📚 Translation in A-Level WJEC Biology | A-Level WJEC 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 polypeptide chain. This step of protein synthesis occurs at the ribosome and requires the participation of transfer RNA (tRNA) molecules, each carrying a specific amino acid. Understanding translation is essential for grasping how genotype determines phenotype, and it is a core topic in the WJEC A-Level Biology specification.
翻译是指 mRNA 所携带的遗传密码被解码，以合成特定多肽链的过程。蛋白质合成的这一阶段在核糖体上进行，需要转运 RNA（tRNA）分子的参与，每个 tRNA 携带特定的氨基酸。理解翻译过程对于掌握基因型如何决定表型至关重要，它是 WJEC A-Level 生物课程大纲中的核心主题。
1. The Genetic Code and Codons | 遗传密码与密码子
The genetic code is a set of rules that defines how a sequence of nucleotide bases in mRNA is translated into the amino acid sequence of a protein. Each three-base sequence on mRNA is called a codon, and most codons specify a particular amino acid. The code is degenerate, meaning that multiple codons can code for the same amino acid, but it is unambiguous – each codon codes for only one amino acid.
遗传密码是一套规则，它定义了 mRNA 中的核苷酸碱基序列如何被翻译成蛋白质的氨基酸序列。mRNA 上每三个碱基组成一个密码子，大多数密码子对应特定的氨基酸。密码子具有简并性，即多个密码子可以编码同一种氨基酸，但它又是明确的——每个密码子只编码一种氨基酸。
- Start codon: AUG (codes for methionine) signals the beginning of translation. 起始密码子：AUG（编码甲硫氨酸）标志着翻译的起始。
- Stop codons: UAA, UAG, UGA do not code for any amino acid; they terminate translation. 终止密码子：UAA、UAG、UGA 不编码任何氨基酸；它们使翻译终止。
In the WJEC exam, you may be asked to deduce the amino acid sequence from a given mRNA sequence using the genetic code table. The degeneracy of the code is often tested in the context of silent mutations.
在 WJEC 考试中，你可能会被要求根据给定的 mRNA 序列，利用遗传密码表推导氨基酸序列。密码子的简并性常在沉默突变的背景下被考查。
2. The Role of tRNA and Aminoacylation | tRNA 的作用与氨酰化
Transfer RNA (tRNA) molecules act as adaptors between mRNA codons and amino acids. Each tRNA has a cloverleaf secondary structure and is folded into an L-shaped tertiary structure. At the 3' end, there is an acceptor stem where the specific amino acid is covalently attached. At the opposite end, the anticodon loop contains a triplet of bases called the anticodon, which is complementary to an mRNA codon.
转运 RNA（tRNA）分子充当 mRNA 密码子和氨基酸之间的接头。每个 tRNA 具有三叶草形的二级结构，并折叠成 L 形的三级结构。在其 3' 端有一个接纳茎，特定的氨基酸通过共价键连接于此。在另一端，反密码子环上含有一个三碱基序列，称为反密码子，它与 mRNA 的密码子互补配对。
Aminoacylation (also called tRNA charging) is the process by which a specific amino acid is attached to its corresponding tRNA. This reaction is catalysed by an aminoacyl-tRNA synthetase, an enzyme that is highly specific for both the amino acid and the tRNA. ATP provides the energy, forming an aminoacyl-AMP intermediate before the amino acid is transferred to the tRNA.
氨酰化（也称 tRNA 负载）是指将特定的氨基酸连接到相应 tRNA 的过程。该反应由氨酰-tRNA 合成酶催化，这种酶对氨基酸和 tRNA 都具有高度特异性。ATP 提供能量，先形成氨酰-AMP 中间体，随后氨基酸被转移到 tRNA 上。
Key WJEC point: You should be able to explain how the specificity of the enzyme ensures the fidelity of translation. If a tRNA is charged with the wrong amino acid, it will still bind to its correct codon, leading to a misincorporated amino acid.
WJEC 关键考点：你应该能够解释该酶的特异性如何确保翻译的准确性。如果 tRNA 携带了错误的氨基酸，它仍会与其正确的密码子结合，导致氨基酸掺入错误。
3. Ribosome Structure and Assembly | 核糖体的结构与组装
Ribosomes are the molecular machines that carry out translation. They consist of two subunits, a large subunit and a small subunit, both made of ribosomal RNA (rRNA) and proteins. In prokaryotes, the subunits are 50S and 30S, forming a 70S ribosome; in eukaryotes, they are 60S and 40S, forming an 80S ribosome. (WJEC may focus on eukaryotic cytoplasmic ribosomes, but prokaryotic ribosomes are relevant for antibiotic action.)
核糖体是执行翻译的分子机器。它们由大、小两个亚基组成，亚基由核糖体 RNA（rRNA）和蛋白质构成。在原核生物中，亚基分别 50S 和 30S，形成 70S 核糖体；在真核生物中，亚基是 60S 和 40S，形成 80S 核糖体。（WJEC 可能侧重真核细胞质核糖体，但原核核糖体与抗生素作用相关。）
The small subunit binds to the mRNA and is responsible for codon–anticodon recognition. The large subunit has peptidyl transferase activity (23S rRNA in prokaryotes, 28S rRNA in eukaryotes), catalysing peptide bond formation. It also contains three binding sites for tRNA molecules:
小亚基与 mRNA 结合，负责密码子–反密码子识别。大亚基具有肽基转移酶活性（原核生物为 23S rRNA，真核生物为 28S rRNA），催化肽键的形成。它还含有三个 tRNA 结合位点：
| Site 位点 | Function 功能 |
|---|---|
| A (aminoacyl) site | Binds incoming aminoacyl‑tRNA. 结合即将进入的氨酰-tRNA。 |
| P (peptidyl) site | Holds the tRNA carrying the growing polypeptide chain. 容纳携带延伸中多肽链的 tRNA。 |
| E (exit) site | Where deacylated tRNA departs from the ribosome. 脱酰 tRNA 离开核糖体的位点。 |
4. Initiation of Translation | 翻译的起始
Translation initiation in eukaryotes involves several steps. The small ribosomal subunit binds to the 5' cap of the mRNA and scans along until it finds the start codon AUG. This process is assisted by eukaryotic initiation factors (eIFs). The initiator tRNA, which carries methionine (Met-tRNAᵢᴹᵉᵗ), pairs with the AUG codon in the P site of the small subunit. The large subunit then joins, completing the initiation complex. GTP hydrolysis provides the energy for these steps.
真核生物的翻译起始包括多个步骤。小核糖体亚基先与 mRNA 的 5' 帽子结构结合，并沿着 mRNA 扫描，直至找到起始密码子 AUG。这一过程由真核起始因子（eIFs）协助完成。携带甲硫氨酸的起始 tRNA（Met-tRNAᵢᴹᵉᵗ）与位于小亚基 P 位点的 AUG 密码子配对。随后大亚基加入，形成完整的起始复合物。GTP 水解为这些步骤提供能量。
In prokaryotes, initiation is simpler: the small subunit recognises the Shine–Dalgarno sequence upstream of the start codon, base-pairing with the 16S rRNA. This positions the initiator tRNA correctly. Polycistronic mRNA (common in prokaryotes, tested in some WJEC options) allows multiple proteins to be translated from a single mRNA.
在原核生物中，起始过程更为简单：小亚基识别起始密码子上游的 Shine–Dalgarno 序列，与 16S rRNA 进行碱基配对，从而正确放置起始 tRNA。多顺反子 mRNA（原核生物中常见，部分 WJEC 选项会考查）使得一条 mRNA 可以翻译出多种蛋白质。
5. Elongation: The Peptide Chain Grows | 延伸：肽链生长
Elongation is a cyclic process involving three stages: codon recognition, peptide bond formation, and translocation.
延伸是一个循环过程，包括三个阶段：密码子识别、肽键形成和移位。
Codon recognition (entry): An aminoacyl‑tRNA with an anticodon complementary to the A‑site codon binds to the A site. This requires elongation factor Tu (EF‑Tu) and GTP hydrolysis.
密码子识别（进入）：反密码子与 A 位点密码子互补的氨酰-tRNA 结合到 A 位点。这需要延伸因子 Tu（EF‑Tu）和 GTP 水解。
Peptide bond formation: The ribosome's peptidyl transferase centre catalyses the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site. The polypeptide chain is transferred to the tRNA in the A site. No ATP or GTP is required for this step; it is catalysed by rRNA.
肽键形成：核糖体的肽基转移酶中心催化 P 位点氨基酸与 A 位点氨基酸之间形成肽键。多肽链随之转移到 A 位点的 tRNA 上。此步骤不需要 ATP 或 GTP，由 rRNA 催化。
Translocation: The ribosome moves (translocates) one codon along the mRNA in the 5'→3' direction. The peptidyl‑ cylated tRNA moves to the E site and exits. Translocation requires elongation factor G (EF‑G) and GTP hydrolysis.
移位：核糖体沿 mRNA 的 5'→3' 方向移动一个密码子。肽基-tRNA 从 A 位点移至 P 位点，脱酰 tRNA 移至 E 位点并离开。移位需要延伸因子 G（EF‑G）和 GTP 水解。
This cycle repeats, adding around 6–10 amino acids per second in eukaryotes. The peptide grows from the N‑terminus to the C‑terminus.
该循环不断重复，真核生物中每秒可添加约 6–10 个氨基酸。肽链从 N 端向 C 端延伸。
6. Termination of Translation | 翻译的终止
Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site. There are no tRNA molecules with anticodons complementary to stop codons. Instead, release factors (RF1, RF2, RF3 in prokaryotes; eRF1 in eukaryotes) recognise the stop codon. The release factor binds to the A site and triggers the hydrolysis of the bond linking the polypeptide to the tRNA in the P site. This releases the newly synthesised polypeptide. The ribosomal subunits then dissociate, and the mRNA is released.
当终止密码子（UAA、UAG 或 UGA）进入 A 位点时，翻译终止。没有 tRNA 具有与终止密码子互补的反密码子。相反，释放因子（原核生物中的 RF1、RF2、RF3；真核生物中的 eRF1）识别终止密码子。释放因子与 A 位点结合，触发 P 位点 tRNA 与多肽链之间酯键的水解，从而释放新合成的多肽。随后核糖体亚基解离，mRNA 也被释放。
WJEC may ask about the consequence of a mutation converting an amino acid codon into a stop codon (nonsense mutation). This leads to a truncated, often non‑functional protein.
WJEC 可能会问及将某个氨基酸密码子突变成终止密码子（无义突变）的后果。这会导致截短、通常无功能的蛋白质。
7. Post‑translational Modifications | 翻译后修饰
The newly synthesised polypeptide is often not yet fully functional. Post‑translational modifications (PTMs) alter the protein’s properties. Common modifications include:
新合成的多肽往往尚不完全具有功能。翻译后修饰（PTMs）会改变蛋白质的性质。常见的修饰包括：
- Folding: Assisted by chaperone proteins, the polypeptide folds into its specific three‑dimensional conformation. 折叠：在分子伴侣蛋白的协助下，多肽折叠成特定的三维构象。
- Cleavage tor methionine, signal peptides, or activation of proenzymes (e.g., proinsulin → insulin). 切割：去除起始甲硫氨酸、信号肽，或酶原的激活（如前胰岛素→胰岛素）。
- Chemical modifications: Phosphorylation (addition of phosphate groups), glycosylation (addition of sugar groups), acetylation, methylation, etc. These can affect enzyme activity, localisation, or stability. 化学修饰：磷酸化（添加磷酸基团）、糖基化（添加糖基）、乙酰化、甲基化等。这些可影响酶活性、定位或稳定性。
- Quaternary structure assembly: Multiple polypeptide subunits may come together (e.g., haemoglobin). 四级结构组装：多条多肽亚基可聚合在一起（如血红蛋白）。
Emphasising the link between translation and protein function is important. For example, the signal peptide on a nascent protein directs it to the rough endoplasmic reticulum – a key point for WJEC, connecting translation to the secretory pathway.
强调翻译与蛋白质功能之间的联系非常重要。例如，新生蛋白质上的信号肽将其引导至粗面内质网——这是 WJEC 的关键点，将翻译与分泌途径联系起来。
8. Comparison: Prokaryotic vs. Eukaryotic Translation | 原核与真核翻译的比较
WJEC often expects you to compare translation in prokaryotes and eukaryotes. Key differences:
WJEC 常要求比较原核生物与真核生物的翻译。关键差异：
| Feature 特征 | Prokaryotes 原核生物 | Eukaryotes 真核生物 |
|---|---|---|
| Ribosome 核糖体 | 70S (50S + 30S) | 80S (60S + 40S) |
| mRNA 信使 RNA | Often polycistronic; no 5' cap or poly‑A tail | Monocistronic; 5' cap and poly‑A tail |
| Initiation 起始 | Shine–Dalgarno sequence aligns start codon | 5' cap recognition; scanning for AUG |
| Initiator tRNA 起始 tRNA | Formylmethionine (fMet‑tRNAᶠᴹᵉᵗ) | Methionine (Met‑tRNAᵢᴹᵉᵗ) |
| Coupled transcription–translation 转录翻译偶联 | Yes – occurs simultaneously | No – separated by nuclear membrane |
| Antibiotics 抗生素 | Many antibiotics target 70S ribosome (e.g., tetracycline, chloramphenicol) | Not directly affected; host ribosome is 80S |
This comparison is frequently examined, particularly the fact that simultaneous transcription and translation is possible in prokaryotes because there is no nuclear membrane. Also, the antibiotic point demonstrates how selective toxicity can be achieved.
这种比较经常在考试中出现，尤其是原核生物因为不存在核膜，转录和翻译可以同时进行这一事实。此外，抗生素的例子展示了如何实现选择性毒性。
9. The Role of GTP and Energy Costs | GTP 的作用与能量消耗
ensive process. Formation of a single peptide bond requires the hydrolysis of several high‑ onds. In activation (aminoacylation), one ATP is hydrolysed to AMP, equivalent to two high‑energy phosphate bonds. During elongation, one GTP is consumed per codon recognition (EF‑Tu) and another GTP per translocation (EF‑G). Therefore, adding one amino acid costs at least four high‑energy phosphate bonds (~4 × 30.5 kJ mol⁻¹).
翻译是一个能耗很高的过程。形成一个肽键需要水解数个高能键。在活化（氨酰化）阶段，一个 ATP 水解为 AMP，相当于两个高能磷酸键。在延伸过程中，每次密码子识别消耗一个 GTP（EF‑Tu），每次移位又消耗一个 GTP（EF‑G）。因此，每添加一个氨基酸至少消耗四个高能磷酸键（约 4 × 30.5 kJ mol⁻¹）。
In WJEC, you may be asked to calculate or compare energy requirements for transcription vs translation, or to explain why proteins are not synthesised unless they are needed – the high energy cost acts as a regulatory check.
在 WJEC 中，你可能被要求计算或比较转录与翻译的能量需求，或解释为什么只有在需要时才合成蛋白质——高能量消耗起到了调节关口的作用。
10. Mutations Affecting Translation | 影响翻译的突变
Different types of mutation in DNA can alter the translated product:
DNA 中的不同类型突变可改变翻译产物：
- Substitution (point) mutation: A single base change. If it results in a codon for the same amino acid, it is a silent mutation (degeneracy). A missense mutation changes one amino acid, which may or may not alter protein function. A nonsense mutation creates a premature stop codon, truncating the protein. 替换（点）突变：单个碱基改变。若产生编码同种氨基酸的密码子，则为沉默突变（简并性）。错义突变改变一个氨基酸，可能影响也可能不影响蛋白质功能。无义突变产生提前的终止密码子，造成蛋白质截短。
- Insertion or deletion (indel) mutation: If not a multiple of three, it causes a frameshift. All downstream codons are read differently, usually producing a completely different and shorter polypeptide. 插入或缺失（插入删除）突变：若不是三的倍数，则引起移码。下游所有密码子的读取发生改变，通常产生完全不同且更短的多肽。
Be prepared to calculate the effect of mutations on amino acid sequences using a given genetic code table. WJEC exam questions often present partial sequences and ask for the outcome of a specific change.
要做好准备，能够利用提供的遗传密码表计算突变对氨基酸序列的影响。WJEC 试题常提供部分序列，并要求说明特定改变的后果。
11. Practical Techniques Related to Translation | 与翻译相关的实验技术
While WJEC does not require deep practical detail, you should be aware of techniques that are linked to translation studies:
尽管 WJEC 不要求深入的实验细节，但你应该了解与翻译研究相关的技术：
- Use of radiolabelled amino acids: Tracking ³⁵S‑methionine incorporation to study protein synthesis rates. 放射性标记氨基酸的使用：追踪 ³⁵S‑甲硫氨酸的掺入，以研究蛋白质合成速率。
- Polyribosomes (polysomes): Multiple ribosomes translating a single mRNA simultaneously, visualised by electron microscopy. 多聚核糖体（多核糖体）：多个核糖体同时翻译同一条 mRNA，可通过电子显微镜观察。
- Cell‑free protein synthesis systems: In vitro translation using ribosomes, tRNAs, and other factors to produce proteins. 无细胞蛋白质合成系统：利用核糖体、tRNA 及其他因子在体外进行翻译以生产蛋白质。
- Antibiotics as inhibitors: Puromycin causes premature chain termination; cycloheximide inhibits eukaryotic translation. These tools help dissect translation steps. 抗生素作为抑制剂：嘌呤霉素引起肽链提前终止；放线菌酮抑制真核翻译。这些工具有助于解析翻译步骤。
Connecting these to the core concepts helps solidify understanding and is useful for application questions.
将这些技术与核心概念联系起来有助于巩固理解，并对应用类题目有帮助。
12. Common Mistakes and Exam Tips | 常见错误与应试技巧
Students often confuse transcription with translation, or forget the direction of synthesis. Key reminders:
同学们常混淆转录和翻译，或忘记合成方向。关键提醒：
- mRNA is read 5'→3'; the polypeptide is synthesised N→C. mRNA 沿 5'→3' 方向读取；多肽按 N→C 方向合成。
- 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 – the ribosome’s peptidyl transferase centre catalyses this using rRNA. 肽键形成发生在 P 位点氨基酸的羧基与 A 位点氨基酸的氨基之间——核糖体的肽基转移酶中心利用 rRNA 催化该反应。
- The anticodon is complementary to the codon, but the tRNA carries the amino acid corresponding to that codon – not the anticodon itself. 反密码子与密码子互补，但 tRNA 携带的是与该密码子对应的氨基酸，而非反密码子对应的氨基酸。
When annotating diagrams, clearly label the ribosome subunits, A/P/E sites, mRNA 5' and 3' ends, and the N- and C‑terminus of the growing peptide. For WJEC, always use correct terminology and spellings (e.g., ribose, adenine, thymine, uracil).
在标注示意图时，清晰地标出核糖体亚基、A/P/E 位点、mRNA 的 5' 和 3' 端，以及延伸中肽链的 N 端和 C 端。在 WJEC 考试中，务必使用正确的术语和拼写（例如核糖、腺嘌呤、胸腺嘧啶、尿嘧啶）。
Finally, practice applying the genetic code table quickly. Many marks are lost through simple reading errors. Remember that the table gives mRNA codons (not DNA triplets), so you must first transcribe the DNA if a gene sequence is provided.
最后，练习快速查阅遗传密码表。很多失分源于简单的读取错误。记住，表格给出的是 mRNA 密码子（不是 DNA 三联体），因此如果提供的是基因序列，你必须先将其转录。
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