📚 GCSE WJEC Biology: Translation Essentials | GCSE WJEC 生物：翻译 考点精讲
Translation is the second stage of protein synthesis, in which the sequence of codons on an mRNA molecule is decoded to assemble a specific polypeptide chain. This process takes place in the cytoplasm on ribosomes, involves transfer RNA (tRNA) molecules carrying amino acids, and relies entirely on base-pairing rules. Understanding translation is essential for WJEC GCSE Biology, as it connects the genetic code to the functional proteins that determine cellular activities.
翻译是蛋白质合成的第二阶段，在此过程中，mRNA 分子上的密码子序列被解读，从而组装出特定的多肽链。这一过程在细胞质的核糖体上进行，涉及携带氨基酸的转运 RNA (tRNA) 分子，并完全依赖碱基配对规则。理解翻译对 WJEC GCSE 生物至关重要，因为它将遗传密码与决定细胞活动的功能蛋白联系起来。
1. What is Translation? | 什么是翻译？
Translation is the cellular process that converts the genetic information carried by messenger RNA (mRNA) into a chain of amino acids – a polypeptide. It is the second part of the central dogma of molecular biology, following transcription. Every three nucleotides on the mRNA, called a codon, specifies one amino acid. The ribosome reads the mRNA codons sequentially and facilitates the binding of tRNA molecules whose anticodons are complementary, thus adding amino acids to a growing polypeptide chain.
翻译是将信使 RNA (mRNA) 携带的遗传信息转化为氨基酸链——多肽的细胞过程。它是分子生物学中心法则的第二部分，紧随转录之后。mRNA 上每三个核苷酸组成一个密码子，指定一种氨基酸。核糖体依次读取 mRNA 密码子，并促进反密码子与之互补的 tRNA 分子结合，从而将氨基酸添加到不断延长的多肽链上。
2. Key Players: mRNA, Ribosomes and tRNA | 关键参与者：mRNA、核糖体与 tRNA
Successful translation depends on three main types of RNA: mRNA, ribosomal RNA (rRNA), and tRNA. mRNA carries the coded message from DNA; ribosomes – made of rRNA and protein – serve as the machinery; and tRNA molecules act as adaptors, each carrying a specific amino acid. Each tRNA has an anticodon loop with three unpaired bases that are complementary to an mRNA codon. Amino acids are attached to the 3′ end of tRNA by enzymes called aminoacyl-tRNA synthetases.
成功的翻译依赖于三种主要的 RNA：mRNA、核糖体 RNA (rRNA) 和 tRNA。mRNA 携带来自 DNA 的编码信息；由 rRNA 和蛋白质组成的核糖体充当翻译机器；tRNA 分子则作为接头，每种 tRNA 携带特定的氨基酸。每个 tRNA 都有一个反密码子环，上面有三个未配对的碱基，与 mRNA 的密码子互补。氨基酸由氨酰-tRNA 合成酶连接到 tRNA 的 3′ 末端。
3. The Genetic Code and Codons | 遗传密码与密码子
The genetic code is a set of rules by which mRNA nucleotide triplets (codons) specify amino acids. It is degenerate – multiple codons can code for the same amino acid – but unambiguous, as each codon codes for only one amino acid. The code is read in the 5′ to 3′ direction. Of the 64 possible codons, 61 code for amino acids and 3 are stop signals (UAA, UAG, UGA). The codon AUG serves as the start signal and codes for methionine.
遗传密码是一套规则，规定 mRNA 的三联核苷酸（密码子）如何指定氨基酸。它具有简并性——多个密码子可以编码同一种氨基酸——但具有无歧义性，因为每个密码子只编码一种氨基酸。密码子沿 5′ 到 3′ 方向被阅读。在 64 种可能的密码子中，61 个编码氨基酸，3 个是终止信号（UAA、UAG、UGA）。密码子 AUG 充当起始信号并编码甲硫氨酸。
| Codon Type | Examples | Role |
|---|---|---|
| Start codon | AUG | Signals initiation; codes for Met |
| Sense codons | e.g. GCU (Ala), GGA (Gly) | Encode specific amino acids |
| Stop codons | UAA, UAG, UGA | Signal termination; no tRNA binds |
上表总结了三种密码子的功能：起始密码子、有义密码子和终止密码子。
4. Anticodons and tRNA Charging | 反密码子与 tRNA 装载
Each tRNA molecule carries an anticodon – a triplet of unpaired bases that can form complementary base pairs with a specific mRNA codon. The anticodon binds to the codon in an antiparallel fashion. Before a tRNA can participate in translation, it must be ‘charged’ with the correct amino acid. Aminoacyl-tRNA synthetase enzymes catalyse this ATP‑dependent reaction, ensuring high fidelity: each enzyme is specific for one amino acid and its corresponding tRNA(s).
每个 tRNA 分子都携带一个反密码子——三个未配对的碱基，能够与特定的 mRNA 密码子形成互补碱基对。反密码子以反向平行的方式与密码子结合。tRNA 在参与翻译之前，必须被“装载”上正确的氨基酸。氨酰-tRNA 合成酶催化这一依赖 ATP 的反应，确保高度忠实性：每种酶只对一种氨基酸及其相应 tRNA 具有特异性。
5. Step 1: Initiation of Translation | 第一步：翻译的起始
In eukaryotes, initiation begins when the small ribosomal subunit binds to the mRNA near the 5′ cap. The ribosome scans the mRNA until it reaches the start codon AUG. A special initiator tRNA carrying methionine then binds to the AUG codon via complementary base pairing. The large ribosomal subunit joins the complex, forming a functional ribosome with the initiator tRNA occupying the P site (peptidyl site). This assembly requires initiation factors and energy from GTP.
在真核细胞中，起始过程从小核糖体亚基与靠近 5′ 帽子结构的 mRNA 结合开始。核糖体沿 mRNA 扫描，直至到达起始密码子 AUG。携带甲硫氨酸的特化起始 tRNA 随后通过互补碱基配对与 AUG 密码子结合。大核糖体亚基加入该复合物，形成一个功能性的核糖体，起始 tRNA 占据 P 位点（肽酰位点）。这一组装过程需要起始因子和 GTP 提供的能量。
6. Step 2: Elongation – Codon Recognition | 第二步：延伸—密码子识别
Once the entire ribosome is assembled, the next mRNA codon after AUG is exposed in the A site (aminoacyl site). A charged tRNA whose anticodon is complementary to this codon enters the A site. Base pairing between codon and anticodon is specific; for example, if the codon is GCU, a tRNA with anticodon CGA (remembering antiparallel reading) binds. This codon–anticodon interaction is stabilized by the ribosome, which proofreads the match via conformational changes.
一旦完整的核糖体组装完毕，AUG 之后的下一个 mRNA 密码子就暴露在 A 位点（氨酰位点）。一个反密码子与该密码子互补的载氨酰 tRNA 进入 A 位点。密码子与反密码子之间的碱基配对是特异性的；例如，如果密码子是 GCU，则反密码子为 CGA（注意反向平行读码）的 tRNA 与之结合。这种密码子–反密码子相互作用由核糖体稳定，并通过构象变化校对匹配情况。
7. Step 3: Elongation – Peptide Bond Formation | 第三步：延伸—肽键形成
With two tRNA molecules now bound to the ribosome (one in the P site carrying the growing peptide chain, the other in the A site carrying a new amino acid), the ribosome catalyses the formation of a peptide bond between the amino acid in the A site and the carboxyl end of the polypeptide chain in the P site. This reaction is catalysed by peptidyl transferase, an activity of the large ribosomal subunit rRNA (a ribozyme). The growing chain is transferred to the tRNA in the A site, extending the polypeptide by one amino acid.
此时两个 tRNA 分子已结合在核糖体上（一个位于 P 位点，携带正在延长的肽链；另一个位于 A 位点，携带新的氨基酸），核糖体催化 A 位点氨基酸与 P 位点多肽链羧基端之间形成肽键。该反应由肽酰转移酶催化，这是大亚基 rRNA 的活性（一种核酶）。延长的肽链被转移到 A 位点的 tRNA 上，使多肽链延长一个氨基酸。
8. Step 4: Elongation – Translocation | 第四步：延伸—移位
After peptide bond formation, the ribosome moves (translocates) along the mRNA by exactly one codon in the 5’→3′ direction. This movement shifts the A‑site tRNA (now holding the peptide chain) into the P site, and the empty tRNA that was in the P site moves into the E site (exit site) and is released. The A site is now vacant and ready to accept the next charged tRNA. Translocation requires elongation factors and GTP hydrolysis, and the cycle repeats for each codon until a stop codon is reached.
肽键形成后，核糖体沿 mRNA 向 5’→3′ 方向精确移动一个密码子（移位）。这一移动将 A 位点的 tRNA（此时携带肽链）移入 P 位点，而原来在 P 位点的空 tRNA 移入 E 位点（出口位点）并释放出去。A 位点此时再次空出，准备好接受下一个载氨酰 tRNA。移位过程需要延伸因子和 GTP 水解，这一循环对每个密码子重复进行，直至遇到终止密码子。
9. Termination and Release | 终止与释放
When the ribosome encounters one of the three stop codons – UAA, UAG or UGA – no tRNA exists with a complementary anticodon. Instead, release factor proteins bind to the A site. The release factor triggers the hydrolysis of the bond linking the completed polypeptide chain to the tRNA in the P site, freeing the polypeptide. The ribosomal subunits, mRNA, and remaining tRNAs then dissociate. This termination step ensures the polypeptide is released at the correct length.
当核糖体遇到三个终止密码子之一——UAA、UAG 或 UGA——没有 tRNA 具有与之互补的反密码子。取而代之的是释放因子蛋白结合到 A 位点。释放因子触发连接已完成多肽链与 P 位点 tRNA 的键水解，释放多肽。随后核糖体亚基、mRNA 和剩余的 tRNA 解离。这一终止步骤确保多肽链以正确长度被释放。
10. Post-Translational Modifications | 翻译后修饰
Newly synthesised polypeptides are often not immediately functional. They undergo post‑translational modifications such as folding into specific 3D shapes with the help of chaperone proteins, cleavage of signal peptides, addition of carbohydrate or lipid groups, and assembly into quaternary structures. In WJEC GCSE Biology, you should recognise that the final protein shape is crucial for its function – e.g. enzymes require an active site, and any error in translation can lead to a non‑functional protein.
新合成的多肽往往并不立即具有功能。它们会经历翻译后修饰，例如在伴侣蛋白帮助下折叠成特定的三维构象、切除信号肽、添加糖基或脂基，以及组装成四级结构。在 WJEC GCSE 生物中，你需要认识到蛋白质的最终形状对其功能至关重要——例如酶需要活性位点，翻译中的任何错误都可能导致蛋白质失活。
11. Comparison: Prokaryotic vs Eukaryotic Translation | 原核与真核翻译对比
Although the basic mechanism of translation is conserved, there are key differences between prokaryotes and eukaryotes. In prokaryotes, translation can begin even while transcription is still in progress because there is no nuclear membrane separating the processes. Prokaryotic mRNA lacks a 5′ cap and often contains multiple ribosome binding sites (Shine–Dalgarno sequences) allowing polycistronic messages. Eukaryotic mRNA is monocistronic, has a 5′ cap and poly-A tail, and undergoes splicing before translation. WJEC may ask you to compare these features.
虽然翻译的基本机制是保守的，但原核生物与真核生物之间存在关键差异。在原核生物中，由于没有核膜将两个过程隔开，转录尚未完成时翻译便可开始。原核生物的 mRNA 缺乏 5′ 帽子，且通常含有多个核糖体结合位点（SD 序列），允许多顺反子信息。真核生物 mRNA 是单顺反子的，具有 5′ 帽子和 poly-A 尾巴，并且在翻译前会经历剪接。WJEC 可能会要求你比较这些特征。
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Site of translation | Cytoplasm (coupled with transcription) | Cytoplasm (after transcription in nucleus) |
| mRNA structure | Polycistronic, no 5′ cap | Monocistronic, 5′ cap and poly-A tail |
| Ribosome size | 70S (50S + 30S) | 80S (60S + 40S) |
| Initiation codon | AUG (formylmethionine) | AUG (methionine) |
12. Key Exam Tips and Common Mistakes | 考试技巧及常见错误
When tackling WJEC GCSE questions on translation, be precise with terminology: never confuse transcription with translation, and remember that mRNA codons are read 5′ to 3′. Avoid saying ‘tRNA matches with mRNA antisense strand’ – tRNA anti‑codon pairs with codons, not the template strand. Use diagrams to illustrate ribosome movement, and always label the P site, A site, and peptide bonds clearly. Common pitfalls include mixing up the roles of different RNA types and forgetting that several ribosomes can translate a single mRNA simultaneously (polysomes). Also recall that the genetic code is universal – this is a key piece of evidence for evolution.
在解答 WJEC GCSE 有关翻译的试题时，术语要精准：切勿混淆转录与翻译，并记住 mRNA 密码子是从 5′ 到 3′ 阅读。不要说“tRNA 与 mRNA 反义链匹配”——tRNA 反密码子与密码子配对，而非模板链。用图解说明核糖体移动，并清晰标记 P 位点、A 位点和肽键。常见的错误包括混淆不同 RNA 类型的作用，以及忘记多个核糖体可以同时翻译同一条 mRNA（多聚核糖体）。还要记住遗传密码是通用的——这是进化的重要证据之一。
Peptide bond formation: amino acidₙ –COOH + H₂N–amino acidₙ₊₁ → amino acidₙ –CO–NH–amino acidₙ₊₁ + H₂O
肽键形成：氨基酸ₙ –COOH + H₂N–氨基酸ₙ₊₁ → 氨基酸ₙ –CO–NH–氨基酸ₙ₊₁ + H₂O
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