📚 IB Biology: Translation – Key Concepts & Exam Tips | IB 生物：翻译 考点精讲
Translation is the process by which ribosomes decode mRNA sequences to synthesize polypeptides. It sits at the heart of gene expression, linking the language of nucleotides to the language of proteins. For IB Biology, mastering translation means understanding the roles of codons, tRNA, ribosomes, and the elongation cycle in detail, as well as being able to compare prokaryotic and eukaryotic mechanisms.
翻译是核糖体解码 mRNA 序列以合成多肽的过程。它处于基因表达的核心，将核苷酸的语言与蛋白质的语言联系起来。对于 IB 生物考试来说，掌握翻译意味着需要深入理解密码子、tRNA、核糖体和延伸循环的角色，同时能够比较原核生物与真核生物的翻译机制。
1. Overview of Translation | 翻译概述
Translation is the second major step of gene expression, following transcription. It converts the linear sequence of codons in mRNA into the amino acid sequence of a polypeptide. The process occurs on ribosomes and requires transfer RNAs (tRNAs) as adaptor molecules, each carrying a specific amino acid. Energy is supplied by GTP and ATP. In IB exams, you are expected to explain that the ribosome moves in the 5′ to 3′ direction along the mRNA, reading triplets one by one.
翻译是基因表达的第二个主要步骤，紧随转录之后。它将 mRNA 中的线性密码子序列转化为多肽的氨基酸序列。该过程发生在核糖体上，并需要转运 RNA（tRNA）作为接头分子，每个 tRNA 携带特定的氨基酸。能量由 GTP 和 ATP 提供。在 IB 考试中，你需要解释核糖体沿 mRNA 从 5′ 向 3′ 方向移动，逐一读取三联体密码。
2. The Genetic Code and Codons | 遗传密码与密码子
The genetic code is a set of rules by which nucleotide triplets, called codons, specify amino acids. It is degenerate (more than one codon can code for the same amino acid) and universal (shared across almost all organisms). The start codon AUG codes for methionine (Met) and signals the beginning of translation. Three stop codons – UAA, UAG, UGA – do not code for amino acids; instead, they trigger the release of the polypeptide. You should be able to use a codon table to deduce amino acid sequences from an mRNA sequence.
遗传密码是一套规则，核苷酸三联体（称为密码子）依据这些规则决定氨基酸。密码子是简并的（多个密码子可以编码同一种氨基酸）并且几乎通用的（几乎所有生物共享同一套密码）。起始密码子 AUG 编码甲硫氨酸（Met），并发出翻译起始信号。三个终止密码子——UAA、UAG、UGA——不编码氨基酸，它们触发多肽的释放。你应该能够使用密码子表从 mRNA 序列推导出氨基酸序列。
3. Key Players: mRNA, tRNA and Ribosomes | 关键角色：mRNA、tRNA 与核糖体
mRNA carries the genetic message from DNA and presents codons to the ribosome. tRNA molecules are folded into a cloverleaf shape and contain an anticodon loop that base-pairs with the complementary codon. The 3′ end of tRNA carries the corresponding amino acid. Ribosomes are the catalytic machines. Prokaryotic ribosomes are 70S (composed of a 50S large subunit and 30S small subunit), while eukaryotic ribosomes are 80S (60S + 40S). IB students often need to label the A site (aminoacyl), P site (peptidyl) and E site (exit) on the ribosome.
mRNA 携带来自 DNA 的遗传信息，并将密码子呈递给核糖体。tRNA 分子折叠成三叶草形状，含有一个反密码子环，与互补的密码子进行碱基配对。tRNA 的 3′ 端携带相应氨基酸。核糖体是催化机器。原核生物核糖体为 70S（由 50S 大亚基和 30S 小亚基组成），而真核生物核糖体为 80S（60S + 40S）。IB 学生通常需要标注核糖体上的 A 位（氨酰位）、P 位（肽基位）和 E 位（出口位）。
4. Aminoacyl-tRNA Synthetases | 氨酰 tRNA 合成酶
Each tRNA must be correctly charged with its cognate amino acid. This is the job of aminoacyl-tRNA synthetases – a family of enzymes that are highly specific for both the tRNA and the amino acid. The enzyme first activates the amino acid using ATP, forming an aminoacyl-AMP intermediate, then transfers the amino acid to the 3′ end of the tRNA. The accuracy of translation depends on this ‘proofreading’ step. IB questions may ask how the fidelity of protein synthesis is maintained.
每种 tRNA 必须正确装载其对应的氨基酸，这是氨酰 tRNA 合成酶的工作——这类酶对 tRNA 和氨基酸都具有高度特异性。该酶首先用 ATP 激活氨基酸，形成氨酰-AMP 中间物，然后将氨基酸转移到 tRNA 的 3′ 端。翻译的准确性依赖于这一“校对”步骤。IB 考题可能会问蛋白质合成的保真度是如何维持的。
5. Initiation of Translation | 翻译起始
In prokaryotes, initiation begins when the small ribosomal subunit binds to the Shine-Dalgarno sequence upstream of the start codon on mRNA. The initiator tRNA carrying N-formylmethionine (fMet) pairs with the AUG codon, and then the large subunit joins to form a complete 70S initiation complex. In eukaryotes, the small subunit recognizes the 5′ cap and scans along the mRNA until it finds the AUG start codon, where the initiator tRNA (carrying methionine) binds. Initiation factors (IFs or eIFs) assist in assembling the complex, and GTP is hydrolysed.
在原核生物中，起始步骤始于小核糖体亚基与 mRNA 上起始密码子上游的 Shine-Dalgarno 序列结合。携带 N-甲酰甲硫氨酸（fMet）的起始 tRNA 与 AUG 密码子配对，然后大亚基加入，形成完整的 70S 起始复合物。在真核生物中，小亚基识别 5′ 帽子并沿 mRNA 扫描，直至找到 AUG 起始密码子，此时携带甲硫氨酸的起始 tRNA 与之结合。起始因子（IF 或 eIF）协助组装复合物，同时 GTP 被水解。
6. Elongation: The Ribosome Cycle | 延伸：核糖体循环
Elongation proceeds through a repeating three-step cycle: codon recognition, peptide bond formation, and translocation. A charged tRNA with the matching anticodon enters the A site (codon recognition). The growing polypeptide chain on the tRNA in the P site is transferred to the amino acid on the tRNA in the A site via peptide bond formation. The ribosome then translocates – moving one codon along the mRNA – so that the spent tRNA moves into the E site and exits, while the peptidyl-tRNA moves into the P site, freeing the A site for the next incoming tRNA. EF-Tu (in prokaryotes) and eEF-1 (in eukaryotes) are elongation factors that deliver aminoacyl-tRNAs, and EF-G/eEF-2 catalyses translocation using GTP.
延伸以重复的三步循环进行：密码子识别、肽键形成和移位。带有匹配反密码子的负载 tRNA 进入 A 位（密码子识别）。随后，位于 P 位 tRNA 上延伸中的多肽链通过肽键形成转移至 A 位 tRNA 上的氨基酸。核糖体随后移位——沿 mRNA 移动一个密码子——使空载 tRNA 进入 E 位并离开，而肽基 tRNA 移入 P 位，释放出 A 位供下一个进入的 tRNA 使用。原核生物中的 EF-Tu 和真核生物中的 eEF-1 是输送氨酰 tRNA 的延伸因子，而 EF-G/eEF-2 利用 GTP 催化移位。
7. Peptide Bond Formation | 肽键形成
Peptide bond formation occurs between the carboxyl group of the amino acid in the P site and the amino group of the amino acid in the A site, releasing a water molecule. This reaction is catalysed by peptidyl transferase, an enzymatic activity of the large ribosomal subunit’s rRNA (a ribozyme), not a protein. The resulting dipeptide (or growing polypeptide) remains attached to the tRNA in the A site through the newly formed peptide bond. This detail highlights the evolutionary significance of RNA catalysis and often appears in IB data-based questions.
肽键形成发生在 P 位氨基酸的羧基与 A 位氨基酸的氨基之间，同时释放一分子水。该反应由肽基转移酶催化，这是大亚基 rRNA 的酶活性（一种核酶），而非蛋白质。形成的二肽（或延伸中的多肽）通过新生成的肽键仍然附着在 A 位的 tRNA 上。这一细节突显了 RNA 催化的进化意义，常在 IB 数据分析题中出现。
8. Termination of Translation | 翻译终止
Termination occurs when a stop codon (UAA, UAG or UGA) enters the A site. No tRNA can recognize these codons. Instead, release factors (RF-1, RF-2 in prokaryotes; eRF1 in eukaryotes) bind to the A site, prompting the peptidyl transferase to transfer the polypeptide to a water molecule, hydrolysing the bond and releasing the newly synthesized protein. The ribosomal subunits dissociate and can be recycled for another round of translation. IB questions often ask why suppressor mutations in tRNA genes can produce full-length proteins despite premature stop codons.
当终止密码子（UAA、UAG 或 UGA）进入 A 位时，翻译终止。没有任何 tRNA 能识别这些密码子。取而代之的是释放因子（原核生物中的 RF-1、RF-2；真核生物中的 eRF1）与 A 位结合，促使肽基转移酶将多肽转移到一分子水上，水解酯键并释放新合成的蛋白质。核糖体亚基解离，并可被循环用于下一轮翻译。IB 考题常问为什么 tRNA 基因中的抑制突变能在提前终止密码子存在时仍产生全长蛋白质。
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 arrangement allows a cell to rapidly produce many copies of a protein from one mRNA transcript. Polysomes are visible under electron microscopes and are a key indicator of active protein synthesis. In IB diagrams, you may be asked to interpret the direction of translation from the lengths of nascent polypeptide chains on a polysome.
多个核糖体可以同时翻译同一个 mRNA 分子，形成多聚核糖体（多体）。这种排布使细胞能够从一个 mRNA 转录本快速合成大量蛋白质拷贝。多聚核糖体在电子显微镜下可见，是活跃蛋白质合成的一个关键指示。在 IB 图解中，你可能需要根据多聚核糖体上新生多肽链的长度推断翻译方向。
10. Prokaryotic vs Eukaryotic Translation | 原核与真核翻译对比
Prokaryotic translation occurs in the cytoplasm, often coupling with transcription, because there is no nuclear envelope. Eukaryotic translation is separated in space and time: transcription occurs in the nucleus, mRNA is processed (5′ cap, poly-A tail, splicing) and then exported to the cytoplasm for translation. Prokaryotes use 70S ribosomes, formyl-methionine as the first amino acid, and mRNA with Shine-Dalgarno sequences. Eukaryotes use 80S ribosomes, methionine as the first amino acid, and the 5′ cap and scanning mechanism for initiation. These differences are exploited by some antibiotics, such as tetracycline and erythromycin, which selectively inhibit prokaryotic ribosomes.
原核生物的翻译在细胞质中进行，常与转录偶联，因为没有核膜。真核生物翻译在空间和时间上分离：转录发生在细胞核，mRNA 经过加工（5′ 帽、poly-A 尾、剪接）后输出到细胞质进行翻译。原核生物使用 70S 核糖体，甲酰甲硫氨酸作为第一个氨基酸，mRNA 含有 Shine-Dalgarno 序列。真核生物使用 80S 核糖体，甲硫氨酸作为第一个氨基酸，并通过 5′ 帽和扫描机制起始。这些差异被一些抗生素利用，例如四环素和红霉素能选择性抑制原核生物核糖体。
11. Post-Translational Modifications | 翻译后修饰
Newly synthesized polypeptides often undergo folding and modifications before becoming functional proteins. Chaperone proteins assist in proper folding. Chemical modifications can include phosphorylation, glycosylation, acetylation, or cleavage of signal peptides. In eukaryotic cells, proteins destined for secretion carry a signal sequence that directs the ribosome to the rough endoplasmic reticulum, where translation continues and the protein is released into the ER lumen. IB students should be able to relate these modifications to protein targeting and function.
新合成的多肽在成为功能性蛋白质之前，通常需要经过折叠和修饰。分子伴侣蛋白协助正确折叠。化学修饰可包括磷酸化、糖基化、乙酰化或信号肽的切割。在真核细胞中，分泌性蛋白质携有一段信号序列，将核糖体引导至粗面内质网，翻译继续进行，蛋白质被释放到内质网腔中。IB 学生应能将这些修饰与蛋白质靶向和功能联系起来。
12. Exam Tips and Common Pitfalls | 考试提示与常见易错点
In IB Biology, translation questions frequently appear in Paper 1 and Paper 2, including data analysis involving codon tables or polysome images. Remember to distinguish between transcription and translation clearly: transcription uses DNA template, produces mRNA; translation uses mRNA, produces polypeptide. Avoid confusing codons (on mRNA) with anticodons (on tRNA). Do not forget that the genetic code is read in the 5′ to 3′ direction. When describing elongation, always name the A, P, E sites and mention GTP hydrolysis. And remember: prokaryotes have no introns, so translation can begin immediately after transcription; eukaryotic mRNA splicing is a separate process.
在 IB 生物考试中，翻译题常出现在 Paper 1 和 Paper 2 中，包括涉及密码子表或多聚核糖体图像的数据分析。请记住清晰区分转录与翻译：转录使用 DNA 模板，合成 mRNA；翻译使用 mRNA，合成多肽。避免将密码子（在 mRNA 上）与反密码子（在 tRNA 上）混淆。不要忘记遗传密码的阅读方向是 5′ 向 3’。在描述延伸时，务必说出 A 位、P 位、E 位，并提及 GTP 水解。另外记住：原核生物没有内含子，因此转录后翻译可以立即开始；而真核生物的 mRNA 剪接是一个独立过程。
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