Translation Key Points | 翻译 考点精讲

📚 Translation Key Points | 翻译 考点精讲

Translation is the second stage of protein synthesis, during which the genetic code carried by messenger RNA (mRNA) is decoded to assemble a specific polypeptide chain. This process takes place at ribosomes in the cytoplasm and involves transfer RNA (tRNA) molecules that match amino acids to the codons on the mRNA. Understanding translation is essential for the IGCSE OCR Biology specification, as it explains how the sequence of bases in DNA ultimately determines the structure and function of proteins.

翻译是蛋白质合成的第二个阶段，在此过程中，信使 RNA（mRNA）携带的遗传密码被解码，从而组装出特定的多肽链。这一过程发生在细胞质中的核糖体上，并涉及转运 RNA（tRNA）分子，它们根据 mRNA 上的密码子匹配相应的氨基酸。理解翻译对于 IGCSE OCR 生物学考试至关重要，因为它解释了 DNA 碱基序列如何最终决定蛋白质的结构与功能。

1. Overview of Translation | 翻译概述

Translation converts the nucleotide language of mRNA into the amino acid language of proteins. It occurs on ribosomes and requires the coordinated action of mRNA, tRNA, ribosomes, and various enzymes. The process can be divided into three main stages: initiation, elongation, and termination. Each stage ensures that amino acids are joined in the correct order specified by the mRNA template.

翻译将 mRNA 的核苷酸语言转化为蛋白质的氨基酸语言。它发生在核糖体上，需要 mRNA、tRNA、核糖体和多种酶的协同作用。该过程可分为三个主要阶段：起始、延伸和终止。每个阶段都确保氨基酸按照 mRNA 模板指定的正确顺序连接起来。

2. The Role of mRNA | mRNA 的作用

Messenger RNA (mRNA) is a single-stranded copy of a gene that carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm. In eukaryotes, the mRNA molecule undergoes processing before translation, including the addition of a 5′ cap and a poly-A tail. The coding region of mRNA consists of a series of three-base sequences called codons, each of which corresponds to a specific amino acid or a stop signal.

信使 RNA（mRNA）是基因的单链拷贝，它将遗传密码从细胞核中的 DNA 传送到细胞质中的核糖体。在真核生物中，mRNA 分子在翻译前会经历加工过程，包括添加 5′ 帽和 poly-A 尾。mRNA 的编码区由一系列三个碱基的序列组成，称为密码子，每个密码子对应一个特定的氨基酸或终止信号。

3. Ribosomes: The Protein Factories | 核糖体：蛋白质工厂

Ribosomes are complex molecular machines made of ribosomal RNA (rRNA) and proteins. They consist of a large subunit and a small subunit. The small subunit binds to the mRNA, while the large subunit has three sites for tRNA molecules: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. Ribosomes catalyze the formation of peptide bonds between adjacent amino acids and move along the mRNA in a 5′ to 3′ direction.

核糖体是由核糖体 RNA（rRNA）和蛋白质组成的复杂分子机器。它们由大亚基和小亚基构成。小亚基与 mRNA 结合，大亚基则拥有三个 tRNA 分子结合位点：A 位（氨酰位）、P 位（肽酰位）和 E 位（出口位）。核糖体催化相邻氨基酸之间肽键的形成，并沿着 mRNA 以 5′ 到 3′ 的方向移动。

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

Transfer RNA (tRNA) molecules are adapters that link specific amino acids to the corresponding codons on mRNA. Each tRNA has a cloverleaf secondary structure and folds into an L-shaped tertiary structure. At the 3′ end, there is a CCA sequence where the amino acid is attached. The anticodon loop contains a triplet of bases called the anticodon, which is complementary to the mRNA codon. Aminoacyl-tRNA synthetases are enzymes that charge tRNAs by attaching the correct amino acid.

转运 RNA（tRNA）分子是适配器，将特定的氨基酸与 mRNA 上相应的密码子联系起来。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 3′ 端，有一个 CCA 序列，氨基酸附着于此。反密码子环包含一个三碱基序列，称为反密码子，与 mRNA 密码子互补。氨酰 tRNA 合成酶是负责给 tRNA 装载正确氨基酸的酶。

5. Codons and Anticodons | 密码子与反密码子

A codon is a sequence of three nucleotides on mRNA that codes for a specific amino acid. Since there are four different bases, there are 4³ = 64 possible codons. Of these, 61 code for amino acids, and 3 are stop codons (UAA, UAG, UGA) that signal the end of translation. The anticodon on tRNA is complementary to the codon and base-pairs with it during translation. The start codon is usually AUG, which codes for methionine and initiates translation.

密码子是 mRNA 上编码特定氨基酸的三个核苷酸序列。由于有四种不同的碱基，因此共有 4³ = 64 个可能的密码子。其中 61 个编码氨基酸，3 个是终止密码子（UAA、UAG、UGA），标志着翻译的结束。tRNA 上的反密码子与密码子互补配对。起始密码子通常是 AUG，它编码甲硫氨酸并启动翻译。

Key codons: AUG (start), UAA, UAG, UGA (stop)

关键密码子：AUG（起始），UAA，UAG，UGA（终止）

6. Initiation of Translation | 翻译的起始

Translation begins when the small ribosomal subunit binds to the mRNA near the 5′ cap (in eukaryotes) or at a ribosome-binding site (in prokaryotes). The initiator tRNA, carrying methionine, binds to the start codon AUG at the P site of the small subunit. This complex then recruits the large ribosomal subunit, forming a complete ribosome with the initiator tRNA in the P site and the A site ready to accept the next tRNA. GTP hydrolysis provides energy for initiation.

当小核糖体亚基与 mRNA 的 5′ 帽附近（真核生物中）或核糖体结合位点（原核生物中）结合时，翻译便开始了。携带甲硫氨酸的起始 tRNA 与小亚基 P 位上的起始密码子 AUG 结合。然后该复合物招募大亚基，形成一个完整的核糖体，其中起始 tRNA 位于 P 位，A 位准备接受下一个 tRNA。GTP 水解释放能量驱动起始过程。

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

During elongation, amino acids are added one by one to the growing polypeptide chain. The process involves three steps: codon recognition, peptide bond formation, and translocation. A tRNA with the matching anticodon enters the A site and base-pairs with the mRNA codon. The ribosome catalyzes the formation of a peptide bond between the amino acid in the A site and the polypeptide chain held by the tRNA in the P site. The ribosome then moves one codon forward, shifting the tRNAs to the E and P sites, and the empty A site is ready for the next tRNA. This cycle requires GTP and elongation factors.

在延伸阶段，氨基酸逐一添加到生长的多肽链上。该过程包括三个步骤：密码子识别、肽键形成和移位。一个带有匹配反密码子的 tRNA 进入 A 位，与 mRNA 密码子碱基配对。核糖体催化 A 位氨基酸与 P 位 tRNA 所携带的多肽链之间形成肽键。随后核糖体向前移动一个密码子，将 tRNA 移至 E 位和 P 位，空的 A 位准备接受下一个 tRNA。此循环需要 GTP 和延伸因子。

8. Peptide Bond Formation | 肽键形成

The chemical reaction that links amino acids is catalyzed by peptidyl transferase, which is an enzymatic activity of the large ribosomal subunit (specifically the rRNA, making it a ribozyme). The amino group of the amino acid in the A site attacks the carbonyl carbon of the amino acid attached to the tRNA in the P site. This condensation reaction results in the formation of a peptide bond and the release of a water molecule. As the polypeptide lengthens, it folds into its three-dimensional shape assisted by chaperone proteins.

连接氨基酸的化学反应由肽基转移酶催化，这是大核糖体亚基（具体说是 rRNA）的酶活性，因此核糖体是一种核酶。A 位氨基酸的氨基攻击 P 位 tRNA 上氨基酸的羰基碳。这一缩合反应导致肽键形成并释放一个水分子。随着多肽链延长，它会在分子伴侣的帮助下折叠成三维形状。

Peptide bond: -NH-CO- formed by condensation reaction

肽键：-NH-CO-，通过缩合反应形成

9. Termination of Translation | 翻译的终止

Translation terminates when a stop codon (UAA, UAG, or UGA) enters the A site of the ribosome. There are no tRNAs with anticodons complementary to stop codons; instead, release factors (proteins) bind to the A site. These factors cause the peptidyl transferase to add a water molecule instead of an amino acid, hydrolyzing the bond between the polypeptide and the tRNA in the P site. This releases the completed polypeptide, and the ribosomal subunits, mRNA, and tRNA dissociate. The polypeptide then undergoes folding and post-translational modifications to become a functional protein.

当一个终止密码子（UAA、UAG 或 UGA）进入核糖体的 A 位时，翻译终止。没有任何 tRNA 具有与终止密码子互补的反密码子；相反，释放因子（蛋白质）会与 A 位结合。这些因子促使肽基转移酶加入一个水分子而非氨基酸，从而水解 P 位上多肽与 tRNA 之间的键。这使得完成的多肽链被释放，核糖体亚基、mRNA 和 tRNA 各自解离。随后多肽链进行折叠和翻译后修饰，成为有功能的蛋白质。

10. The Genetic Code is Universal | 遗传密码是通用的

The genetic code is nearly universal, meaning that the same codons specify the same amino acids in almost all living organisms. This provides strong evidence for a common evolutionary ancestor. The code is also degenerate, as most amino acids are encoded by more than one codon. The degeneracy often lies in the third base of the codon, which is known as the wobble position. This reduces the deleterious effects of point mutations.

遗传密码几乎是通用的，意味着在几乎所有生物中，相同的密码子指定相同的氨基酸。这为共同进化祖先提供了有力证据。该密码还具有简并性，因为大多数氨基酸由不止一个密码子编码。简并性通常体现在密码子的第三位碱基上，这被称为摆动位置。这降低了点突变的有害影响。

11. From Polypeptide to Functional Protein | 从多肽到功能蛋白

The newly synthesized polypeptide chain is not yet a functional protein. It must fold into a specific three-dimensional conformation, often with the help of chaperone proteins. Some proteins also require post-translational modifications, such as phosphorylation, glycosylation, or cleavage of signal peptides. Proteins can have primary, secondary, tertiary, and quaternary structures, which determine their function. Errors in translation can lead to non-functional proteins and disease.

新合成的多肽链还不是有功能的蛋白质。它必须折叠成特定的三维构象，通常需要分子伴侣的帮助。一些蛋白质还需要翻译后修饰，如磷酸化、糖基化或信号肽的切除。蛋白质可具有一级、二级、三级和四级结构，这些结构决定其功能。翻译中的错误会导致蛋白质功能丧失并引发疾病。

12. Summary: Comparing Transcription and Translation | 总结：转录与翻译的比较

While both transcription and translation are essential steps of gene expression, they differ in location, molecules, and products. Transcription occurs in the nucleus (eukaryotes), produces mRNA from DNA, and uses RNA polymerase. Translation occurs in the cytoplasm on ribosomes, produces a polypeptide from mRNA, and uses tRNA, ribosomes, and various factors. The table below highlights the key differences.

尽管转录和翻译都是基因表达的关键步骤，但它们在发生位置、分子和产物上均不同。转录发生在细胞核（真核生物），以 DNA 为模板产生 mRNA，使用 RNA 聚合酶。翻译在细胞质的核糖体上进行，以 mRNA 为模板产生多肽，使用 tRNA、核糖体和多种因子。下表总结了主要差异。

Feature | 特征	Transcription | 转录	Translation | 翻译
Location | 位置	Nucleus (eukaryotes) | 细胞核	Cytoplasm / ribosome | 细胞质/核糖体
Template | 模板	DNA	mRNA
Product | 产物	mRNA	Polypeptide (protein) | 多肽
Key enzyme/molecule | 关键酶/分子	RNA polymerase	Ribosome (peptidyl transferase)
Components | 组分	Nucleotides (ATP, UTP, GTP, CTP)	Amino acids, tRNA, ATP, GTP

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