terminator sequence. In prokaryotes, terminators often form a hairpin loop in the RNA that causes the polymerase to detach. In eukaryotes, the process is more complex and involves cleavage of the RNA and release of the polymerase.
延伸一直持续到 RNA 聚合酶遇到终止子序列。在原核生物中，终止子常常使 RNA 形成一个发夹结构，导致聚合酶脱离。真核生物的终止更复杂，涉及 RNA 的切割和聚合酶的释放。
Once termination occurs, the completed RNA strand is released from the DNA template, and the RNA polymerase dissociates. This marks the end of transcription for that gene.
一旦终止发生，完整的 RNA 链就从 DNA 模板上释放，RNA 聚合酶也随之解离。这标志着该基因转录的结束。
7. Post-Transcriptional Modifications in Eukaryotes | 真核生物的转录后修饰
In eukaryotic cells, the primary RNA transcript (pre-mRNA) undergoes several processing steps inside the nucleus before it becomes mature mRNA. These modifications are essential for stability, export, and translation.
在真核细胞中，初级 RNA 转录本（前体 mRNA）在细胞核内要经过几个加工步骤才能成为成熟 mRNA。这些修饰对于 mRNA 的稳定性、出核和翻译至关重要。
Three key modifications occur:
主要发生三种修饰：
- Capping: a modified guanine nucleotide (7-methylguanosine) is added to the 5′ end. This cap protects the mRNA from degradation and helps ribosome binding. | 加帽：在 5′ 端添加一个修饰过的鸟嘌呤核苷酸（7-甲基鸟苷）。帽子结构保护 mRNA 不被降解，并协助核糖体结合。
- Poly-A tail: a string of 100-250 adenine nucleotides is added to the 3′ end. The poly-A tail enhances stability and facilitates nuclear export. | 多聚腺苷酸尾巴：在 3′ 端添加一长串腺嘌呤核苷酸（100–250 个）。poly-A 尾巴增强稳定性并促进出核。
- Splicing: non-coding introns are removed, and coding exons are joined together by a spliceosome. This produces a continuous coding sequence. | 剪接：非编码的内含子被切除，编码的外显子由剪接体连接在一起，形成连续的编码序列。
Prokaryotic mRNA typically does not undergo these modifications and is ready for translation immediately after transcription.
原核生物的 mRNA 通常不经历这些修饰，转录结束后立刻就能进行翻译。
8. Transcription vs DNA Replication | 转录与 DNA 复制的比较
| Feature | 特点 | Transcription | 转录 | DNA Replication | DNA 复制 |
|---|---|---|
| Purpose | 目的 | To produce an RNA copy of a gene | 产生一个基因的 RNA 拷贝 | To duplicate the entire genome for cell division | 为细胞分裂复制整个基因组 |
| Enzyme | 酶 | RNA polymerase | RNA 聚合酶 | DNA polymerase (along with helicase, primase) | DNA 聚合酶 (及解旋酶、引物酶) |
| Template | 模板 | Only template strand of a gene | 仅基因的模板链 | Both strands of the whole DNA molecule | 整个 DNA 分子的两条链 |
| Product | 产物 | Single-stranded mRNA, tRNA, rRNA | 单链 mRNA、tRNA、rRNA | Two identical double-stranded DNA molecules | 两条相同的双链 DNA 分子 |
| Primer needed | 需引物 | No | 否 | Yes (RNA primer) | 是 (RNA 引物) |
| Base pairing | 碱基配对 | A-U, T-A, C-G, G-C | A-U、T-A、C-G、G-C | A-T, T-A, C-G, G-C | A-T、T-A、C-G、G-C |
9. The Central Role of Transcription in Gene Expression | 转录在基因表达中的核心作用
Transcription is the key regulatory step in gene expression. By controlling which genes are transcribed and how often, a cell can respond to signals, differentiate, and maintain homeostasis without altering the underlying DNA sequence.
转录是基因表达的关键调控环节。通过控制哪些基因被转录以及转录的速率，细胞能响应信号、进行分化并维持稳态，而无需改变底层的 DNA 序列。
The mRNA produced by transcription carries the genetic code in the form of codons – groups of three bases that specify one amino acid. Thus, transcription bridges the information stored in the DNA molecule and the protein synthesis machinery in the cytoplasm.
转录产生的 mRNA 以密码子的形式携带遗传密码——三个碱基一组，指定一种氨基酸。因此，转录在 DNA 中存储的信息和细胞质中的蛋白质合成机器之间架起了桥梁。
10. Common Misconceptions and Exam Tips | 常见误区与考试提示
Students often confuse template and coding strands. Remember, the mRNA sequence matches the coding strand (with U for T), not the template strand. The enzyme reads the template strand 3′ → 5′ but builds RNA 5′ → 3′.
学生经常混淆模板链和编码链。要记住，mRNA 序列与编码链相同（U 替代 T），而不是模板链。酶沿模板链 3′ → 5′ 读取，但以 5′ → 3′ 方向合成 RNA。
Another common error is to write thymine pairing with uracil. Thymine in DNA pairs with adenine in RNA, not with uracil. Uracil only appears in RNA and pairs with adenine on the DNA template.
另一个常见错误是写胸腺嘧啶与尿嘧啶配对。DNA 中的胸腺嘧啶与 RNA 中的腺嘌呤配对，而不是尿嘧啶。尿嘧啶只出现在 RNA 中，并与 DNA 模板上的腺嘌呤配对。
When drawing transcription diagrams, label the DNA template strand, RNA polymerase, the growing mRNA strand, the direction of synthesis, and the promoter region. Always check that base pairing is correct and that the RNA is single-stranded.
在绘制转录示意图时，应标注 DNA 模板链、RNA 聚合酶、正在延伸的 mRNA 链、合成方向以及启动子区域。务必检查碱基配对是否正确以及 RNA 为单链结构。
For the exam, be prepared to explain the difference between transcription and translation, and to state that in eukaryotes, post-transcriptional modification (capping, tailing, splicing) occurs before translation, while in prokaryotes it does not.
考试时要能解释转录与翻译的区别，并能说明在真核生物中，翻译前会发生转录后修饰（加帽、加尾、剪接），而原核生物不会。
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