📚 Translation: Protein Synthesis Step-by-Step | 翻译：蛋白质合成逐步解析
Translation is the process by which the genetic code carried by messenger RNA (mRNA) is decoded to produce a specific sequence of amino acids in a polypeptide chain. This stage of protein synthesis occurs on ribosomes in the cytoplasm and involves the coordinated function of transfer RNA (tRNA), ribosomal RNA (rRNA), various enzymes, and protein factors. For CCEA A-Level Biology, you need to be able to describe the roles of these molecules, outline the steps of initiation, elongation, and termination, and explain how the genetic code is interpreted with precision. Mastery of this topic also requires linking the process back to gene structure and understanding the consequences of mutations on the final protein product.
翻译是信使RNA（mRNA）携带的遗传密码被解码，以产生多肽链中特定氨基酸序列的过程。这一蛋白质合成阶段发生在细胞质的核糖体上，涉及转运RNA（tRNA）、核糖体RNA（rRNA）、多种酶和蛋白质因子的协同作用。对于CCEA A-Level生物学科，你需要能够描述这些分子的作用，概述起始、延伸和终止的步骤，并解释遗传密码如何被精确解读。掌握这一主题还需要将过程与基因结构联系起来，并理解突变对最终蛋白质产物的影响。
1. Overview of Translation | 翻译概述
Translation is the second major step in the central dogma of molecular biology, following transcription. While transcription converts a gene's DNA sequence into a complementary mRNA copy, translation converts the nucleotide language of mRNA into the amino acid language of proteins. The site of translation is the ribosome, a complex molecular machine composed of rRNA and proteins. Ribosomes move along the mRNA molecule, reading its codons one at a time and adding the corresponding amino acids to a growing polypeptide chain. The overall process can be divided into three phases: initiation, elongation, and termination.
翻译是分子生物学中心法则中继转录之后的第二个主要步骤。转录将基因的DNA序列转化为互补的mRNA拷贝，而翻译则将mRNA的核苷酸语言转化为蛋白质的氨基酸语言。翻译的场所是核糖体，一种由rRNA和蛋白质组成的复杂分子机器。核糖体沿着mRNA分子移动，一次一个地读取其密码子，并将相应的氨基酸添加到不断增长的多肽链中。整个过程可以分为三个阶段：起始、延伸和终止。
2. The Genetic Code | 遗传密码
The genetic code consists of 64 possible three-base sequences (codons) in mRNA, each of which specifies an amino acid or a stop signal. It is degenerate, meaning that most amino acids are encoded by more than one codon; for example, leucine has six codons. The code is also universal across almost all life forms and is non-overlapping — ribosomes read bases three at a time in a continuous sequence. The start codon AUG codes for methionine in eukaryotes and also signals the beginning of translation. The three stop codons — UAA, UAG, and UGA — do not code for any amino acid but instead trigger the release of the completed polypeptide chain.
遗传密码由mRNA中64种可能的三碱基序列（密码子）组成，每一个密码子指定一个氨基酸或一个终止信号。它是简并的，意味着大多数氨基酸由多个密码子编码；例如亮氨酸有六个密码子。该密码在几乎所有生命形式中都是通用的，并且不重叠——核糖体一次读取三个碱基，形成连续的序列。起始密码子AUG在真核生物中编码甲硫氨酸，同时也指示翻译的起始。三个终止密码子——UAA、UAG和UGA——不编码任何氨基酸，而是触发已完成多肽链的释放。
3. Components Required for Translation | 翻译所需的成分
Several key molecules are essential for translation. mRNA carries the genetic information from DNA to the ribosome. tRNA molecules act as adaptors, with an anticodon at one end that base-pairs with a complementary codon on the mRNA and an attachment site at the other end for a specific amino acid. Each tRNA is charged with its correct amino acid by an aminoacyl-tRNA synthetase enzyme, a process that requires energy from ATP. Ribosomes consist of a large and a small subunit; in eukaryotic cells these are 60S and 40S respectively, forming an 80S ribosome. Ribosomes possess three binding sites for tRNA: the A (aminoacyl) site, P (peptidyl) site, and E (exit) site. Additionally, various protein initiation factors, elongation factors, and release factors regulate the process, and GTP provides energy for several steps.
翻译过程中几种关键分子不可或缺。mRNA将遗传信息从DNA携带至核糖体。tRNA分子充当接头，其一端为反密码子，可与mRNA上的互补密码子碱基配对，另一端则为特定氨基酸的连接位点。每种tRNA由氨酰-tRNA合成酶装载上正确的氨基酸，这一过程需要ATP供能。核糖体由一个大亚基和一个小亚基组成；在真核细胞中，它们分别是60S和40S，形成80S核糖体。核糖体具有三个tRNA结合位点：A（氨酰位）、P（肽酰位）和E（出口位）。此外，多种蛋白质起始因子、延伸因子和释放因子调控该过程，GTP为若干步骤提供能量。
4. Initiation of Translation | 翻译的起始
In eukaryotes, initiation begins when the small ribosomal subunit binds to the 5' cap of the mRNA. It then scans along the mRNA in the 5' to 3' direction until it encounters the start codon, AUG. An initiator tRNA carrying methionine (Met-tRNAi) binds to the start codon in the P site of the small subunit. This process is assisted by initiation factors (eIFs) and uses GTP. Once the start codon–anticodon pairing is established, the large ribosomal subunit joins the complex, and the initiator tRNA occupies the P site of the now-complete ribosome. The A site is positioned over the next codon, ready for elongation to begin.
在真核生物中，起始阶段始于小核糖体亚基与mRNA的5'帽结构结合。然后它沿着mRNA从5'到3'方向扫描，直至遇到起始密码子AUG。一个携带甲硫氨酸的起始tRNA（Met-tRNAi）结合到小亚基P位点的起始密码子上。这一过程由起始因子（eIFs）辅助，并消耗GTP。一旦起始密码子-反密码子配对建立，大核糖体亚基加入复合体，起始tRNA占据完整核糖体的P位点。A位点位于下一个密码子上方，准备开始延伸。
5. Elongation: Codon Recognition | 延伸：密码子识别
Elongation is a cyclic process where amino acids are added one by one to the growing polypeptide chain. The first step is codon recognition: a charged tRNA with an anticodon complementary to the mRNA codon in the A site enters the ribosome. This delivery is mediated by an elongation factor (EF-Tu in prokaryotes, eEF1 in eukaryotes) that binds the tRNA and GTP. When the correct tRNA–codon match is achieved, GTP is hydrolyzed, and the tRNA is fully accommodated in the A site. Accurate codon–anticodon pairing ensures that the correct amino acid is incorporated according to the genetic code.
延伸是一个循环过程，氨基酸被逐一添加到不断增长的多肽链中。第一步是密码子识别：一个带电荷的tRNA，其反密码子与A位点上的mRNA密码子互补，进入核糖体。这一运送由与tRNA和GTP结合的延伸因子（原核生物中为EF-Tu，真核生物中为eEF1）介导。当正确的tRNA-密码子匹配完成时，GTP被水解，tRNA完全进入A位点。精确的密码子-反密码子配对照遗传密码保证了正确的氨基酸被引入。
6. Elongation: Peptide Bond Formation | 延伸：肽键形成
Once the A site is occupied by the correct aminoacyl-tRNA, a peptide bond is formed between the amino acid in the A site and the growing polypeptide chain that is attached to the tRNA in the P site. This reaction is catalyzed by peptidyl transferase, an enzymatic activity provided by the rRNA of the large ribosomal subunit (a ribozyme). The polypeptide is transferred from the P-site tRNA to the amino acid on the A-site tRNA, extending the chain by one residue. This step requires no additional energy input beyond the energy already stored in the aminoacyl-tRNA bond.
一旦A位点被正确的氨酰-tRNA占据，A位点的氨基酸与连接在P位点tRNA上的增长中的多肽链之间就会形成一个肽键。这一反应由肽基转移酶催化，该酶活性由大核糖体亚基的rRNA提供（一种核酶）。多肽从P位点tRNA转移到A位点tRNA的氨基酸上，使链延长一个残基。除了氨酰-tRNA键中已储存的能量外，此步骤不需要额外的能量输入。
7. Elongation: Translocation | 延伸：移位
Following peptide bond formation, the ribosome must move exactly one codon along the mRNA to bring the next codon into the A site. This process is called translocation. The tRNA that was in the P site, now without an amino acid, moves to the E site and then exits the ribosome. The tRNA carrying the newly extended polypeptide moves from the A site to the P site. Translocation is driven by elongation factor EF-G (in prokaryotes) or eEF2 (in eukaryotes) with the hydrolysis of GTP. After translocation, the A site is empty and ready to receive the next aminoacyl-tRNA, starting a new elongation cycle.
肽键形成后，核糖体必须沿mRNA精确移动一个密码子，使下一个密码子进入A位点。这一过程称为移位。原来位于P位点的tRNA（此时已无氨基酸）移入E位点，随后离开核糖体。携带新生多肽的tRNA从A位点移入P位点。移位由延伸因子EF-G（原核生物）或eEF2（真核生物）驱动，并伴随GTP的水解。移位后，A位点变空，准备接收下一个氨酰-tRNA，开始新一轮延伸循环。
8. Termination of Translation | 翻译的终止
Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA molecules have anticodons complementary to these codons. Instead, release factors (eRF1 and eRF3 in eukaryotes) bind to the stop codon. This binding triggers peptidyl transferase to hydrolyze the bond linking the completed polypeptide chain to the tRNA in the P site, releasing the polypeptide. The ribosomal subunits, mRNA, and remaining tRNA then dissociate, ready to be reused for another round of translation.
当终止密码子（UAA、UAG或UGA）进入A位点时，翻译终止。没有tRNA分子具有与这些密码子互补的反密码子。此时，释放因子（真核生物中为eRF1和eRF3）结合到终止密码子上。这一结合触发肽基转移酶将连接已完成多肽链与P位点tRNA的键水解，释放多肽链。核糖体亚基、mRNA及剩余的tRNA随即解离，准备重新用于另一轮翻译。
9. Post-Translational Modifications | 翻译后修饰
Newly synthesized polypeptides are often not immediately functional. They must fold into a specific three-dimensional conformation, sometimes assisted by chaperone proteins. Many proteins undergo post-translational modifications, such as the removal of the initial methionine residue, phosphorylation, glycosylation, or cleavage into smaller active peptides (as seen with insulin). These modifications can occur in the rough endoplasmic reticulum or Golgi apparatus before the protein is transported to its final destination. Understanding these steps is essential in CCEA questions that link translation to protein targeting and secretion.
新合成的多肽通常并非立即具有功能。它们必须折叠成特定的三维构象，有时需要分子伴侣蛋白的辅助。许多蛋白质会经历翻译后修饰，例如切除起始的甲硫氨酸残基、磷酸化、糖基化，或切割为较小的活性肽（如胰岛素）。这些修饰可在粗面内质网或高尔基体中进行，之后蛋白质被运送到其最终目的地。理解这些步骤对于回答CCEA试题中联系翻译与蛋白质靶向及分泌的问题至关重要。
10. Energy Requirements and Enzymes in Translation | 翻译中的能量需求与酶
Translation is energetically expensive. Charging tRNA molecules with their specific amino acids consumes one ATP per amino acid (converted to AMP + PPi). During elongation, GTP is hydrolyzed at two points: first during codon recognition when the aminoacyl-tRNA is loaded into the A site, and again during translocation. Therefore, forming each peptide bond effectively requires the hydrolysis of two GTP molecules in addition to the ATP used in tRNA charging. Key enzymes and catalytic activities include aminoacyl-tRNA synthetases (ligases), peptidyl transferase (a ribozyme), and the various GTPase activities of elongation and termination factors.
翻译的能量消耗很大。用特定氨基酸装载tRNA分子每添加一个氨基酸消耗一个ATP（转化为AMP + PPi）。在延伸过程中，GTP在两个节点被水解：第一次是在密码子识别时氨酰-tRNA被装载进A位点，第二次是在移位时。因此，每形成一个肽键实际上需要水解两个GTP分子，外加tRNA装载时消耗的ATP。关键的酶及催化活性包括氨酰-tRNA合成酶（连接酶）、肽基转移酶（一种核酶），以及延伸因子和终止因子的各种GTPase活性。
11. Comparison: Transcription vs. Translation | 比较：转录与翻译
| Feature | Transcription | Translation |
|---|---|---|
| Location in eukaryotes | Nucleus | Cytoplasm (ribosomes) |
| Template | DNA | mRNA |
| Product | mRNA (or tRNA, rRNA) | Polypeptide (protein) |
| Enzyme/ catalyst | RNA polymerase | Peptidyl transferase (ribozyme) |
| Energy source | NTPs (ATP, GTP, etc.) | GTP and ATP |
For CCEA exams, it is important to distinguish clearly between these two processes. Transcription produces an RNA copy of a gene, while translation uses that RNA copy to synthesize a protein. Errors in either process can lead to non-functional proteins, and the compartmentalisation in eukaryotes ensures that RNA processing (e.g., splicing) occurs before translation.
对CCEA考试而言，清晰区分这两个过程非常重要。转录产生基因的RNA拷贝，而翻译则利用该RNA拷贝合成蛋白质。两个过程中的任何一个出错都可能导致无功能的蛋白质，而真核生物中的区室化确保了RNA加工（如剪接）在翻译之前进行。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Students often confuse the roles of mRNA codons and tRNA anticodons, or fail to state clearly that peptidyl transferase activity resides in the rRNA of the large subunit, not a separate protein. When answering CCEA exam questions, always refer to translation as occurring in the 5' to 3' direction of the mRNA and describe the A, P, and E sites explicitly. Use precise language: say “the ribosome moves along the mRNA”, not “the mRNA moves through the ribosome”. Also, be prepared to interpret diagrams of ribosome structure or tRNA charging reactions, and to explain how a point mutation in a gene could alter the amino acid sequence of the final protein if it occurs in the coding region.
学生们经常混淆mRNA密码子和tRNA反密码子的作用，或未能明确指出肽基转移酶活性位于大亚基的rRNA中，而非一个单独的蛋白质。在回答CCEA试题时，务必说明翻译沿mRNA的5'到3'方向进行，并明确描述A、P和E位点。使用准确的表述：说“核糖体沿mRNA移动”，而不是“mRNA穿过核糖体”。此外，要准备好解读核糖体结构或tRNA装载反应示意图，并能够解释基因中的点突变如果发生在编码区，是如何改变最终蛋白质的氨基酸序列的。
Published by TutorHao | CCEA Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图