📚 Translation: CIE A-Level Biology Key Concepts | 翻译：CIE A-Level 生物考点精讲
Translation is the process by which the sequence of codons in mRNA is decoded into the sequence of amino acids in a polypeptide chain. This topic is central to CIE A-Level Biology, linking the molecular machinery of the cell to the expression of genes. Mastery of the steps, components, and exam technique is essential for high marks in both structured questions and multiple-choice tests.
翻译是 mRNA 中的密码子序列被解码为多肽链中氨基酸序列的过程。该主题是 CIE A-Level 生物的核心，它将细胞的分子机器与基因表达联系起来。掌握步骤、组分和考试技巧对于在简答题和选择题中获得高分至关重要。
1. Central Dogma and the Role of Translation | 中心法则与翻译的作用
The central dogma of molecular biology states that DNA is transcribed into mRNA, which is then translated into protein. Translation occurs in the cytoplasm on ribosomes and represents the final step of gene expression, directly determining the primary structure of a polypeptide.
分子生物学的中心法则指出，DNA 转录成 mRNA，随后 mRNA 翻译为蛋白质。翻译在细胞质的核糖体上进行，是基因表达的最后一步，直接决定了多肽的一级结构。
Errors in translation, such as incorporation of the wrong amino acid, can lead to non-functional proteins and have severe consequences for the cell. Therefore, high fidelity is maintained throughout the process by proofreading mechanisms.
翻译中的错误，如掺入错误的氨基酸，可能导致无功能的蛋白质，并对细胞产生严重后果。因此，整个翻译过程通过校对机制维持高保真度。
2. Components of Translation: mRNA, tRNA, Ribosomes | 翻译的组分：mRNA、tRNA 与核糖体
Messenger RNA (mRNA) carries the genetic code from the nucleus to the ribosome. It is read in groups of three bases called codons, each specifying one amino acid. The coding region is flanked by untranslated regions (UTRs) that play roles in regulation.
信使 RNA (mRNA) 将遗传密码从细胞核带到核糖体。它以三个碱基为一组（称为密码子）被读取，每个密码子指定一种氨基酸。编码区两侧为非翻译区 (UTR)，在调控中发挥作用。
Transfer RNA (tRNA) molecules act as adaptors. Each tRNA has an anticodon loop complementary to a codon and carries a specific amino acid esterified to its 3′ end. The anticodon pairs with the mRNA codon via hydrogen bonds in an antiparallel fashion.
转移 RNA (tRNA) 分子充当适配器。每个 tRNA 具有与密码子互补的反密码子环，并在其 3’端携带一个特定的氨基酸。反密码子通过氢键与 mRNA 密码子反平行配对。
Ribosomes are large ribonucleoprotein complexes composed of a small subunit and a large subunit. In eukaryotes, the small subunit (40S) binds mRNA, while the large subunit (60S) catalyses peptide bond formation. The ribosome has three tRNA binding sites: the A (aminoacyl), P (peptidyl), and E (exit) sites.
核糖体是由小亚基和大亚基组成的大型核糖核蛋白复合物。在真核生物中，小亚基 (40S) 结合 mRNA，大亚基 (60S) 催化肽键形成。核糖体具有三个 tRNA 结合位点：A 位（氨酰位）、P 位（肽基位）和 E 位（出口位）。
3. The Genetic Code: Codons and Anticodons | 遗传密码：密码子与反密码子
The genetic code is degenerate (most amino acids are specified by more than one codon), non-overlapping, and comma-less. It is almost universal across all organisms. The start codon is typically AUG, which codes for methionine in eukaryotes and formylmethionine in prokaryotes.
遗传密码是简并的（大多数氨基酸由多种密码子编码）、无重叠、无标点的，并且在所有生物中几乎通用。起始密码子通常为 AUG，在真核生物中编码甲硫氨酸，在原核生物中编码甲酰甲硫氨酸。
Three stop codons—UAA, UAG, and UGA—do not code for any amino acid; instead, they signal the termination of translation. Anticodons on tRNA base-pair with codons following complementary rules, with some flexibility in the third base known as wobble.
三个终止密码子——UAA、UAG 和 UGA——不编码任何氨基酸，而是发出翻译终止的信号。tRNA 上的反密码子按照互补规则与密码子配对，第三个碱基具有一定的灵活性，称为摆动。
| Codon | Signal / Amino Acid |
|---|---|
| AUG | Start / Methionine |
| UAA | Stop |
| UAG | Stop |
| UGA | Stop |
4. tRNA Charging: Aminoacyl-tRNA Synthetase | tRNA 的负载：氨酰-tRNA 合成酶
Before a tRNA can participate in translation, it must be covalently linked to its cognate amino acid. This process, called charging or aminoacylation, is catalysed by aminoacyl-tRNA synthetases (aaRS). There is at least one specific aaRS for each amino acid.
在 tRNA 参与翻译之前，它必须与其对应的氨基酸共价连接。这一过程称为负载或氨酰化，由氨酰-tRNA 合成酶 (aaRS) 催化。每种氨基酸至少有一种特异性的 aaRS。
The reaction occurs in two steps: first, the amino acid is activated by ATP, forming an aminoacyl-AMP intermediate; second, the activated amino acid is transferred to the 2′ or 3′ hydroxyl of the terminal adenine of tRNA. The enzyme has proofreading activity to ensure extremely high accuracy.
反应分两步进行：首先，氨基酸被 ATP 活化，形成氨酰-AMP 中间体；其次，活化的氨基酸转移到 tRNA 末端腺苷的 2’ 或 3’ 羟基上。该酶具有校对活性，确保极高的准确性。
5. Initiation: Assembly of the Translation Complex | 起始：翻译复合物的组装
In eukaryotes, initiation begins with the assembly of the pre-initiation complex. The small ribosomal subunit (40S) associates with initiation factors (eIFs) and an initiator tRNA charged with methionine (Met-tRNAi). This complex binds to the 5′ cap of mRNA and scans along the molecule until it encounters the first AUG codon in a favourable Kozak sequence.
在真核生物中，起始以预起始复合物的组装开始。小核糖体亚基 (40S) 与起始因子 (eIFs) 和负载甲硫氨酸的起始 tRNA (Met-tRNAi) 结合。该复合物与 mRNA 的 5’ 帽结合，并沿着分子扫描，直到在有利的 Kozak 序列中遇到第一个 AUG 密码子。
Once the start codon is recognised, the large ribosomal subunit (60S) joins, driven by GTP hydrolysis. This forms the complete 80S ribosome with the initiator tRNA occupying the P site. The A site is now ready to accept the next aminoacyl-tRNA.
一旦识别出起始密码子，在 GTP 水解的驱动下，大核糖体亚基 (60S) 加入，形成完整的 80S 核糖体，起始 tRNA 占据 P 位。此时 A 位准备接受下一个氨酰-tRNA。
6. Elongation: Peptide Bond Formation and Translocation | 延伸：肽键形成与移位
Elongation proceeds through a cycle of three steps: codon recognition, peptide bond formation, and translocation. An aminoacyl-tRNA whose anticodon matches the codon in the A site enters the A site, a process facilitated by elongation factors (eEF-1 in eukaryotes) and GTP hydrolysis.
延伸通过三个步骤的循环进行：密码子识别、肽键形成和移位。其反密码子与 A 位密码子匹配的氨酰-tRNA 进入 A 位，该过程由延伸因子（真核生物中为 eEF-1）和 GTP 水解协助。
The large ribosomal subunit possesses peptidyl transferase activity, which catalyses the formation of a peptide bond between the amino acid in the P site and the incoming amino acid in the A site. This activity is actually due to the ribosomal RNA (rRNA), making the ribosome a ribozyme. The polypeptide chain is transferred to the tRNA in the A site.
大核糖体亚基具有肽基转移酶活性，能催化 P 位氨基酸与 A 位新进入氨基酸之间形成肽键。该活性实际上归因于核糖体 RNA (rRNA)，使核糖体成为一种核酶。多肽链被转移到 A 位的 tRNA 上。
Translocation then occurs: the ribosome moves one codon along the mRNA in the 5′ to 3′ direction. This requires elongation factor eEF-2 and GTP. The deacylated tRNA in the P site moves to the E site and exits, the peptidyl-tRNA moves from A to P, and the A site becomes vacant for the next round.
随后发生移位：核糖体沿 mRNA 从 5’ 到 3’ 方向移动一个密码子。这需要延伸因子 eEF-2 和 GTP。P 位脱酰基的 tRNA 移至 E 位并离开，肽基-tRNA 从 A 位移至 P 位，A 位空出以进行下一轮循环。
7. Termination: Release Factors and Disassembly | 终止：释放因子与解聚
When the ribosome encounters a stop codon (UAA, UAG, or UGA) in the A site, no tRNA recognises it. Instead, class I release factors (eRF1 in eukaryotes) bind to the A site. This triggers the peptidyl transferase to hydrolyse the ester bond linking the polypeptide to the tRNA in the P site, releasing the newly synthesised polypeptide.
当核糖体在 A 位遇到终止密码子（UAA、UAG 或 UGA）时，没有 tRNA 能识别它。相反，I 类释放因子（真核生物为 eRF1）结合至 A 位。这会触发肽基转移酶水解连接多肽与 P 位 tRNA 的酯键，释放新合成的多肽。
Following peptide release, class II release factors (eRF3) aided by GTP hydrolysis promote the dissociation of the ribosomal subunits, mRNA, and remaining tRNA. These components can be reused in further rounds of translation.
多肽释放后，II 类释放因子 (eRF3) 在 GTP 水解的帮助下促进核糖体亚基、mRNA 和剩余 tRNA 的解离。这些组分可被重新用于后续的翻译过程。
8. Polysomes and Efficiency | 多聚核糖体与效率
A single mRNA molecule can be translated by multiple ribosomes simultaneously. This structure, visible under electron microscopy, is called a polysome or polyribosome. Each ribosome translates the same mRNA independently, producing multiple copies of the polypeptide in a short time.
单个 mRNA 分子可同时被多个核糖体翻译。这种在电子显微镜下可见的结构称为多聚核糖体或聚核糖体。每个核糖体独立地翻译同一条 mRNA，在短时间内产生多份多肽拷贝。
Polysomes are found in both prokaryotic and eukaryotic cells. Because transcription and translation are coupled in prokaryotes, polysomes often form on nascent mRNAs. This arrangement dramatically increases the efficiency of protein synthesis.
多聚核糖体存在于原核和真核细胞中。由于原核生物中转录和翻译是偶联的，聚核糖体常在新生的 mRNA 上形成。这种排列大大提高了蛋白质合成的效率。
9. Post-translational Modifications | 翻译后修饰
The polypeptide released from the ribosome often undergoes modifications before becoming a fully functional protein. These modifications may include cleavage of the initiator methionine, formation of disulfide bonds, phosphorylation, glycosylation, or addition of prosthetic groups.
从核糖体释放的多肽在成为完全功能性蛋白质之前常常会进行修饰。这些修饰可能包括切除起始甲硫氨酸、形成二硫键、磷酸化、糖基化或添加辅基。
Many proteins require chaperone-assisted folding to achieve their correct three-dimensional conformation. Some proteins, such as insulin, are proteolytically cleaved to remove signal peptides or internal segments, a step essential for biological activity.
许多蛋白质需要分子伴侣辅助折叠，才能获得正确的三维构象。某些蛋白质，如胰岛素，需要通过蛋白酶解切除信号肽或内部片段，这一步对其生物活性至关重要。
10. Exam Focus: Key Diagrams and Marking Points | 考试要点：关键图示与得分点
In CIE A-Level exams, translation questions frequently ask you to describe the steps in sequence using proper terminology (e.g. aminoacylation, peptidyl transferase, translocation). Ensure you mention the roles of ATP and GTP, the direction of ribosome movement (5′ to 3′), and the sites (A, P, E).
在 CIE A-Level 考试中，翻译题目经常要求你使用正确的术语（如氨酰化、肽基转移酶、移位）按顺序描述步骤。务必提及 ATP 和 GTP 的作用、核糖体移动的方向（5’ 到 3’）以及位点（A、P、E）。
Learn to interpret diagrams of the ribosome showing tRNA positions and codon–anticodon interactions. Common mistakes include confusing transcription with translation, writing ‘U’ in DNA, or misidentifying the template strand. Practice using the codon table to deduce amino acid sequences.
学会解释展示 tRNA 位置以及密码子-反密码子相互作用的核糖体示意图。常见错误包括混淆转录与翻译、在 DNA 中写 ‘U’，或错误识别模板链。练习使用密码子表推导氨基酸序列。
When drawing flowcharts for revision, link each stage to the energy source (ATP for charging, GTP for initiation and translocation). Remember that peptide bond formation itself does not require ATP/GTP input but is an energetic process facilitated by the ester bond energy from the charged tRNA.
在绘制复习流程图时，将每个阶段与能量来源联系起来（负载消耗 ATP，起始和移位消耗 GTP）。请记住，肽键形成本身不需要 ATP/GTP 输入，但它是一个由负载 tRNA 的酯键能量所推动的放能过程。
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