IB Edexcel Biology: Translation Key Points | 翻译考点精讲

📚 IB Edexcel Biology: Translation Key Points | 翻译考点精讲

Translation is the second major step of gene expression, during which the genetic information carried by mRNA is decoded by ribosomes to synthesise a specific polypeptide. It is a highly coordinated process involving tRNA molecules, ribosomes, enzymes and various protein factors. Mastery of translation is essential for IB and Edexcel Biology exams, as questions frequently test the roles of codons, anticodons, ribosome sites and the differences between prokaryotic and eukaryotic translation.

翻译是基因表达的第二个主要步骤，在此过程中，mRNA 携带的遗传信息被核糖体解码，合成特定的多肽链。这是一个高度有序的过程，涉及 tRNA 分子、核糖体、酶以及多种蛋白质因子。掌握翻译是 IB 和 Edexcel 生物考试的关键，考题常涉及密码子、反密码子、核糖体位点的功能以及原核与真核翻译的差异。

1. Overview of Translation | 翻译概述

Translation occurs on ribosomes in the cytoplasm (or on the rough ER in eukaryotes). It converts the linear sequence of mRNA codons into the amino acid sequence of a protein, following the central dogma of molecular biology: DNA → RNA → Protein. The process requires three main players: mRNA, which carries the code; tRNA, which delivers amino acids; and ribosomes, which catalyse peptide bond formation.

翻译发生在细胞质中的核糖体上（真核生物中也可在粗面内质网上进行）。它按照分子生物学的中心法则（DNA → RNA → 蛋白质），将 mRNA 密码子的线性序列转变为蛋白质的氨基酸序列。这一过程需要三个主要参与者：携带密码的 mRNA、运送氨基酸的 tRNA，以及催化肽键形成的核糖体。

Each three‑nucleotide codon on the mRNA specifies one amino acid, and the genetic code is virtually universal. Translation can be divided into three stages: initiation, elongation and termination, with each stage being tightly regulated by specific factors.

mRNA 上每三个核苷酸构成的密码子对应一种氨基酸，遗传密码几乎是通用的。翻译可分为三个步骤：起始、延伸和终止，每一步都受到特定因子的严格调控。

2. The Genetic Code and Codons | 遗传密码与密码子

The genetic code is a set of rules that defines how nucleotide triplets (codons) are translated into amino acids. There are 64 possible codons, but only 20 standard amino acids, making the code degenerate – several codons can code for the same amino acid. The code is read in the 5′ to 3′ direction on mRNA, and each codon is non‑overlapping.

遗传密码是一套规则，定义了核苷酸三联体（密码子）如何被翻译为氨基酸。一共有 64 种可能的密码子，但只有 20 种标准氨基酸，因此密码子具有简并性——多个密码子可以编码同一种氨基酸。密码以 5′ 到 3′ 的方向在 mRNA 上读取，且密码子之间不重叠。

Codon Type	Codon Sequence(s)	Function
Start	AUG	Codes for methionine (Met) and signals the beginning of translation
Stop	UAA, UAG, UGA	Do not code for any amino acid; they recruit release factors to terminate translation

The start codon AUG is crucial because it establishes the reading frame. A shift of one or two nucleotides leads to a completely different polypeptide, which is why frameshift mutations are often catastrophic. In multiple‑choice and data‑based questions, you may be asked to predict the amino acid sequence from a given mRNA sequence, so memorising the properties of the code is highly recommended.

起始密码子 AUG 至关重要，因为它决定了阅读框。移动一或两个核苷酸就会造成完全不同的多肽链，这也是框移突变往往具有灾难性后果的原因。在选择题和数据分析题中，常要求根据已知 mRNA 序列推导氨基酸序列，因此强烈建议熟记遗传密码的特点。

3. Structure and Charging of tRNA | tRNA 的结构与装载

tRNA molecules are adaptors that match amino acids with the corresponding mRNA codons. A typical tRNA has a cloverleaf secondary structure with three main loops: the D loop, the T loop and the anticodon loop. The anticodon loop contains a triplet of bases (the anticodon) that is complementary to a specific mRNA codon. At the 3′ end, there is a single‑stranded CCA tail where the amino acid is attached.

tRNA 分子是一种接头，能将氨基酸与对应的 mRNA 密码子匹配。典型的 tRNA 具有三叶草状二级结构，包括三个主要环：D 环、T 环和反密码子环。反密码子环含有与特定 mRNA 密码子互补的三碱基序列（反密码子）。在 3′ 端有一段单链 CCA 尾巴，用于连接氨基酸。

Before participating in translation, tRNA must be ‘charged’ with the correct amino acid by an enzyme called aminoacyl‑tRNA synthetase. This enzyme uses ATP to form a high‑energy bond between the amino acid and the tRNA’s 3′ end. There is at least one specific synthetase for each amino acid, which ensures high fidelity – the enzyme checks both the amino acid and the anticodon.

在参与翻译之前，tRNA 必须被正确的氨基酸“装载”，这一过程由氨酰‑tRNA 合成酶催化。该酶利用 ATP 在氨基酸与 tRNA 的 3′ 端之间形成高能键。每一种氨基酸至少对应一种专一的合成酶，以确保高保真度——酶会同时校对氨基酸和反密码子。

A charged tRNA is called an aminoacyl‑tRNA (e.g., Met‑tRNAⁱⁿⁱᵗ for initiator tRNA in prokaryotes). Exam questions often ask students to identify the anticodon that pairs with a given codon, remembering that the codon and anticodon are antiparallel and complementary.

装载了氨基酸的 tRNA 称为氨酰‑tRNA（例如原核生物中起始 tRNA 的 Met‑tRNAⁱⁿⁱᵗ）。考试题目常要求学生写出与给定密码子配对的反密码子，注意密码子与反密码子是反向平行且互补的。

4. Ribosome Structure and Functional Sites | 核糖体结构与功能位点

Ribosomes are large ribonucleoprotein complexes that provide the platform for protein synthesis. Prokaryotic ribosomes (70S) consist of a 50S large subunit and a 30S small subunit, while eukaryotic ribosomes (80S) have a 60S large subunit and a 40S small subunit. The S values are Svedberg units, which reflect sedimentation rate rather than simple molecular mass.

核糖体是大型核糖核蛋白复合物，为蛋白质合成提供平台。原核生物的核糖体（70S）由 50S 大亚基和 30S 小亚基组成，而真核生物核糖体（80S）由 60S 大亚基和 40S 小亚基组成。S 值代表沉降系数，反映的是沉降速率而非单纯的分子量。

Three key sites exist within ribosomes for tRNA binding: the A site (aminoacyl‑tRNA entry site), the P site (peptidyl‑tRNA binding site) and the E site (exit site). During elongation, an aminoacyl‑tRNA first enters the A site; the growing polypeptide is transferred to this new tRNA in the P site through peptide bond formation; then the ribosome translocates, moving the deacylated tRNA to the E site for exit.

核糖体内有三个关键位点用于结合 tRNA：A 位点（氨酰‑tRNA 进入位点）、P 位点（肽基‑tRNA 结合位点）和 E 位点（出口位点）。在延伸过程中，氨酰‑tRNA 首先进入 A 位点；随后在肽键形成时，新生肽链从 P 位点 tRNA 转移到 A 位点 tRNA 上；接着核糖体易位，将去酰化的 tRNA 移至 E 位点并离开。

The large subunit catalyses peptide bond formation through its peptidyl transferase centre (composed of rRNA, not protein – a crucial ribozyme feature). This fact often appears in exam questions highlighting the catalytic role of rRNA.

大亚基通过其肽基转移酶中心（由 rRNA 组成，而非蛋白质——体现了核酶的重要特征）催化肽键形成。这一知识点常出现在考题中，强调 rRNA 的催化作用。

5. Initiation of Translation | 翻译的起始

Initiation requires the assembly of the small ribosomal subunit, mRNA, the initiator tRNA (carrying methionine) and initiation factors. In prokaryotes, the 30S subunit binds to a purine‑rich Shine‑Dalgarno sequence on mRNA, which aligns the start codon AUG in the correct position. The initiator tRNA, formylmethionine‑tRNA (fMet‑tRNAfMet), then pairs with AUG and the 50S subunit joins to form the 70S initiation complex.

起始需要小亚基、mRNA、起始 tRNA（携带甲硫氨酸）和起始因子的装配。在原核生物中，30S 亚基与 mRNA 上富含嘌呤的 Shine‑Dalgarno 序列结合，使起始密码子 AUG 准确定位。起始 tRNA（甲酰甲硫氨酸‑tRNAⁱⁿⁱᵗ）随后与 AUG 配对，50S 亚基加入，形成 70S 起始复合物。

In eukaryotes, the 40S subunit recognises the 5′ cap of mRNA and scans along until it finds the first AUG within a Kozak consensus sequence. The initiator tRNA is Met‑tRNAⁱⁿⁱᵗ (unformylated), and numerous eukaryotic initiation factors (eIFs) orchestrate the process. The 60S subunit then joins, forming an 80S ribosome ready for elongation.

在真核生物中，40S 亚基识别 mRNA 的 5′ 帽结构并沿链扫描，直至找到位于 Kozak 共有序列中的第一个 AUG。起始 tRNA 为 Met‑tRNAⁱⁿⁱᵗ（未被甲酰化），多种真核起始因子（eIFs）协调整个过程。随后 60S 亚基加入，形成 80S 核糖体，准备进入延伸阶段。

6. Elongation: The Polypeptide Chain Grows | 延伸：多肽链的延长

Elongation is a cyclic process that adds amino acids one by one to the growing polypeptide. Each cycle involves three steps: (i) codon‑directed binding of an aminoacyl‑tRNA to the A site, (ii) peptide bond formation, and (iii) translocation of the ribosome along the mRNA by one codon.

延伸是一个循环过程，逐个将氨基酸添加到生长中的多肽链上。每个循环包含三个步骤：(i) 由密码子指导氨酰‑tRNA 结合至 A 位点，(ii) 肽键形成，(iii) 核糖体沿 mRNA 易位一个密码子的距离。

An elongation factor (EF‑Tu in prokaryotes, eEF1α in eukaryotes) delivers the aminoacyl‑tRNA to the A site, and GTP hydrolysis ensures accurate codon‑anticodon pairing. Once the correct tRNA is in place, the peptidyl transferase centre catalyses the formation of a peptide bond between the carboxyl group of the polypeptide (attached to the tRNA in the P site) and the amino group of the amino acid in the A site. This reaction releases the tRNA in the P site, leaving a deacylated tRNA.

延伸因子（原核生物为 EF‑Tu，真核生物为 eEF1α）将氨酰‑tRNA 运送至 A 位点，GTP 水解保证了密码子‑反密码子配对的准确性。正确的 tRNA 到位后，肽基转移酶中心催化 P 位点 tRNA 上多肽的羧基与 A 位点氨基酸的氨基之间形成肽键。该反应使 P 位点的 tRNA 被释放，留下一分子去酰化 tRNA。

Translocation is driven by another elongation factor (EF‑G in prokaryotes) with GTP hydrolysis. The ribosome moves exactly three nucleotides along the mRNA, shifting the peptidyl‑tRNA from the A site to the P site and the deacylated tRNA from the P site to the E site, from which it quickly disassociates. The A site is now vacant and ready for the next aminoacyl‑tRNA.

易位由另一种延伸因子（原核生物为 EF‑G）和 GTP 水解驱动。核糖体沿 mRNA 精确移动三个核苷酸，将肽基‑tRNA 从 A 位点移至 P 位点，并将去酰化 tRNA 从 P 位点移至 E 位点，后者迅速脱离。此时 A 位点空出，准备接受下一个氨酰‑tRNA。

7. Peptide Bond Formation – A Closer Look | 肽键形成——深入解析

The formation of a peptide bond is a condensation reaction that releases one water molecule. The carboxyl group (–COOH) of the nascent polypeptide attacks the amino group (–NH₂) of the incoming amino acid, forming a covalent C‑N bond. This reaction is catalysed by the 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) in the large subunit – a classic example of a ribozyme.

肽键的形成是一个缩合反应，释放出一分子水。新生多肽链的羧基（–COOH）攻击进入的氨基酸的氨基（–NH₂），形成共价 C‑N 键。该反应由大亚基中的 23S rRNA（原核生物）或 28S rRNA（真核生物）催化——这是核酶的经典范例。

Because rRNA, not protein, provides the catalytic activity, the ribosome is considered a ribozyme. The energy for peptide bond formation comes from the high‑energy ester linkage between the tRNA and its amino acid in the A site, not from GTP. GTP is used during tRNA binding and translocation.

由于起催化作用的是 rRNA 而非蛋白质，核糖体被视为一种核酶。形成肽键的能量来自 A 位点 tRNA 与氨基酸之间的高能酯键，而非来自 GTP。GTP 在 tRNA 结合和易位过程中被消耗。

8. Termination of Translation | 翻译的终止

Termination occurs when a stop codon (UAA, UAG or UGA) moves into the A site. There are no corresponding tRNAs for stop codons; instead, release factors (RF‑1, RF‑2, RF‑3 in prokaryotes; eRF1 in eukaryotes) recognise the stop codon and bind to the A site. These factors trigger the peptidyl transferase to hydrolyse the bond between the polypeptide and the tRNA in the P site, releasing the completed protein.

当终止密码子（UAA、UAG 或 UGA）进入 A 位点时，翻译终止。终止密码子没有对应的 tRNA；取而代之的是释放因子（原核生物中的 RF‑1、RF‑2、RF‑3；真核生物中的 eRF1）识别终止密码子并结合到 A 位点。这些因子促使肽基转移酶水解 P 位点 tRNA 与多肽之间的键，释放出完整蛋白质。

Following polypeptide release, the ribosomal subunits, mRNA and remaining deacylated tRNA dissociate, often aided by ribosome recycling factors. The subunits can then initiate another round of translation. Improper termination, such as read‑through of stop codons, can produce elongated or misfolded proteins, which is a potential topic in experimental analysis questions.

多肽释放后，核糖体亚基、mRNA 和剩余的去酰化 tRNA 会解离，这通常需要核糖体再循环因子的协助。随后亚基可进入下一轮翻译。不正确的终止（如通读终止密码子）会产生延长或错误折叠的蛋白质，这是实验分析类题目中的潜在考点。

9. Polysomes and Efficiency | 多聚核糖体与翻译效率

Multiple ribosomes can translate a single mRNA molecule simultaneously, forming a structure called a polysome or polyribosome. This greatly amplifies protein synthesis efficiency, as several polypeptide chains can be produced in quick succession from one mRNA template. Polysomes are visible in electron micrographs and are often used as evidence that translation is occurring in a specific cellular region.

多个核糖体可以同时翻译同一条 mRNA 分子，形成称为多聚核糖体（polysome）的结构。这极大提高了蛋白质合成效率，因为从一个 mRNA 模板上可以快速连续地产生多条多肽链。多聚核糖体在电镜照片中清晰可见，常被用作特定细胞区域发生翻译的证据。

In prokaryotes, because there is no nuclear membrane, transcription and translation are coupled – ribosomes can begin translating mRNA while it is still being synthesised. In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm, so co‑transcriptional translation is not possible, though polysomes still form in the cytoplasm.

原核生物没有核膜，因此转录和翻译可以偶联——核糖体能在 mRNA 尚未完全合成时就开始翻译。在真核生物中，转录在细胞核内进行，翻译在细胞质中进行，因此不存在共转录翻译，但细胞质中仍然可以形成多聚核糖体。

10. Post‑Translational Modifications | 翻译后修饰

After release from the ribosome, many proteins undergo post‑translational modifications (PTMs) to become fully functional. These include folding with the aid of chaperones, cleavage of signal peptides or pro‑regions, and covalent addition of chemical groups such as phosphorylation, glycosylation or ubiquitination. PTMs can regulate protein activity, localisation and stability.

从核糖体释放后，许多蛋白质需经过翻译后修饰才能发挥完整功能。这些修饰包括在分子伴侣辅助下折叠，切除信号肽或前体区，以及共价添加化学基团，如磷酸化、糖基化或泛素化。翻译后修饰可调控蛋白质的活性、定位和稳定性。

For example, insulin is first synthesised as preproinsulin, which undergoes cleavage to become proinsulin and finally mature insulin. Phosphorylation of enzymes is a key regulatory mechanism in many signalling pathways. Exam questions may ask you to interpret experimental data showing protein size shifts due to modification.

例如，胰岛素最初合成为前胰岛素原，经剪切变为胰岛素原，最终成为成熟胰岛素。酶的磷酸化是许多信号通路中的关键调控机制。考题可能要求你根据实验数据判断由修饰引起的蛋白质大小变化。

11. Comparing Prokaryotic and Eukaryotic Translation | 原核与真核翻译的比较

Although the core mechanism of translation is conserved, there are notable differences that are frequently examined:

Ribosomes: 70S in prokaryotes vs 80S in eukaryotes.
Initiation signals: Shine‑Dalgarno sequence in prokaryotes; 5′ cap and Kozak context in eukaryotes.
Initiator tRNA: Formylmethionine (fMet) in prokaryotes; methionine (Met) in eukaryotes.
Cellular location: Coupled transcription‑translation in prokaryotes; separate in eukaryotes.
Inhibitors: Antibiotics like tetracycline and chloramphenicol target bacterial 70S ribosomes without affecting eukaryotic 80S ribosomes, making them useful drugs.

尽管翻译的核心机制保守，但原核与真核生物之间存在显著差异，这些差异常被考查：

核糖体：原核生物为 70S，真核生物为 80S。
起始信号：原核生物依赖 Shine‑Dalgarno 序列；真核生物依赖 5′ 帽结构和 Kozak 共有序列。
起始 tRNA：原核生物为甲酰甲硫氨酸（fMet），真核生物为甲硫氨酸（Met）。
细胞位置：原核生物中转录与翻译偶联；真核生物中二者分隔进行。
抑制剂：四环素、氯霉素等抗生素可作用于细菌 70S 核糖体，而不影响真核 80S 核糖体，因此成为有效的药物。

12. Common Exam Pitfalls and Tips | 常见失分点与应试技巧

Anticodon‑codon pairing: Students often write the anticodon in the same 5’→3′ direction as the codon. Remember to write the anticodon antiparallel and complementary. For example, if the codon is 5’‑AUG‑3′, the anticodon is 3’‑UAC‑5′, typically written as 5’‑CAU‑3′ in the standard 5’→3′ notation.

反密码子‑密码子配对：学生常将反密码子按与密码子相同的 5’→3′ 方向书写。务必记住反密码子是反向平行且互补的。例如，密码子为 5’‑AUG‑3’，反密码子为 3’‑UAC‑5’，按标准 5’→3′ 方向通常写作 5’‑CAU‑3’。

Direction of translation: The ribosome reads mRNA 5’→3′, and the polypeptide is synthesised from the N‑terminus to the C‑terminus. Confusing these directions leads to erroneous answers in questions about mutation effects or sequence outputs.

翻译方向：核糖体沿 5’→3′ 方向读取 mRNA，多肽链从 N 端向 C 端合成。混淆这些方向会导致在突变效应或序列推导题中得出错误答案。

Energy consumption: Note that GTP is used during initiation, tRNA binding and translocation, but not in peptide bond formation itself. The energy for forming the peptide bond is already stored in the aminoacyl‑tRNA ester linkage.

能量消耗：注意 GTP 在起始、tRNA 结合和易位过程中被消耗，但肽键形成本身不消耗 GTP。形成肽键的能量已经储存在氨酰‑tRNA 的酯键中。

Stop codon recognition: Release factors, not tRNA, bind to stop codons. Never state that a tRNA anticodon pairs with UAA, UAG or UGA.

终止密码子的识别：与终止密码子结合的是释放因子，而非 tRNA。切勿说 tRNA 的反密码子与 UAA、UAG 或 UGA 配对。

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IB Edexcel Biology: Translation Key Points | 翻译 考点精讲