Translation in Protein Synthesis | 蛋白质合成中的翻译考点精讲

📚 Translation in Protein Synthesis | 蛋白质合成中的翻译考点精讲

Translation is the second stage of protein synthesis, where the genetic code carried by messenger RNA (mRNA) is decoded to assemble amino acids into a specific polypeptide chain. This process takes place at ribosomes and is essential for producing the proteins that determine cell structure and function. Understanding translation is a key requirement for IGCSE AQA Biology, and this article will break down each step with clear explanations and exam-focused details.

翻译是蛋白质合成的第二个阶段，在这个阶段中，信使RNA（mRNA）携带的遗传密码被解码，从而将氨基酸组装成特定的多肽链。这个过程在核糖体上进行，对于制造决定细胞结构和功能的蛋白质至关重要。理解翻译是IGCSE AQA生物学的重要考点，本文将用清晰的解释和考试重点细节逐一拆解每一步。

1. What is Translation? | 什么是翻译？

Translation is the process by which the sequence of nucleotide bases in mRNA is converted into a sequence of amino acids in a polypeptide. It follows transcription, where a gene on DNA is copied into mRNA in the nucleus. In translation, the ribosome reads the mRNA codons, and transfer RNA (tRNA) molecules bring the corresponding amino acids to build the protein chain. Without translation, the genetic information stored in DNA could not be expressed as functional proteins.

翻译是将mRNA中的核苷酸碱基序列转化为多肽中氨基酸序列的过程。它发生在转录之后——转录是在细胞核中将DNA上的一段基因复制为mRNA。在翻译过程中，核糖体读取mRNA上的密码子，转运RNA（tRNA）分子携带相应的氨基酸来构建蛋白质链。没有翻译，储存在DNA中的遗传信息就无法表达为功能蛋白质。

2. The Site of Translation: Ribosomes | 翻译的场所：核糖体

Ribosomes are the molecular machines where translation occurs. They can be found free in the cytoplasm or attached to the rough endoplasmic reticulum (RER). A ribosome is composed of ribosomal RNA (rRNA) and proteins, and consists of a small subunit and a large subunit. The small subunit binds to mRNA, while the large subunit has sites for tRNA binding and catalyses the formation of peptide bonds between amino acids. In eukaryotes, the ribosome has a sedimentation coefficient of 80S, made up of 40S and 60S subunits.

核糖体是发生翻译的分子机器。它们可以游离在细胞质中，也可以附着在粗面内质网（RER）上。核糖体由核糖体RNA（rRNA）和蛋白质组成，包含一个小亚基和一个大亚基。小亚基与mRNA结合，而大亚基拥有tRNA结合位点，并催化氨基酸之间肽键的形成。在真核生物中，核糖体的沉降系数为80S，由40S和60S亚基组成。

3. The Role of mRNA in Translation | mRNA在翻译中的作用

Messenger RNA (mRNA) carries the genetic instructions from DNA to the ribosome. It is a single-stranded molecule produced during transcription. In translation, each group of three bases on the mRNA, known as a codon, specifies one particular amino acid. The mRNA also contains start and stop codons that signal the beginning and end of the polypeptide coding sequence. The sequence of codons determines the primary structure of the protein.

信使RNA（mRNA）将遗传指令从DNA携带到核糖体。它是一条单链分子，在转录过程中生成。在翻译过程中，mRNA上每三个碱基的组合（称为密码子）对应一个特定的氨基酸。mRNA上还含有起始密码子和终止密码子，用于指示多肽编码序列的起点和终点。密码子的排列顺序决定了蛋白质的一级结构。

4. The Genetic Code: Codons | 遗传密码：密码子

The genetic code is the set of rules by which information in mRNA is translated into amino acids. There are 64 possible codons (4³ combinations), but only 20 amino acids, meaning the code is degenerate—more than one codon can code for the same amino acid. For example, the amino acid leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). The code is also universal, shared by almost all organisms, and non-overlapping; each base is part of only one codon. The start codon AUG codes for methionine and initiates translation.

遗传密码是将mRNA中的信息翻译为氨基酸的一套规则。共有64种可能的密码子（4³种组合），但只有20种氨基酸，这意味着密码具有简并性——多个密码子可以编码同一种氨基酸。例如，亮氨酸由六种不同的密码子编码（UUA、UUG、CUU、CUC、CUA、CUG）。遗传密码还具有通用性（几乎所有生物共用同一套密码）和非重叠性（每个碱基仅属于一个密码子）。起始密码子AUG编码甲硫氨酸，并启动翻译。

Number of amino acids coded = number of bases in coding sequence ÷ 3

编码的氨基酸数 = 编码序列碱基数 ÷ 3

5. Transfer RNA (tRNA) Structure and Function | 转运RNA（tRNA）的结构与功能

Transfer RNA (tRNA) is a small RNA molecule that acts as an adapter between the mRNA codon and the corresponding amino acid. Each tRNA has a cloverleaf-like secondary structure and a specific three-base sequence called the anticodon at one end. The anticodon is complementary to the mRNA codon. At the opposite end (the 3′ end), there is an attachment site for a specific amino acid, catalyzed by an enzyme called aminoacyl-tRNA synthetase. Each tRNA molecule is specific for one amino acid.

转运RNA（tRNA）是一种小RNA分子，它充当mRNA密码子与相应氨基酸之间的适配器。每个tRNA具有类似三叶草的二级结构，一端含有一个特定的三碱基序列，称为反密码子。反密码子与mRNA上的密码子互补配对。在对侧端（3’端），有一个特定氨基酸的连接位点，此过程由氨酰tRNA合成酶催化。每个tRNA分子专门对应一种氨基酸。

6. Initiation of Translation | 翻译的起始

Translation begins when the small ribosomal subunit binds to the mRNA molecule near the 5′ end. In eukaryotes, this binding is facilitated by the 5′ cap. The small subunit moves along the mRNA until it encounters the start codon AUG. A tRNA molecule carrying methionine, with the anticodon UAC, then binds to the start codon through complementary base pairing. Once this initiation complex is formed, the large ribosomal subunit attaches, completing the functional ribosome. The initiator tRNA sits in the P site (peptidyl site) of the ribosome.

翻译开始时，核糖体小亚基结合到mRNA分子靠近5’端的位置。在真核生物中，这种结合借助5’帽结构进行。小亚基沿着mRNA移动，直到遇到起始密码子AUG。携带甲硫氨酸且反密码子为UAC的tRNA分子随即通过碱基互补配对结合在起始密码子上。一旦形成这一起始复合物，核糖体大亚基便附着上来，形成有功能的核糖体。起始tRNA占据核糖体的P位（肽酰位点）。

7. Elongation: Building the Polypeptide Chain | 延伸：构建多肽链

During elongation, amino acids are added one by one to the growing polypeptide chain. The ribosome has three binding sites for tRNA: the A site (aminoacyl site), the P site (peptidyl site), and the E site (exit site). A new tRNA molecule carrying the next amino acid enters the A site, and its anticodon pairs with the complementary mRNA codon. Then 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, a reaction known as peptidyl transfer. The ribosome translocates, moving the mRNA by one codon: the tRNA in the P site shifts to the E site and is released, while the tRNA in the A site moves to the P site, leaving the A site empty for the next tRNA. This process repeats, powered by GTP (guanosine triphosphate).

在延伸过程中，氨基酸逐个添加到延伸中的多肽链上。核糖体拥有三个tRNA结合位点：A位（氨酰位）、P位（肽酰位）和E位（出口位）。一个携带下一个氨基酸的新tRNA分子进入A位，其反密码子与mRNA上的互补密码子配对。随后核糖体催化P位上的氨基酸与A位上的氨基酸之间形成肽键，这一反应称为肽基转移。核糖体发生移位，将mRNA移动一个密码子的距离：P位的tRNA移动到E位并被释放，同时A位的tRNA移动到P位，使A位空出以接纳下一个tRNA。这一过程在GTP（鸟苷三磷酸）提供的能量推动下反复进行。

Amino acid₍ₙ₎ + Amino acid₍ₙ₊₁₎ → Dipeptide + H₂O (peptide bond formation)

氨基酸₍ₙ₎ + 氨基酸₍ₙ₊₁₎ → 二肽 + 水（肽键形成）

8. Termination of Translation | 翻译的终止

Elongation continues until a stop codon on the mRNA enters the A site. Stop codons—UAA, UAG, or UGA—do not code for any amino acid and are not recognised by tRNA. Instead, a protein called a release factor binds to the stop codon, triggering the ribosome to break the bond between the completed polypeptide and the tRNA in the P site. The polypeptide is released, and the ribosomal subunits, mRNA, and release factor dissociate. The newly formed polypeptide then folds and may undergo further modification to become a functional protein.

延伸持续进行，直到mRNA上的一个终止密码子进入A位。终止密码子——UAA、UAG或UGA——不编码任何氨基酸，也不被任何tRNA识别。取而代之的是一种称为释放因子的蛋白质与终止密码子结合，触发核糖体断开完整多肽与P位tRNA之间的连接。多肽被释放，核糖体亚基、mRNA和释放因子随之解离。新合成的多肽随后进行折叠，并可能经历进一步修饰，成为有功能的蛋白质。

9. The Polypeptide Folds into a Functional Protein | 多肽折叠成功能性蛋白质

After translation, the linear chain of amino acids undergoes folding, driven by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. The primary structure (sequence of amino acids) determines the final three-dimensional shape, which is crucial for the protein’s function. Some proteins, like haemoglobin, have a quaternary structure formed by multiple polypeptide chains. In addition, proteins may be modified in the Golgi apparatus or the endoplasmic reticulum before they become fully active.

翻译完成后，线性氨基酸链会发生折叠，这一过程由氢键、离子键、疏水相互作用和二硫键驱动。一级结构（氨基酸序列）决定了最终的三维构象，这对于蛋白质的功能至关重要。有些蛋白质（如血红蛋白）具有由多条多肽链组成的四级结构。此外，蛋白质在成为完全活性形式之前，可能还需要在高尔基体或内质网中进行修饰。

10. Key Steps and Comparison to Transcription | 关键步骤总结及与转录的比较

The main stages of translation can be summarised as: initiation (ribosome assembly on mRNA, start codon recognition), elongation (codon–anticodon pairing, peptide bond formation, translocation), and termination (stop codon recognition, polypeptide release). Unlike transcription, which occurs in the nucleus and uses DNA as a template to produce mRNA, translation occurs in the cytoplasm, uses mRNA as the template, and builds a polypeptide. Both processes involve complementary base pairing: in transcription, DNA bases pair with RNA nucleotides (A–U, T–A, C–G, G–C); in translation, mRNA codons pair with tRNA anticodons (A–U, U–A, C–G, G–C).

翻译的主要阶段可总结为：起始（核糖体在mRNA上组装，识别起始密码子）、延伸（密码子与反密码子配对、肽键形成、移位）和终止（识别终止密码子、多肽释放）。与转录在细胞核中进行、以DNA为模板产生mRNA不同，翻译发生在细胞质中，以mRNA为模板，构建多肽。两个过程都涉及碱基互补配对：在转录中，DNA碱基与RNA核苷酸配对（A–U、T–A、C–G、G–C）；在翻译中，mRNA密码子与tRNA反密码子配对（A–U、U–A、C–G、G–C）。

11. Common Exam Questions on Translation | 常见翻译考试题型

IGCSE AQA Biology exam questions on translation often require you to: identify the components of translation (ribosome, mRNA, tRNA, amino acids), explain the roles of codon and anticodon, describe the sequence of events during elongation, or use a given DNA template strand to determine the amino acid sequence (by working out the mRNA and then using a codon table). You may also be asked to explain the significance of the genetic code being degenerate or to predict the effect of a mutation on the polypeptide. Be prepared for data analysis questions involving mRNA sequences and amino acid charts.

IGCSE AQA生物学考试中关于翻译的题目通常要求你：识别翻译的组成部分（核糖体、mRNA、tRNA、氨基酸），解释密码子和反密码子的作用，描述延伸过程中的事件顺序，或者根据给定的DNA模板链确定氨基酸序列（先推导mRNA，然后使用密码子表）。你还可能需要解释遗传密码简并性的意义，或预测突变对多肽的影响。准备好应对涉及mRNA序列和氨基酸图表的分析题。

12. Key Terms and Definitions | 关键术语和定义

Below is a quick-reference table of essential terminology for translation:

下表是翻译必需术语的快速参考：

Term	术语	Definition
Translation	翻译	The process of synthesising a polypeptide from mRNA at the ribosome.
Codon	密码子	A sequence of three mRNA bases that codes for one amino acid.
Anticodon	反密码子	A sequence of three tRNA bases complementary to a codon.
Ribosome	核糖体	The organelle composed of rRNA and protein that carries out translation.
A site	A位	Aminoacyl site: where an incoming tRNA carrying an amino acid binds.
P site	P位	Peptidyl site: where the tRNA holding the growing polypeptide chain is located.
E site	E位	Exit site: where the empty tRNA leaves the ribosome.
Peptide bond	肽键	The covalent bond formed between two amino acids during translation.
Start codon	起始密码子	AUG, which signals the beginning of translation and codes for methionine.
Stop codon	终止密码子	UAA, UAG, or UGA; signals the end of translation.

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Translation in Protein Synthesis | 蛋白质合成中的翻译 考点精讲