📚 IB WJEC Biology: Translation – Key Points | IB WJEC 生物：翻译 考点精讲
Translation is the process by which the genetic information carried by messenger RNA (mRNA) is decoded to synthesise a specific polypeptide chain. This vital stage of gene expression occurs on ribosomes and requires the coordinated action of transfer RNA (tRNA) molecules, numerous enzymes, and protein factors. For IB and WJEC Biology students, a detailed understanding of the molecular events – initiation, elongation, and termination – is essential, as well as the ability to compare prokaryotic and eukaryotic translation and to apply the genetic code table to predict amino acid sequences.
翻译是指将信使 RNA（mRNA）所携带的遗传信息解码并合成特定多肽链的过程。这一基因表达的关键阶段发生在核糖体上，需要转运 RNA（tRNA）、多种酶以及蛋白质因子的协同作用。对于 IB 和 WJEC 生物学科的学生来说，不仅需要深入理解起始、延伸和终止的分子事件，还必须能够比较原核与真核翻译的差异，并运用遗传密码表预测氨基酸序列。
1. Overview of Translation | 翻译概述
Translation is the second major step of the central dogma of molecular biology. Following transcription, the mRNA transcript is used as a template to assemble amino acids into a polypeptide. This process takes place in the cytoplasm in both prokaryotes and eukaryotes, though in prokaryotes it can begin while the mRNA is still being synthesised. The ribosome reads the nucleotide sequence in groups of three bases, called codons. Each codon specifies a particular amino acid or a stop signal. Translation requires energy in the form of GTP and ATP, and relies on the precise base-pairing between the mRNA codon and the anticodon of a tRNA molecule charged with the corresponding amino acid.
翻译是分子生物学中心法则的第二大步骤。转录完成后，mRNA 转录本被用作模板将氨基酸装配成多肽。无论在原核生物还是真核生物中，该过程都发生在细胞质中，但在原核生物中，翻译可以在 mRNA 合成完成前就开始。核糖体以三个碱基为一组读取核苷酸序列，每组称为一个密码子。每个密码子对应一个特定的氨基酸或终止信号。翻译需要 GTP 和 ATP 提供能量，并依赖于 mRNA 密码子与携带相应氨基酸的 tRNA 反密码子之间的精确碱基配对。
2. The Genetic Code | 遗传密码
The genetic code is a set of rules that defines how the four-letter language of nucleic acids (A, U, G, C in RNA) is translated into the twenty-letter language of proteins. The code is degenerate, meaning that most amino acids are encoded by more than one codon. For example, leucine is specified by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This degeneracy minimises the impact of mutations. The code is unambiguous – each codon codes for only one amino acid. Among the 64 possible codons, 61 code for amino acids, and three are stop codons (UAA, UAG, UGA) that signal the end of translation. The codon AUG serves as the start codon, specifying methionine (Met) in eukaryotes and a modified form, N-formylmethionine (fMet), in prokaryotes. The reading frame of the ribosome must be correct from the start codon; any shift by one or two bases leads to an entirely different amino acid sequence.
遗传密码是一套规则，定义了如何将核酸的四字母语言（RNA 中的 A、U、G、C）翻译成蛋白质的二十字母语言。密码子具有简并性，即大多数氨基酸由不止一个密码子编码。例如亮氨酸由六个不同的密码子（UUA、UUG、CUU、CUC、CUA、CUG）编码。这种简并性降低了突变的影响。遗传密码是明确无歧义的——每个密码子只编码一种氨基酸。64 个可能的密码子中，61 个编码氨基酸，3 个是终止密码子（UAA、UAG、UGA），标志着翻译的结束。AUG 是起始密码子，真核生物中编码甲硫氨酸（Met），原核生物中编码其修饰形式——N-甲酰甲硫氨酸（fMet）。核糖体的阅读框必须从起始密码子开始保持正确；任何一两个碱基的移位都会导致完全不同的氨基酸序列。
3. Roles of mRNA, tRNA, and Ribosomes | mRNA、tRNA 和核糖体的作用
Messenger RNA (mRNA) carries the genetic blueprint from DNA to the ribosome. In eukaryotes, the mature mRNA has a 5′ cap and a 3′ poly-A tail that enhance stability and aid in ribosome binding. The coding sequence is flanked by untranslated regions (UTRs). Transfer RNA (tRNA) serves as an adaptor molecule. It possesses a cloverleaf secondary structure, with a three-nucleotide anticodon loop at one end and an amino acid attachment site at the 3′ end (the sequence CCA). The anticodon base-pairs with the complementary codon on the mRNA. Each tRNA is specific for a particular amino acid and is charged by a specific aminoacyl-tRNA synthetase. Ribosomes are large ribonucleoprotein complexes consisting of a small and a large subunit. In prokaryotes, the 70S ribosome is made of a 50S large subunit and a 30S small subunit. Eukaryotes have an 80S ribosome with a 60S large subunit and a 40S small subunit. The ribosome has three distinct binding sites for tRNA: the A (aminoacyl) site, where the incoming aminoacyl-tRNA binds; the P (peptidyl) site, where the tRNA carrying the growing polypeptide chain is located; and the E (exit) site, from which deacylated tRNA leaves the ribosome.
信使 RNA（mRNA）将遗传蓝图从 DNA 携带至核糖体。真核生物中，成熟的 mRNA 具有 5′ 帽结构和 3′ 多聚腺苷酸尾，可增强其稳定性并辅助核糖体结合。编码序列两侧含有非翻译区（UTR）。转运 RNA（tRNA）充当接头分子。它具有三叶草形的二级结构，一端为三核苷酸的反密码子环，另一端是氨基酸结合位点（位于 3′ 端的 CCA 序列）。反密码子与 mRNA 上的互补密码子发生碱基配对。每种 tRNA 仅对一种特定氨基酸具有特异性，并由特定的氨酰-tRNA 合成酶进行装载。核糖体是由大亚基和小亚基构成的巨大核糖核蛋白复合物。原核生物的 70S 核糖体由 50S 大亚基和 30S 小亚基组成；真核生物的 80S 核糖体由 60S 大亚基和 40S 小亚基组成。核糖体上有三个不同的 tRNA 结合位点：A 位（氨酰位），是进入的氨酰-tRNA 结合的位置；P 位（肽酰位），是携带延伸中多肽链的 tRNA 所在的位置；E 位（出口位），脱酰后的 tRNA 从此处离开核糖体。
4. Amino Acid Activation | 氨基酸的活化
Before an amino acid can be incorporated into a polypeptide, it must be attached to its cognate tRNA. This process, called amino acid activation or tRNA charging, is catalysed by aminoacyl-tRNA synthetases. There is a specific synthetase for each amino acid. The enzyme first catalyses the formation of an aminoacyl-adenylate intermediate (amino acid-AMP) using ATP, releasing pyrophosphate (PPᵢ). The activated amino acid is then transferred to the 2′ or 3′ hydroxyl group of the ribose at the 3′ end of the tRNA, forming an aminoacyl-tRNA with a high-energy ester bond. This ester bond stores the energy that will later be used for peptide bond formation. The accuracy of translation depends critically on the fidelity of tRNA charging; the synthetase has proofreading activity to correct misattached amino acids.
氨基酸在参入多肽之前必须先与对应的 tRNA 连接。这一过程称为氨基酸活化或 tRNA 装载，由氨酰-tRNA 合成酶催化。每种氨基酸都有其特定的合成酶。该酶首先利用 ATP 催化生成氨酰-腺苷酸中间体（氨基酸-AMP），同时释放焦磷酸（PPᵢ）。随后，活化的氨基酸被转移到 tRNA 3′ 末端核糖的 2′ 或 3′ 羟基上，形成一个带有高能酯键的氨酰-tRNA。这个酯键储存的能量将在后续的肽键形成中被使用。翻译的精确性高度依赖于 tRNA 装载的忠实度；合成酶具有校正活性，可纠正错误连接的氨基酸。
5. Initiation of Translation | 翻译的起始
Initiation is a tightly regulated stage that assembles the ribosome at the start codon. In prokaryotes, the small 30S ribosomal subunit binds to the Shine-Dalgarno sequence, a purine-rich region upstream of the start codon in the mRNA. This interaction aligns the start codon with the P site. The initiator tRNA, carrying N-formylmethionine (fMet-tRNAᶠᴹᵉᵗ), binds to the start codon with the help of initiation factors (IF-1, IF-2, IF-3). GTP is required. The large 50S subunit then joins, forming the 70S initiation complex with fMet-tRNA occupying the P site. In eukaryotes, the 40S small subunit, along with initiator Met-tRNAᵢ and eukaryotic initiation factors (eIFs), binds at the 5′ cap of the mRNA and scans in the 5′ to 3′ direction until it encounters the start codon in a favourable sequence context known as the Kozak sequence (ACCAUGG). Once the start codon is recognised, the 60S subunit joins, forming the 80S initiation complex. The initiator tRNA is in the P site, and the A site is ready to accept the next aminoacyl-tRNA.
起始是一个受到严格调控的阶段，负责将核糖体装配在起始密码子处。在原核生物中，小 30S 亚基结合到 mRNA 起始密码子上游的一段富含嘌呤的 Shine-Dalgarno 序列上。这一相互作用使起始密码子与 P 位对齐。携带 N-甲酰甲硫氨酸的起始 tRNA（fMet-tRNAᶠᴹᵉᵗ）在起始因子（IF-1、IF-2、IF-3）的帮助下结合到起始密码子上，并需要 GTP。随后大 50S 亚基加入，形成 70S 起始复合物，此时 fMet-tRNA 占据 P 位。在真核生物中，40S 小亚基与起始甲硫氨酸 tRNA（Met-tRNAᵢ）和多种真核起始因子（eIFs）一起结合到 mRNA 的 5′ 帽结构上，并沿 5′ 到 3′ 方向扫描，直到在合适的序列上下文（称为 Kozak 序列，如 ACCAUGG）中遇到起始密码子。识别起始密码子后，60S 亚基加入，形成 80S 起始复合物。起始 tRNA 位于 P 位，A 位准备接受下一个氨酰-tRNA。
6. Elongation | 延伸
Elongation is the cyclic addition of amino acids to the growing polypeptide chain. The process involves three main steps, repeated for each codon. Step 1 – Codon recognition: an aminoacyl-tRNA with the correct anticodon binds to the complementary codon in the A site of the ribosome. This binding requires elongation factor Tu (EF-Tu in prokaryotes, eEF1α in eukaryotes) and GTP. Upon correct codon-anticodon pairing, GTP is hydrolysed and the elongation factor is released. Step 2 – Peptide bond formation: peptidyl transferase, an enzymatic activity of the large subunit ribosomal RNA (a ribozyme), catalyses the formation of a peptide bond between the amino group of the aminoacyl-tRNA in the A site and the carboxyl end of the polypeptide chain attached to the tRNA in the P site. As a result, the polypeptide becomes attached to the tRNA in the A site, and the tRNA in the P site becomes deacylated. Step 3 – Translocation: the ribosome shifts along the mRNA by one codon (three bases) in the 5′ to 3′ direction. The movement requires elongation factor G (EF-G in prokaryotes, eEF2 in eukaryotes) and GTP. The tRNA carrying the growing chain moves from the A site to the P site, the deacylated tRNA moves from the P site to the E site and then exits, while the next codon enters the now-empty A site. The entire elongation cycle consumes two GTP molecules per amino acid added.
延伸是氨基酸循环添加至生长中的多肽链的过程。该过程包含三个主要步骤，每个密码子循环一次。第一步——密码子识别：带有正确反密码子的氨酰-tRNA 结合到核糖体 A 位中的互补密码子上。此结合需要延伸因子 Tu（原核生物中为 EF-Tu，真核生物中为 eEF1α）和 GTP。当密码子与反密码子正确配对后，GTP 被水解释放，延伸因子离开。第二步——肽键形成：大亚基 rRNA 具有肽基转移酶活性（一种核酶），催化 A 位氨酰-tRNA 的氨基与 P 位 tRNA 上多肽链的羧基端之间形成肽键。结果多肽链转移至 A 位 tRNA 上，而 P 位 tRNA 变为脱酰状态。第三步——移位：核糖体沿 mRNA 向 5′ 至 3′ 方向移动一个密码子（三个碱基）。此移动需要延伸因子 G（原核生物中为 EF-G，真核生物中为 eEF2）和 GTP。携带多肽链的 tRNA 从 A 位移至 P 位，脱酰的 tRNA 从 P 位移至 E 位并随后离开，同时下一个密码子进入已空出的 A 位。每添加一个氨基酸，整个延伸循环消耗两分子 GTP。
7. Termination | 终止
Translation terminates when a stop codon (UAA, UAG, or UGA) enters the A site of the ribosome. No normal tRNA carries an anticodon matching these codons. Instead, proteins called release factors (RFs) recognise the stop codon. In prokaryotes, RF-1 recognises UAA and UAG, while RF-2 recognises UAA and UGA; RF-3 helps with the dissociation. In eukaryotes, a single release factor, eRF1, recognises all three stop codons, and eRF3 functions as a GTPase. The release factor binds to the A site and triggers hydrolysis of the ester bond linking the completed polypeptide chain to the tRNA in the P site, releasing the polypeptide. Subsequently, the ribosomal subunits, mRNA, and deacylated tRNA dissociate, powered by GTP hydrolysis and with the help of ribosome recycling factors. The released polypeptide then folds into its native three-dimensional conformation, often assisted by chaperone proteins, and may undergo further modifications.
当终止密码子（UAA、UAG 或 UGA）进入核糖体 A 位时，翻译终止。正常的 tRNA 都不携带与这些密码子匹配的反密码子。取而代之的是称为释放因子（RF）的蛋白质对终止密码子进行识别。原核生物中，RF-1 识别 UAA 和 UAG，RF-2 识别 UAA 和 UGA，RF-3 协助解离。真核生物中，单一释放因子 eRF1 识别全部三种终止密码子，eRF3 则具有 GTP 酶功能。释放因子结合到 A 位后，触发连接完整多肽链与 P 位 tRNA 的酯键水解，释放多肽链。随后，在 GTP 水解和核糖体再循环因子的帮助下，核糖体亚基、mRNA 和脱酰 tRNA 解离。释放出的多肽链将折叠成其天然的三维构象，此过程常需伴侣蛋白协助，并可能经历进一步的修饰。
8. Polysomes | 多聚核糖体
A single mRNA molecule can be translated by multiple ribosomes simultaneously. A ribosome bound at the start codon begins translation, and as it moves along, a new ribosome can attach behind it. The entire structure, composed of an mRNA strand with several ribosomes spaced along it, is called a polysome or polyribosome. This arrangement greatly increases the efficiency of protein synthesis, allowing many copies of the polypeptide to be generated from one mRNA in a short time. Polysomes are observed in both prokaryotes and eukaryotes. In prokaryotes, because there is no nuclear barrier, ribosomes can attach to nascent mRNA while it is still being transcribed, leading to coupled transcription–translation.
一条 mRNA 分子可被多个核糖体同时翻译。一个核糖体在起始密码子处结合并开始翻译，沿 mRNA 移动时，后方可以结合新的核糖体。这种由一条 mRNA 与分布其上的多个核糖体共同组成的结构称为多聚核糖体或多体。这种排列大大提高了蛋白质合成的效率，使一条 mRNA 在短时间内即可产生大量多肽拷贝。多聚核糖体在原核和真核生物中均存在。在原核生物中，由于没有核膜屏障，核糖体甚至可以附着到仍在转录中的新生 mRNA 上，实现转录与翻译的偶联。
9. Comparison of Prokaryotic and Eukaryotic Translation | 原核与真核翻译的比较
Although the fundamental mechanism of translation is highly conserved, there are several significant differences between prokaryotes and eukaryotes. Prokaryotic ribosomes are 70S (50S + 30S); eukaryotic are 80S (60S + 40S). Initiation in prokaryotes relies on the Shine-Dalgarno sequence and the initiator tRNA carries fMet; in eukaryotes, the 5′ cap and Kozak sequence are required, and the initiator tRNA carries methionine. Prokaryotes can have multiple genes on one mRNA (polycistronic mRNA), each with its own ribosome binding site, whereas eukaryotic mRNAs are typically monocistronic. Prokaryotic transcription and translation can occur simultaneously in the cytoplasm; in eukaryotes, transcription in the nucleus is separated from translation in the cytoplasm, and the mRNA undergoes extensive processing (capping, splicing, polyadenylation) before it is exported. Antibiotics can selectively target bacterial 70S ribosomes, exploiting these differences. The elongation and termination mechanisms share similarities but involve distinct factors.
尽管翻译的基本机制高度保守，原核生物与真核生物之间仍存在若干显著差异。原核核糖体为 70S（50S + 30S），真核核糖体为 80S（60S + 40S）。原核起始依靠 Shine-Dalgarno 序列，起始 tRNA 携带 fMet；真核生物需要 5′ 帽结构和 Kozak 序列，起始 tRNA 携带甲硫氨酸。原核生物的 mRNA 可以是多顺反子，一条 mRNA 含有多个基因，每个基因具有各自的核糖体结合位点；而真核 mRNA 通常为单顺反子。原核生物转录与翻译可在细胞质中同时进行；真核生物中，转录发生在细胞核，翻译在细胞质，两者分隔，且 mRNA 在输出前需经历广泛的加工（加帽、剪接、加尾）。抗生素正是利用这些差异，选择性地作用于细菌 70S 核糖体。延伸和终止机制相似，但参与因子有所不同。
| Feature | 特征 | Prokaryotes | 原核生物 | Eukaryotes | 真核生物 |
|---|---|---|
| Ribosome size | 核糖体大小 | 70S | 80S |
| Initiation site | 起始位点 | Shine-Dalgarno sequence | 5′ cap + Kozak sequence |
| Initiator tRNA | 起始 tRNA | fMet-tRNAᶠᴹᵉᵗ | Met-tRNAᵢ |
| mRNA structure | mRNA 结构 | Often polycistronic | 常为多顺反子 | Monocistronic | 单顺反子 |
| Compartment | 发生部位 | Cytoplasm, coupled with transcription | 细胞质，与转录偶联 | Cytoplasm, after mRNA processing and export | 细胞质，mRNA 加工输出后 |
10. Post-translational Modifications | 翻译后修饰
Newly synthesised polypeptides are often functionally inactive and require post-translational modifications (PTMs) to become mature, functional proteins. Modifications can include folding, assisted by molecular chaperones such as Hsp70 and chaperonins, to achieve the correct three-dimensional conformation. Chemical modifications add functional groups: phosphorylation (addition of phosphate groups by kinases), glycosylation (addition of oligosaccharides to form glycoproteins), acetylation, methylation, and hydroxylation. Proteolytic cleavage removes specific segments; for example, signal peptides are cleaved from secretory proteins, and insulin is produced by cleaving proinsulin. Some proteins require the addition of cofactors or prosthetic groups, such as haem in haemoglobin. Proper targeting to organelles (e.g., mitochondria, chloroplast, endoplasmic reticulum) is directed by signal sequences within the polypeptide. Disulfide bridges form between cysteine residues, stabilising tertiary and quaternary structures.
新合成的多肽链通常不具有功能，需要经历翻译后修饰（PTM）才能成为成熟的功能蛋白。修饰包括折叠，在分子伴侣（如 Hsp70 和伴侣蛋白）的协助下形成正确的三维构象。化学修饰可添加功能基团：磷酸化（由激酶添加磷酸基团）、糖基化（添加寡糖形成糖蛋白）、乙酰化、甲基化和羟基化。蛋白水解切割可去除特定区段，例如分泌蛋白的信号肽被切除，胰岛素由胰岛素原切割而成。某些蛋白质需要添加辅因子或辅基，如血红蛋白中的血红素。细胞内正确的定位（如线粒体、叶绿体、内质网）由多肽内部的信号序列指导。二硫键在半胱氨酸残基之间形成，可稳定蛋白质的三级和四级结构。
11. Antibiotics and Translation | 抗生素与翻译
Many clinically important antibiotics work by inhibiting bacterial translation, exploiting the structural differences between 70S prokaryotic and 80S eukaryotic ribosomes. Tetracyclines block the binding of aminoacyl-tRNA to the A site of the bacterial ribosome. Streptomycin binds to the 30S subunit, causing misreading of the genetic code and inhibiting initiation. Chloramphenicol binds to the 50S subunit and inhibits peptidyl transferase activity, thereby blocking peptide bond formation. Erythromycin binds to the 50S subunit and prevents translocation. Puromycin resembles aminoacyl-tRNA and causes premature chain termination in both prokaryotes and eukaryotes; it is used as a research tool. Understanding these mechanisms not only highlights the importance of translation in cell survival but also explains why these drugs show selective toxicity towards bacteria.
许多临床上重要的抗生素通过抑制细菌的翻译发挥作用，利用了 70S 原核核糖体与 80S 真核核糖体之间的结构差异。四环素类阻断氨酰-tRNA 与细菌核糖体 A 位的结合。链霉素结合于 30S 亚基，导致遗传密码的错读并抑制起始。氯霉素结合于 50S 亚基，抑制肽基转移酶活性，从而阻断肽键形成。红霉素结合于 50S 亚基，阻止移位。嘌呤霉素在结构上类似氨酰-tRNA，可在原核和真核生物中引起肽链提前终止，常被用作研究工具。理解这些机制不仅能凸显翻译对细胞生存的重要性，还能解释这些药物为何对细菌具有选择性毒性。
12. Key Exam Points and Common Mistakes | 考点与常见错误
When preparing for IB and WJEC Biology exams, focus on the roles of mRNA, tRNA, and ribosomes, and be able to label the A, P, and E sites. Students should be comfortable using a genetic code table to deduce the amino acid sequence from an mRNA sequence and to identify the tRNA anticodon (remembering that anticodon is complementary and antiparallel to the codon). A common mistake is forgetting that the start codon also codes for an amino acid – the first methionine may be removed later, but it is initially incorporated. Do not confuse the process of transcription with translation; translation occurs on ribosomes, uses tRNA, and produces a polypeptide. Be able to compare prokaryotic and eukaryotic initiation, including the Shine-Dalgarno sequence, 5′ cap, and the nature of the initiator tRNA. Explain how the degeneracy of the code reduces the effect of point mutations, and recognise that a frameshift mutation alters the entire reading frame downstream. Practice describing the elongation cycle in concise steps: codon recognition, peptide bond formation, translocation. Know that peptide bond formation is catalysed by rRNA (ribozyme), not by protein enzymes. Finally, connect the process of translation to broader biological concepts, such as gene expression regulation, the effect of antibiotics, and the significance of polysomes in efficient protein production.
在备考 IB 和 WJEC 生物考试时，应重点关注 mRNA、tRNA 和核糖体的作用，并能够标出 A 位、P 位和 E 位。学生应熟练使用遗传密码表从 mRNA 序列推导氨基酸序列，并识别 tRNA 反密码子（记住反密码子与密码子互补且反向平行）。常见的错误是忘记起始密码子也编码一个氨基酸——第一个甲硫氨酸日后可能被切除，但它最初是被加入肽链的。不要混淆转录和翻译的过程；翻译发生在核糖体上，使用 tRNA，产物是多肽。要能够比较原核与真核翻译的起始过程，包括 Shine-Dalgarno 序列、5′ 帽结构以及起始 tRNA 的特性。解释密码子的简并性如何降低点突变的影响，并认识到移码突变会改变下游的整个阅读框。练习用简洁的步骤描述延伸循环：密码子识别、肽键形成、移位。要明确肽键形成是由 rRNA（核酶）催化，而非蛋白性质的酶。最后，将翻译过程与更广泛的生物学概念联系起来，例如基因表达调控、抗生素的作用机制，以及多聚核糖体对高效蛋白质生产的意义。
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