📚 A-Level OCR Biology: Translation Exam Focus | A-Level OCR 生物：翻译 考点精讲
Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids in a polypeptide chain. It is a crucial part of gene expression and occurs on ribosomes in the cytoplasm. Understanding translation is essential for A-Level OCR Biology, as it links the genetic code to protein synthesis and often appears in exam questions.
翻译是利用mRNA携带的遗传密码，解码产生特定氨基酸序列多肽链的过程。它是基因表达的关键部分，发生在细胞质的核糖体上。理解翻译对于A-Level OCR生物至关重要，因为它将遗传密码与蛋白质合成联系起来，常在考试中出现。
1. Overview of Translation | 翻译概述
Translation is the second stage of protein synthesis, following transcription. It involves the conversion of the nucleotide sequence of mRNA into the amino acid sequence of a polypeptide. The process requires mRNA, ribosomes, transfer RNA (tRNA) molecules, amino acids, and several protein factors. The ribosome reads the mRNA in the 5′ to 3′ direction, and the resulting polypeptide grows from the N-terminus to the C-terminus.
翻译是蛋白质合成的第二阶段，紧随转录之后。它涉及将mRNA的核苷酸序列转化为多肽的氨基酸序列。该过程需要mRNA、核糖体、转运RNA (tRNA) 分子、氨基酸及多种蛋白质因子。核糖体沿5’向3’方向阅读mRNA，生成的多肽从N末端向C末端延伸。
2. The Machinery: mRNA, tRNA and Ribosomes | 分子机器：mRNA、tRNA与核糖体
mRNA carries the genetic code from DNA in the form of codons—triplets of nucleotides. tRNA molecules have a specific anticodon at one end and an amino acid attachment site at the other. Each tRNA is specific to one amino acid. Ribosomes consist of two subunits (large and small) made of ribosomal RNA (rRNA) and proteins. In eukaryotes, the small subunit is 40S and the large is 60S, forming an 80S ribosome; prokaryotes have 30S, 50S and 70S. The ribosome provides three sites: A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA binding.
mRNA以密码子（三个核苷酸一组）的形式携带来自DNA的遗传信息。tRNA分子一端具有特异的反密码子，另一端是氨基酸结合位点。每种tRNA特异对应一种氨基酸。核糖体由大、小两个亚基组成，由核糖体RNA (rRNA) 和蛋白质构成。真核生物中小亚基为40S，大亚基60S，形成80S核糖体；原核生物则为30S、50S和70S。核糖体提供三个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 and Codon–Anticodon Recognition | 遗传密码与密码子–反密码子识别
The genetic code is degenerate (several codons can code for the same amino acid), unambiguous (each codon specifies only one amino acid), and universal (with rare exceptions). The start codon is AUG, which encodes methionine. Stop codons (UAA, UAG, UGA) do not code for any amino acid; they signal termination. During translation, the tRNA anticodon (three nucleotides) base-pairs with the complementary mRNA codon. The pairing is antiparallel: codon 5′-AUG-3′ pairs with anticodon 3′-UAC-5′. Wobble base pairing at the third position allows some tRNAs to recognize more than one codon.
遗传密码是简并的（多个密码子编码同一种氨基酸）、明确无歧义的（每个密码子只指定一种氨基酸）、并且几乎普遍通用（极少数例外）。起始密码子是AUG，编码甲硫氨酸。终止密码子（UAA、UAG、UGA）不编码任何氨基酸，它们发出终止信号。翻译时，tRNA的反密码子（三个核苷酸）与mRNA上互补的密码子碱基配对。配对是反向平行的：密码子5′-AUG-3’与反密码子3′-UAC-5’结合。第三位碱基的摆动配对允许某些tRNA识别多个密码子。
4. Amino Acid Activation and tRNA Charging | 氨基酸活化与tRNA装载
Before translation, amino acids must be attached to their corresponding tRNA molecules. This is catalysed by aminoacyl-tRNA synthetases, each specific to one amino acid and its tRNA isoacceptors. The enzyme uses ATP to activate the amino acid, forming an aminoacyl-AMP intermediate, and then transfers the amino acid to the 3′ end of the tRNA, forming an aminoacyl-tRNA (charged tRNA). The reaction: Amino acid + ATP + tRNA → Aminoacyl-tRNA + AMP + PPi.
翻译前，氨基酸必须与相应的tRNA结合。这一过程由氨酰-tRNA合成酶催化，每种酶特异对应一种氨基酸及其同工tRNA。该酶利用ATP活化氨基酸，形成氨酰-AMP中间体，然后将氨基酸转移到tRNA的3’末端，形成氨酰-tRNA（即负载tRNA）。反应式为：氨基酸 + ATP + tRNA → 氨酰-tRNA + AMP + PPi。
5. Initiation of Translation | 翻译起始
In eukaryotes, initiation begins when the small ribosomal subunit binds to the 5′ cap of mRNA and scans for the start codon AUG. The initiator tRNA carrying methionine (Met-tRNAi) binds to the start codon via its anticodon. Several initiation factors (eIFs) facilitate the assembly. The large ribosomal subunit then joins, forming a functional ribosome with Met-tRNAi in the P site. In prokaryotes, the small subunit binds to a Shine-Dalgarno sequence upstream of AUG, and the initiator tRNA carries formyl-methionine (fMet).
真核生物中，起始时小核糖体亚基结合到mRNA的5’帽子结构，并扫描寻找起始密码子AUG。携带甲硫氨酸的起始tRNA (Met-tRNAi) 通过反密码子与起始密码子结合。多种起始因子 (eIF) 协助组装。随后大亚基加入，形成具有功能的核糖体，此时Met-tRNAi位于P位点。原核生物中，小亚基结合到AUG上游的Shine-Dalgarno序列，起始tRNA携带甲酰甲硫氨酸 (fMet)。
6. Elongation: Codon Recognition and Peptide Bond Formation | 延伸：密码子识别与肽键形成
Elongation involves a cycle of three steps: (1) a charged tRNA enters the A site, its anticodon pairing with the mRNA codon; (2) a peptide bond is formed between the amino acid in the P site and the newly arrived amino acid in the A site, catalysed by peptidyl transferase (an rRNA ribozyme in the large subunit). The P site tRNA becomes deacylated; (3) the ribosome translocates, moving the tRNA from the A site to the P site and the deacylated tRNA to the E site, reading the next codon. The polypeptide chain grows by one amino acid per cycle.
延伸循环包括三步：(1) 负载tRNA进入A位，其反密码子与mRNA密码子配对；(2) P位上氨基酸与新进入A位的氨基酸之间形成肽键，由肽基转移酶（大亚基中的rRNA核酶）催化，P位tRNA变为脱酰状态；(3) 核糖体易位，将A位tRNA移至P位，脱酰tRNA移至E位，并阅读下一个密码子。每循环一次多肽链延长一个氨基酸。
Peptide bond formation: n(amino acid) → polypeptide + (n−1) H₂O
肽键形成：n个氨基酸 → 多肽 + (n−1) H₂O
7. Elongation: Translocation and GTP Hydrolysis | 延伸：移位与GTP水解
Translocation requires elongation factor G (EF-G in prokaryotes, eEF2 in eukaryotes) and GTP hydrolysis. As the ribosome moves precisely three nucleotides along the mRNA, the uncharged tRNA exits via the E site. The energy from GTP hydrolysis drives the conformational changes. The ribosome then exposes the next codon in the A site, ready for the incoming aminoacyl-tRNA. This process continues, adding amino acids sequentially.
移位需要延伸因子G（原核为EF-G，真核为eEF2）和GTP水解。核糖体沿着mRNA精确移动三个核苷酸，脱酰tRNA通过E位离开。GTP水解产生的能量驱动构象变化。核糖体随后在A位暴露下一个密码子，准备接纳新的氨酰-tRNA。该过程持续进行，依次添加氨基酸。
8. Termination of Translation | 翻译终止
When a stop codon (UAA, UAG, or UGA) enters the A site, no normal tRNA can recognize it. Instead, release factors (eRF in eukaryotes, RF1/RF2 in prokaryotes) bind to the A site. This triggers peptidyl transferase to hydrolyse the bond linking the polypeptide to the tRNA in the P site, releasing the polypeptide. The ribosomal subunits, mRNA, and last tRNA dissociate, often with the help of ribosome recycling factors.
当终止密码子（UAA、UAG或UGA）进入A位时，没有正常的tRNA能识别它。相反，释放因子（真核为eRF，原核为RF1/RF2）会结合到A位。这会触发肽基转移酶水解连接P位tRNA与多肽的键，释放多肽链。随后核糖体亚基、mRNA和最后的tRNA解离，通常有核糖体再循环因子协助。
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 polyribosome (polysome). This allows rapid production of many copies of the polypeptide from one mRNA template. In both prokaryotes and eukaryotes, polysomes can be observed. Electron micrographs often show ribosomes spaced along the mRNA strand.
多个核糖体可以同时翻译同一条mRNA分子，形成多聚核糖体（多聚体）。这使得一个mRNA模板能快速产生多拷贝的多肽。原核和真核生物中均可观察到多聚核糖体。电子显微照片常显示核糖体沿mRNA链间隔排列。
10. Translation on the Rough Endoplasmic Reticulum | 粗面内质网上的翻译
Polypeptides destined for secretion or membrane insertion are synthesised by ribosomes attached to the rough ER. The polypeptide contains a signal peptide at its N-terminus that directs the ribosome to the ER membrane. As translation proceeds, the emerging peptide enters the ER lumen, where chaperones assist folding and post-translational modifications such as glycosylation begin.
需分泌或插入膜的多肽由附着在粗面内质网上的核糖体合成。该多肽在N端含有信号肽，引导核糖体到达内质网膜。随着翻译进行，新生肽链进入内质网腔，伴随伴侣蛋白辅助折叠，并开始糖基化等翻译后修饰。
11. Post-translational Modifications (PTMs) | 翻译后修饰
After translation, many proteins undergo post-translational modifications to become functional. These include phosphorylation, glycosylation, cleavage of signal peptides, and formation of disulfide bonds. Some modifications are essential for protein folding and activity. PTMs can also regulate protein function and stability.
翻译后，许多蛋白经过翻译后修饰才能发挥功能。包括磷酸化、糖基化、信号肽切除、二硫键形成等。某些修饰对蛋白质折叠和活性至关重要。PTM也能调控蛋白质功能和稳定性。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
In OCR A-Level exams, students often mix up transcription and translation, confuse codons with anticodons, or fail to specify the direction of ribosome movement. Remember that the ribosome moves along mRNA, not the other way around. Be clear about the energy source (GTP, not ATP for elongation). Use precise terminology: ‘peptide bond formation is catalysed by peptidyl transferase’. Also, relate the process to gene mutations (e.g., a substitution may lead to a different amino acid if it
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