📚 Translation in IB & OCR Biology: Key Exam Points | IB OCR 生物：翻译 考点精讲
Translation is the process by which the genetic information carried by mRNA is decoded to synthesise a specific polypeptide chain. It occurs on ribosomes and involves the coordinated action of mRNA, tRNA, aminoacyl‑tRNA synthetases, and numerous protein factors. Mastering this topic is essential for both IB and OCR A‑Level Biology, as it frequently appears in structured questions requiring precise molecular details and an understanding of sequence‑to‑phenotype relationships.
翻译是指 mRNA 携带的遗传信息被解码并合成特定多肽链的过程。它发生在核糖体上，涉及 mRNA、tRNA、氨酰‑tRNA 合成酶以及多种蛋白质因子的协同作用。准确掌握这一主题对 IB 和 OCR A‑Level 生物学至关重要，因为考试中经常出现结构性问题，要求考生熟悉分子层面的精确细节并理解序列与表型之间的关系。
1. Introduction to Translation | 翻译简介
Translation is the second major stage of gene expression, following transcription. In this process, the linear sequence of codons in mRNA is read by ribosomes to direct the polymerisation of amino acids into a polypeptide. The reaction requires energy in the form of GTP and ATP, and it is highly regulated to ensure fidelity.
翻译是基因表达的第二大阶段，紧随转录之后。在此过程中，mRNA 中的线性密码子序列被核糖体读取，指导氨基酸聚合成多肽。该反应需要 GTP 和 ATP 形式的能量，并且受到严格调控以确保高保真度。
Unlike DNA replication and transcription, which use nucleic acid templates to direct nucleic acid synthesis, translation converts information from the language of nucleotides to the language of amino acids. This conceptual shift is often tested in exams.
与使用核酸模板指导核酸合成的 DNA 复制和转录不同，翻译将信息从核苷酸语言转换为氨基酸语言。这一概念转变在考试中经常被考查。
2. The Genetic Code: Codons and Anticodons | 遗传密码：密码子与反密码子
The genetic code is a set of rules that defines how a sequence of three nucleotides (a codon) on mRNA corresponds to one amino acid. There are 4³ = 64 possible codons, 61 of which specify amino acids, while three are stop codons (UAA, UAG, UGA) that signal termination. The code is degenerate, meaning most amino acids are encoded by more than one codon, and it is universal across almost all organisms—an important piece of evidence for common ancestry.
遗传密码是一套规则，定义了 mRNA 上三个核苷酸序列（密码子）如何对应一个氨基酸。共有 4³ = 64 种可能的密码子，其中 61 种编码氨基酸，另有三种是终止密码子（UAA、UAG、UGA），发出终止信号。密码子具有简并性，即大多数氨基酸由多个密码子编码，并且几乎所有生物体通用——这是共同祖先的重要证据。
Each tRNA molecule carries a specific trinucleotide sequence called the anticodon, which is complementary to a codon on the mRNA. Correct codon–anticodon base‑pairing, following Watson–Crick rules, ensures that the appropriate amino acid is inserted into the growing polypeptide. The wobble hypothesis explains how some tRNAs can recognise more than one codon due to flexibility at the third base position.
每个 tRNA 分子带有一个称为反密码子的特定三核苷酸序列，与 mRNA 上的密码子互补。正确的密码子‑反密码子碱基配对遵循沃森‑克里克规则，确保将正确的氨基酸插入到延长的多肽链中。摆动假说解释了为何某些 tRNA 由于第三碱基位置的灵活性能够识别多个密码子。
3. Structure of Ribosomes | 核糖体的结构
Ribosomes are ribonucleoprotein complexes that serve as the catalytic machinery for translation. Both prokaryotic and eukaryotic ribosomes consist of two subunits—large and small—composed of ribosomal RNA (rRNA) and ribosomal proteins. Prokaryotic ribosomes are 70S (30S small subunit + 50S large subunit), while eukaryotic cytoplasmic ribosomes are 80S (40S small subunit + 60S large subunit).
核糖体是核糖核蛋白复合物，充当翻译的催化机器。原核生物和真核生物的核糖体均由大小两个亚基组成，包含核糖体 RNA（rRNA）和核糖体蛋白。原核生物核糖体为 70S（30S 小亚基 + 50S 大亚基），而真核生物胞质核糖体为 80S（40S 小亚基 + 60S 大亚基）。
The ribosome contains three key tRNA binding sites: the A (aminoacyl) site, where an incoming aminoacyl‑tRNA binds; the P (peptidyl) site, which holds the tRNA carrying the growing peptide chain; and the E (exit) site, from which an uncharged tRNA leaves. Ribosomes also possess peptidyl transferase activity, catalysed by the rRNA component—making the ribosome a ribozyme.
核糖体包含三个关键的 tRNA 结合位点：A 位（氨基酰位），与新进入的氨基酰‑tRNA 结合；P 位（肽基位），容纳携带增长肽链的 tRNA；E 位（出口位），空载 tRNA 由此离开。核糖体还具有肽基转移酶活性，由 rRNA 组分催化——这使得核糖体成为一种核酶。
4. Transfer RNA (tRNA) Structure and Function | tRNA 的结构与功能
tRNA molecules are relatively short RNA chains (about 75–90 nucleotides) that fold into a characteristic cloverleaf secondary structure maintained by intramolecular hydrogen bonds. The three‑dimensional L‑shape brings the anticodon loop at one end and the amino acid attachment site (CCA‑3′ terminus) at the opposite end, allowing the tRNA to act as an adaptor molecule.
tRNA 分子是较短的 RNA 链（约 75–90 个核苷酸），通过分子内氢键折叠成典型的三叶草二级结构。其三维 L 型构象使一端的反密码子环与另一端的氨基酸连接位点（CCA‑3′ 末端）分离，使 tRNA 能够充当接头分子。
Each tRNA is specific to one amino acid, but due to the degeneracy of the genetic code, a cell may contain several isoacceptor tRNAs for the same amino acid. The amino acid is covalently attached to the 3’‑end adenosine of the tRNA via an ester bond, forming aminoacyl‑tRNA.
每个 tRNA 对一种氨基酸具有特异性，但由于遗传密码的简并性，细胞可能包含针对同一种氨基酸的若干同工受体 tRNA。氨基酸通过酯键共价连接到 tRNA 3′ 末端的腺苷上，形成氨基酰‑tRNA。
5. Aminoacyl‑tRNA Synthetases | 氨酰‑tRNA 合成酶
Aminoacyl‑tRNA synthetases (aaRS) are the enzymes responsible for charging tRNAs with their cognate amino acids. Each aaRS is highly specific for both a particular amino acid and its corresponding set of isoacceptor tRNAs. The charging reaction occurs in two steps: the amino acid is first activated by ATP to form an aminoacyl‑adenylate intermediate, then transferred to the tRNA’s 3’‑terminal ribose.
氨酰‑tRNA 合成酶是负责将 tRNA 与其同源氨基酸连接的酶。每种 aaRS 对特定的氨基酸及其对应的同工受体 tRNA 组具有高度特异性。连接反应分两步进行：氨基酸首先被 ATP 活化形成氨酰‑腺苷酸中间体，然后转移至 tRNA 的 3’‑末端核糖上。
This high fidelity is crucial because once an amino acid is attached to tRNA, the ribosome relies solely on codon–anticodon pairing for incorporation; it cannot check the identity of the amino acid. A mistake by aaRS would result in misincorporation and a potentially non‑functional protein.
这种高保真性至关重要，因为一旦氨基酸与 tRNA 连接，核糖体在掺入时仅依赖密码子‑反密码子配对，无法检查氨基酸的身份。aaRS 出错会导致错误掺入，可能产生无功能的蛋白质。
6. Initiation of Translation | 翻译的起始
In prokaryotes, initiation involves the binding of the small ribosomal subunit to the Shine–Dalgarno sequence on mRNA, located a few nucleotides upstream of the start codon AUG. This interaction aligns the start codon at the P site. The initiator tRNA carrying N‑formylmethionine (fMet) binds to the start codon, assisted by initiation factors (IF‑1, IF‑2, IF‑3), and then the large subunit joins to form the completed 70S initiation complex.
在原核生物中，起始过程涉及小核糖体亚基与 mRNA 上位于起始密码子 AUG 上游几个核苷酸处的 Shine–Dalgarno 序列结合。这种相互作用将起始密码子对准 P 位。携带 N‑甲酰甲硫氨酸（fMet）的起始 tRNA 在起始因子（IF‑1、IF‑2、IF‑3）的帮助下与起始密码子结合，随后大亚基加入，形成完整的 70S 起始复合物。
Eukaryotic initiation is more complex. The small 40S subunit binds to the methylguanosine cap at the 5′ end of mRNA and scans along until it finds the first AUG in a favourable Kozak consensus sequence. Initiation factors such as eIF4E (cap‑binding protein) and the poly‑A‑binding protein facilitate circularisation of the mRNA, enhancing efficiency.
真核生物的起始更为复杂。小亚基 40S 与 mRNA 5′ 端的甲基鸟苷帽结合，并沿着 mRNA 扫描，直至在有利的 Kozak 共有序列中找到第一个 AUG。起始因子如 eIF4E（帽结合蛋白）和多聚 A 结合蛋白促进 mRNA 环化，从而提高翻译效率。
7. Elongation Phase | 延伸阶段
Elongation is a cyclic process of amino acid addition that occurs in three main steps: codon recognition, peptide bond formation, and translocation. During codon recognition, an incoming aminoacyl‑tRNA, delivered by elongation factor EF‑Tu in prokaryotes (or eEF1α in eukaryotes), enters the A site and base‑pairs with the mRNA codon. GTP hydrolysis accompanies this step to ensure correct pairing.
延伸是一个循环添加氨基酸的过程，包括三个主要步骤：密码子识别、肽键形成和易位。在密码子识别阶段，由原核生物的延伸因子 EF‑Tu（或真核生物的 eEF1α）递送的氨基酰‑tRNA 进入 A 位并与 mRNA 密码子进行碱基配对。此步骤伴随 GTP 水解以确保正确配对。
Once the correct aminoacyl‑tRNA is bound, the peptidyl transferase centre (located in the 23S rRNA of the large subunit in prokaryotes, or 28S rRNA in eukaryotes) catalyses the formation of a peptide bond between the amino group of the A‑site amino acid and the carbonyl carbon of the P‑site polypeptide chain. The growing chain is transferred to the tRNA in the A site, leaving a deacylated tRNA in the P site.
一旦正确的氨基酰‑tRNA 结合，肽基转移酶中心（位于原核生物大亚基的 23S rRNA 或真核生物的 28S rRNA 中）催化 A 位氨基酸的氨基与 P 位多肽链的羰基碳之间形成肽键。增长链转移到 A 位的 tRNA 上，而 P 位留下一个空载 tRNA。
8. Translocation and Peptide Bond Formation | 易位与肽键形成
After peptide bond formation, the ribosome translocates along the mRNA by one codon in the 5′ → 3′ direction. This movement is driven by elongation factor EF‑G in prokaryotes (eEF2 in eukaryotes) with GTP hydrolysis. During translocation, the tRNAs shift positions: the deacylated tRNA moves from P to E, the peptidyl‑tRNA moves from A to P, and the A site becomes vacant, ready for the next codon.
肽键形成后，核糖体沿 mRNA 在 5′ → 3′ 方向上移动一个密码子。这一移动由原核生物的延伸因子 EF‑G（真核生物的 eEF2）在 GTP 水解驱动下完成。易位过程中，tRNA 位置发生变化：空载 tRNA 从 P 位移至 E 位，肽基‑tRNA 从 A 位移至 P 位，A 位空出，准备迎接下一个密码子。
The peptide bond (—CO—NH—) is formed by a nucleophilic attack of the α‑amino group of the A‑site aminoacyl‑tRNA on the ester carbonyl carbon linking the peptide chain to the P‑site tRNA. This is a condensation reaction that releases one water molecule. The ribosome’s rRNA provides the catalytic function, and no protein enzyme is responsible for this bond formation.
肽键（—CO—NH—）由 A 位氨基酰‑tRNA 的 α‑氨基对连接肽链与 P 位 tRNA 的酯羰基碳进行亲核攻击而形成。这是一个缩合反应，释放一分子水。核糖体的 rRNA 提供催化功能，且没有任何蛋白酶负责该键的形成。
9. Termination of Translation | 翻译的终止
Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA normally recognises these codons; instead, release factors (RF‑1 and RF‑2 in prokaryotes, eRF1 in eukaryotes) bind to the A site. These proteins mimic tRNA structure and trigger hydrolysis of the peptidyl‑tRNA ester bond, releasing the completed polypeptide chain.
当终止密码子（UAA、UAG 或 UGA）进入 A 位时，翻译终止。通常没有 tRNA 识别这些密码子；相反，释放因子（原核生物的 RF‑1 和 RF‑2，真核生物的 eRF1）与 A 位结合。这些蛋白质模拟 tRNA 结构，触发肽基‑tRNA 酯键的水解，释放完整的多肽链。
Following polypeptide release, the ribosomal subunits, mRNA, and deacylated tRNA dissociate with the help of ribosome recycling factors and GTP. The components can then be reused for additional rounds of translation.
多肽释放后，核糖体亚基、mRNA 和空载 tRNA 在核糖体循环因子和 GTP 的帮助下解离。这些组分随后可被重新用于下一轮翻译。
10. Polysomes and Efficiency | 多聚核糖体与效率
Multiple ribosomes can simultaneously translate a single mRNA molecule, forming a structure known as a polysome or polyribosome. Each ribosome operates independently, synthesising a separate polypeptide chain. This arrangement dramatically increases the overall rate of protein synthesis from one mRNA transcript and is observable in electron micrographs.
多个核糖体可以同时翻译同一 mRNA 分子，形成称为多聚核糖体的结构。每个核糖体独立运作，合成独立的多肽链。这种排列方式极大地提高了从单个 mRNA 转录本合成蛋白质的总速率，并且可在电子显微照片中观察到。
In prokaryotes, because transcription and translation occur in the same cellular compartment (no nucleus), ribosomes can begin translating mRNA while it is still being transcribed. This coupling is not possible in eukaryotes due to the nuclear envelope, and mRNA must be processed and exported before translation.
在原核生物中，由于转录和翻译发生在同一细胞区室（无细胞核），核糖体可在 mRNA 仍在转录时就开始翻译。这种偶联在真核生物中因核膜的存在而无法实现，mRNA 必须先经过加工并输出细胞核后才能进行翻译。
11. Comparison of Prokaryotic and Eukaryotic Translation | 原核与真核翻译的比较
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Ribosome size | 70S (30S + 50S) | 80S (40S + 60S) |
| Initiation | Shine–Dalgarno sequence aligns start codon | Kozak sequence scanning from 5′ cap |
| Initiator tRNA | fMet‑tRNAᶠᴹᵉᵗ | Met‑tRNAⁱᴹᵉᵗ |
| Transcription–translation coupling | Yes | No (nuclear separation) |
| Post‑translational modifications | Rare | Extensive (e.g., glycosylation, phosphorylation) |
| Targeting of ribosome to ER | Absent (no ER) | Signal recognition particle (SRP) pathway |
This comparison is frequently examined under the context of antibiotic action: many antibiotics (e.g., tetracycline, chloramphenicol) target prokaryotic translation specifically because of ribosomal differences, making them clinically useful without harming eukaryotic host cells, although mitochondrial ribosomes (which resemble prokaryotic ribosomes) can be affected.
该比较经常在抗生素作用的情境下被考查：许多抗生素（例如四环素、氯霉素）因核糖体差异而特异性地靶向原核翻译，使其在临床上具有使用价值而不伤害真核宿主细胞，尽管线粒体核糖体（类似于原核核糖体）可能受到影响。
12. Key Exam Points and Common Misconceptions | 考点总结与常见误区
Students often confuse transcription with translation, for example, stating that mRNA is ‘translated into DNA’ or that ribosomes read DNA directly. Remember: translation uses mRNA as a template to produce polypeptide chains, while transcription generates mRNA from DNA. The central dogma is DNA → RNA → protein.
学生常混淆转录与翻译，例如声称 mRNA “翻译成 DNA”或核糖体直接读取 DNA。牢记：翻译以 mRNA 为模板生成多肽链，而转录从 DNA 生成 mRNA。中心法则为 DNA → RNA → 蛋白质。
Another common error is misidentifying the roles of the A, P, and E sites. The A site receives each new aminoacyl‑tRNA (except the initiator), the P site holds the tRNA with the growing chain, and the E site is where uncharged tRNA exits. The initiator tRNA enters directly into the P site during initiation.
另一常见错误是混淆 A、P、E 位点的功能。A 位接收每个新的氨基酰‑tRNA（起始者除外），P 位容纳携带增长链的 tRNA，E 位是空载 tRNA 离开的位置。起始 tRNA 在起始时直接进入 P 位。
Many students underestimate the importance of rRNA catalytic activity. Exam answers should explicitly state that peptidyl transferase is a function of the large subunit rRNA (a ribozyme), not a ribosomal protein. This supports the RNA world hypothesis and is a key piece of molecular evidence for evolution.
许多考生低估了 rRNA 催化活性的重要性。答案应明确指出肽基转移酶是大亚基 rRNA（一种核酶）的功能，而非核糖体蛋白。这支持了 RNA 世界假说，并且是进化的重要分子证据。
Finally, when discussing the degeneracy of the genetic code, link it to the wobble position (third base) and the reduction in the impact of point mutations. Be prepared to deduce amino acid sequences from given mRNA sequences using a genetic code table, noting directionality (5′ to 3′).
最后，在讨论遗传密码的简并性时，要将其与摆动位置（第三碱基）以及点突变影响的降低联系起来。要做好准备，使用遗传密码表从给定的 mRNA 序列推导出氨基酸序列，并注意方向性（5′ 至 3’）。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导