Translation | 翻译 - aleveler.com

📚 Translation | 翻译

Translation is the process by which the sequence of codons on mRNA is decoded to produce a specific polypeptide chain, which subsequently folds into a functional protein. This fundamental step of gene expression takes place at ribosomes in the cytoplasm and requires the coordinated participation of mRNA, tRNA, ribosomes, and various enzymatic factors. Understanding translation is essential for grasping how genetic information stored in DNA ultimately dictates cellular structure and function, and it features prominently in WJEC IGCSE Biology assessments.

翻译是指根据 mRNA 上密码子序列合成特定多肽链，继而折叠成功能蛋白质的过程。这一基因表达的关键步骤在细胞质中的核糖体上进行，需要 mRNA、tRNA、核糖体及多种酶因子的协同参与。理解翻译对于掌握遗传信息如何从 DNA 最终决定细胞结构与功能至关重要，也是 WJEC IGCSE 生物学考试的核心内容。

1. The Central Dogma and the Role of Translation | 中心法则与翻译的角色

The flow of genetic information follows the central dogma: DNA is transcribed into mRNA, and mRNA is translated into protein. Translation bridges the nucleotide language of nucleic acids and the amino acid language of proteins. In WJEC IGCSE, you are expected to explain why translation is a necessary step and how it differs from transcription in terms of location, molecules involved, and final product.

遗传信息的流动遵循中心法则：DNA 转录为 mRNA，mRNA 再翻译为蛋白质。翻译连接了核酸的核苷酸语言与蛋白质的氨基酸语言。在 WJEC IGCSE 考试中，你需要解释为什么翻译是必要的步骤，以及它在发生的场所、参与的分子和最终产物方面与转录有何不同。

Transcription occurs in the nucleus (in eukaryotes); translation occurs in the cytoplasm at ribosomes.
转录发生在细胞核（真核生物）；翻译发生在细胞质的核糖体上。
The product of transcription is an mRNA transcript; the product of translation is a polypeptide.
转录的产物是 mRNA 转录本；翻译的产物是多肽。
Translation uses tRNA and ribosomes, which are not required for transcription.
翻译使用 tRNA 和核糖体，这些在转录中不需要。

2. Key Molecules Involved in Translation | 翻译中的关键分子

Several types of RNA and protein complexes are essential for translation. The main players include messenger RNA (mRNA), transfer RNA (tRNA), ribosomes (composed of ribosomal RNA and proteins), and amino acids. Each component has a clearly defined role, and WJEC often asks students to describe their structures and functions.

翻译需要多种 RNA 和蛋白质复合物。主要角色包括信使 RNA (mRNA)、转运 RNA (tRNA)、核糖体（由核糖体 RNA 和蛋白质组成）以及氨基酸。每个组分都有明确定义的功能，WJEC 经常要求学生描述它们的结构与作用。

Molecule	Function	Key Features for WJEC
mRNA	Carries genetic code from DNA; contains codons	Single-stranded; codon is a triplet of bases
tRNA	Transfers specific amino acids to the ribosome; contains an anticodon complementary to an mRNA codon	Cloverleaf shape; each tRNA carries one type of amino acid
Ribosomes	Site of protein synthesis; consists of large and small subunits	Made of rRNA and proteins; found free in cytoplasm or attached to rough ER

WJEC candidates should be able to label diagrams of tRNA showing the amino acid attachment site and the anticodon, as well as identify the P site and A site on a ribosome schematic.

WJEC 考生应能够标注 tRNA 示意图上的氨基酸附着位点和反密码子，并能在核糖体简图上识别 P 位和 A 位。

3. The Genetic Code and Codons | 遗传密码与密码子

The genetic code is the set of rules that determines how a nucleotide sequence is converted into an amino acid sequence. In mRNA, each group of three consecutive bases is called a codon. Each codon specifies one amino acid, or a start/stop signal. The code is degenerate (most amino acids are coded by more than one codon) but unambiguous (each codon codes for only one amino acid).

遗传密码是一套决定核苷酸序列如何转换为氨基酸序列的规则。在 mRNA 中，每三个连续碱基构成一个密码子。每个密码子指定一种氨基酸，或起始/终止信号。密码子具有简并性（多数氨基酸由不止一个密码子编码），但无歧义（每个密码子只编码一种氨基酸）。

For WJEC Biology, you do not need to memorise the entire codon table, but you should be able to use a provided table to deduce the amino acid sequence from an mRNA strand. Pay attention to the start codon AUG, which codes for methionine, and the three stop codons (UAA, UAG, UGA) that signal termination of translation.

在 WJEC 生物学中，你不需要背诵整个密码子表，但应能利用给定的表格从 mRNA 链推导氨基酸序列。注意起始密码子 AUG 编码甲硫氨酸，而三个终止密码子（UAA、UAG、UGA）发出翻译终止信号。

4. tRNA Structure and Charging | tRNA 的结构与加载

Transfer RNA molecules act as adaptors that bridge the codon in mRNA and the corresponding amino acid. A tRNA molecule has a characteristic cloverleaf secondary structure, but its three-dimensional L-shape is crucial for fitting into the ribosome. At the 3′ end, there is a CCA sequence where the amino acid is covalently attached. At the opposite loop, the anticodon consists of three bases complementary to the mRNA codon.

转运 RNA 分子充当了连接 mRNA 密码子与对应氨基酸的接头。tRNA 分子具有特征性的三叶草二级结构，但其三维 L 型结构对于嵌入核糖体至关重要。在 3′ 端有一条 CCA 序列，氨基酸通过共价键连接于此。在对侧环上，反密码子由与 mRNA 密码子互补的三个碱基组成。

Before participating in translation, tRNA must be ‘charged’ with its specific amino acid. This process is catalysed by enzymes called aminoacyl-tRNA synthetases, which attach the correct amino acid using energy from ATP. There is at least one specific synthetase and one specific tRNA for each of the 20 amino acids. WJEC may ask simple questions about the importance of this specificity.

在参与翻译之前，tRNA 必须被“加载”其特定的氨基酸。这一过程由氨酰-tRNA 合成酶催化，利用 ATP 的能量将正确的氨基酸连接上去。20 种氨基酸每种至少有一种特异的合成酶和一种特异的 tRNA。WJEC 可能会问及这种特异性的重要性。

5. Ribosome Structure and Functional Sites | 核糖体结构与功能位点

Ribosomes consist of a small subunit and a large subunit, both made of ribosomal RNA (rRNA) and proteins. In the assembled ribosome, there are three important binding sites for tRNA molecules: the A site (aminoacyl), the P site (peptidyl), and the E site (exit). During translation, incoming charged tRNAs enter the A site, the growing polypeptide chain is held in the P site, and empty tRNAs leave via the E site.

核糖体由大亚基和小亚基组成，两者均由核糖体 RNA (rRNA) 和蛋白质构成。在组装的核糖体中，有 tRNA 分子的三个重要结合位点：A 位（氨酰位）、P 位（肽酰位）和 E 位（出口位）。翻译过程中，新进入的加载 tRNA 进入 A 位，延伸中的多肽链位于 P 位，空载的 tRNA 由 E 位离开。

Ribosomes can be free in the cytoplasm or bound to the rough endoplasmic reticulum. Proteins destined for secretion or membrane insertion are typically synthesised on RER-bound ribosomes, while cytoplasmic proteins are made on free ribosomes. WJEC often links this to protein targeting.

核糖体可以游离于细胞质中或结合在粗面内质网上。以分泌或嵌入膜为目的的蛋白质通常在粗面内质网结合的核糖体上合成，而细胞质蛋白则由游离核糖体制造。WJEC 经常将这一点与蛋白质靶向联系起来。

6. Initiation of Translation | 翻译的起始

Translation initiation involves the assembly of the ribosome complex at the start codon of the mRNA. In eukaryotes (the focus of WJEC IGCSE), the small ribosomal subunit binds to the 5′ cap of mRNA and scans along until it encounters the AUG start codon. Then, the initiator tRNA carrying methionine pairs with AUG via its anticodon, and the large subunit joins to form the complete ribosome.

翻译起始涉及核糖体复合物在 mRNA 起始密码子处的组装。在真核生物（WJEC IGCSE 的重点）中，小核糖体亚基结合到 mRNA 的 5′ 帽结构上并沿 mRNA 扫描，直至遇到 AUG 起始密码子。接着，携带甲硫氨酸的起始 tRNA 通过其反密码子与 AUG 配对，大亚基加入形成完整的核糖体。

The initiator tRNA sits in the P site of the ribosome. This leaves the A site vacant and ready to accept the next charged tRNA corresponding to the second codon. Initiation factors assist the process and are dissociated once the complete ribosome is formed.

起始 tRNA 位于核糖体的 P 位。这使得 A 位空出，准备接收与第二个密码子对应的加载 tRNA。起始因子协助这一过程，并在完整核糖体形成后解离。

7. Elongation: The Peptide Chain Grows | 延伸：肽链的生长

Elongation is the cyclic process during which amino acids are added one by one to the growing polypeptide chain. It consists of three main steps: codon recognition, peptide bond formation, and translocation. WJEC examiners expect you to describe these events in sequence and explain the roles of the ribosome sites.

延伸是一个循环过程，期间氨基酸逐个添加到延伸中的多肽链上。它包括三个主要步骤：密码子识别、肽键形成和进位转位。WJEC 考官期望你顺序描述这些事件，并解释核糖体位点的作用。

First, a charged tRNA whose anticodon is complementary to the codon exposed in the A site enters and binds (codon recognition). Next, the ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site. This reaction transfers the entire polypeptide chain from the tRNA in the P site to the amino acid on the A-site tRNA. Finally, translocation occurs: the ribosome moves one codon along the mRNA, shifting the now empty tRNA from the P site to the E site for exit, and the tRNA carrying the growing polypeptide moves from the A site to the P site. The A site is free for the next charged tRNA.

首先，反密码子与暴露在 A 位的密码子互补的加载 tRNA 进入并与之结合（密码子识别）。接着，核糖体催化 P 位上的氨基酸与 A 位上的氨基酸之间形成肽键。该反应将整个多肽链从 P 位上的 tRNA 转移到 A 位 tRNA 上的氨基酸。最后，发生转位：核糖体沿 mRNA 移动一个密码子，将此时空载的 tRNA 由 P 位移至 E 位以便离开，而携带延伸中多肽的 tRNA 则由 A 位移至 P 位。A 位腾空，准备迎接下一个加载 tRNA。

8. Termination of Translation | 翻译的终止

Elongation continues until a stop codon (UAA, UAG, or UGA) enters the A site. There are no tRNAs with anticodons complementary to these stop codons. Instead, proteins called release factors recognise the stop codons and bind to the A site. This triggers the ribosome to add a water molecule instead of an amino acid, causing hydrolysis of the bond linking the polypeptide to the tRNA in the P site.

延伸持续进行，直至一个终止密码子（UAA、UAG 或 UGA）进入 A 位。没有 tRNA 具有与这些终止密码子互补的反密码子。相反，称为释放因子的蛋白质识别终止密码子并与之结合于 A 位。这引发核糖体添加一个水分子而非氨基酸，导致连接多肽与 P 位 tRNA 的键发生水解。

The completed polypeptide chain is released, and the ribosomal subunits, mRNA, and release factors dissociate. The polypeptide then undergoes folding and often post-translational modifications to become a functional protein. WJEC expects you to name the three stop codons and describe why translation ends.

完整的多肽链被释放，核糖体亚基、mRNA 与释放因子解离。随后多肽进行折叠，并常经历翻译后修饰以形成功能蛋白质。WJEC 期望你列出三个终止密码子并描述翻译终止的原因。

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 structure called a polysome or polyribosome. This greatly increases the efficiency of protein synthesis, allowing a cell to produce many copies of a protein from one mRNA transcript in a short period. In WJEC questions, you may be asked to interpret electron micrographs or diagrams showing polysomes.

多个核糖体可以同时翻译同一条 mRNA 分子，形成叫做多聚核糖体的结构。这大大提高了蛋白质合成的效率，使细胞能在短时间内从一个 mRNA 转录本产生大量蛋白质拷贝。在 WJEC 题目中，你可能会被要求解读显示多聚核糖体的电镜照片或示意图。

Prokaryotic polysomes are often studied as a model, but the principle applies to eukaryotes as well. The proximity of ribosomes to the mRNA and the speed of elongation allow the next ribosome to initiate soon after the previous one has cleared the initiation region.

原核生物的多聚核糖体通常作为模型研究，但这一原理也适用于真核生物。核糖体与 mRNA 的靠近以及延伸速度使得前一个核糖体刚刚离开起始区域，下一个就能随即起始。

10. Post-translational Modifications and Protein Targeting | 翻译后修饰与蛋白质靶向

After release from the ribosome, polypeptides often require modifications to become fully functional. These can include folding assisted by chaperone proteins, cleavage of signal sequences, addition of carbohydrate groups (glycosylation), phosphorylation, or assembly with other polypeptide chains to form quaternary structures. WJEC IGCSE may ask about these processes in relation to enzymes or hormones like insulin.

从核糖体释放后，多肽常需要修饰才能具备完整功能。这些修饰包括伴侣蛋白辅助的折叠、信号序列的切除、糖基化（添加糖类基团）、磷酸化，或与其他多肽链组装形成四级结构。WJEC IGCSE 可能会结合酶或胰岛素等激素考查这些过程。

Proteins synthesised on the rough ER enter the endomembrane system and are transported via vesicles to the Golgi apparatus for further modification and sorting. Those destined for secretion follow the secretory pathway. Understanding protein targeting helps explain how cells maintain compartmentalisation.

在粗面内质网上合成的蛋白质进入内膜系统，通过囊泡运输到高尔基体进行进一步修饰与分选。以分泌为目标的蛋白质遵循分泌途径。理解蛋白质靶向有助于解释细胞如何维持区室化。

11. Comparing Translation in Prokaryotes and Eukaryotes (WJEC Context) | 原核与真核生物翻译的比较（WJEC 视角）

Although WJEC IGCSE focuses mainly on eukaryotic translation, a brief comparison with prokaryotes can strengthen your understanding and prepare you for extension questions. In prokaryotes, translation can begin while transcription is still occurring because there is no nuclear membrane separating the two processes. Ribosomes can bind to the mRNA at a Shine-Dalgarno sequence upstream of the start codon.

尽管 WJEC IGCSE 主要关注真核生物的翻译，但简要对比原核生物可以加深理解，并为拓展问题做好准备。在原核生物中，转录仍在进行时翻译就可以开始，因为没有核膜将这两个过程分隔。核糖体可以通过起始密码子上游的 Shine-Dalgarno 序列与 mRNA 结合。

Eukaryotic translation occurs in the cytoplasm after mRNA processing (5′ capping, splicing, 3′ polyadenylation) and export from the nucleus. The first amino acid in eukaryotes is methionine, whereas in prokaryotes it is formylmethionine. WJEC might ask why antibiotics that target prokaryotic ribosomes do not harm human cells—because of structural differences between 70S and 80S ribosomes.

真核生物的翻译发生在细胞质中，在 mRNA 经过加工（5′ 加帽、剪接、3′ 多腺苷酸化）并从核输出之后。真核生物的第一个氨基酸是甲硫氨酸，而原核生物是甲酰甲硫氨酸。WJEC 可能会问为什么靶向原核核糖体的抗生素不会伤害人类细胞——因为 70S 与 80S 核糖体存在结构差异。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

When answering WJEC questions on translation, precision with terminology is vital. Always use ‘codon’ for mRNA triplets and ‘anticodon’ for tRNA triplets; avoid mixing them up. Describe the process stepwise, naming the ribosomal sites where appropriate. If asked to translate a DNA or mRNA sequence, remember to transcribe DNA to mRNA first (replacing thymine with uracil), then use the codon table provided. A common error is using the DNA sequence directly as codons.

在回答 WJEC 关于翻译的题目时，术语的精确性至关重要。务必使用“密码子”称呼 mRNA 三联体，用“反密码子”称呼 tRNA 三联体；避免混淆。逐步描述过程，并在适当处指明核糖体位点。如果要求翻译 DNA 或 mRNA 序列，记住先将 DNA 转录为 mRNA（将胸腺嘧啶替换为尿嘧啶），然后使用给出的密码子表。常见错误是直接将 DNA 序列当做密码子使用。

Diagrams may require you to label the amino acid site on tRNA, the direction of ribosome movement, or the location of peptide bond formation. Practise sketching the flow of information from gene to protein. Also, be prepared to explain how a mutation in DNA can lead to a changed amino acid sequence and ultimately affect protein function—a link between the topics of DNA, protein synthesis, and enzymes.

图示题可能要求你标注 tRNA 上的氨基酸位点、核糖体移动方向或肽键形成的位置。练习绘制从基因到蛋白质的信息流示意图。同时，准备好解释 DNA 突变如何导致氨基酸序列改变，并最终影响蛋白质功能——这联系了 DNA、蛋白质合成和酶等主题。

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Translation | 翻译