📚 IGCSE CIE Biology: Translation (Protein Synthesis) | IGCSE CIE 生物：翻译（蛋白质合成）考点精讲
Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain that will fold into a functional protein. It is the second major stage of protein synthesis, following transcription, and takes place entirely in the cytoplasm at the ribosomes. Understanding translation is essential for IGCSE CIE Biology, as it explains how the language of nucleotides is converted into the language of proteins, linking genotype to phenotype.
翻译是根据信使RNA（mRNA）上的遗传密码合成特定氨基酸序列、进而形成多肽链并折叠成功能蛋白质的过程。它是蛋白质合成的第二大阶段，紧随转录之后，完全在细胞质中的核糖体上进行。理解翻译对于IGCSE CIE生物学至关重要，因为它解释了核苷酸语言如何转换为蛋白质语言，从而将基因型与表现型联系起来。
1. Overview of Translation | 翻译概述
Translation is the step in gene expression where ribosomes read the nucleotide sequence of mRNA in groups of three bases, called codons, and assemble the corresponding amino acids into a polypeptide. Each codon specifies a particular amino acid or a stop signal. The process involves several key players: mRNA, ribosomes, transfer RNA (tRNA), amino acids, and various protein factors. Energy for translation is provided by GTP.
翻译是基因表达的步骤，核糖体以三个碱基为一组（即密码子）读取mRNA的核苷酸序列，并将对应的氨基酸组装成多肽。每个密码子指定一种特定的氨基酸或终止信号。该过程涉及多个关键角色：mRNA、核糖体、转运RNA（tRNA）、氨基酸以及多种蛋白质因子。翻译的能量由GTP提供。
2. The Site of Translation: Ribosomes | 翻译的场所：核糖体
Ribosomes are complex molecular machines composed of ribosomal RNA (rRNA) and proteins. They consist of a large subunit and a small subunit. In eukaryotes, the small subunit (40S) and large subunit (60S) form an 80S ribosome; in prokaryotes, 30S and 50S subunits form a 70S ribosome. The ribosome has three binding sites for tRNA: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. During translation, the ribosome moves along the mRNA in the 5′ to 3′ direction, catalysing the formation of peptide bonds.
核糖体是由核糖体RNA（rRNA）和蛋白质组成的复杂分子机器，由大亚基和小亚基构成。真核生物中，小亚基（40S）和大亚基（60S）组成80S核糖体；原核生物中，30S和50S亚基构成70S核糖体。核糖体上有三个tRNA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，核糖体沿mRNA从5’到3’方向移动，催化肽键的形成。
3. mRNA and the Genetic Code | mRNA与遗传密码
Messenger RNA (mRNA) carries a copy of the genetic information from DNA in the nucleus to the cytoplasm. It is a single-stranded molecule with a sequence of bases (A, U, C, G). The genetic code is read in non-overlapping triplets called codons. There are 64 possible codons (4³), but only 20 standard amino acids, so the code is degenerate, meaning multiple codons can encode the same amino acid. The codon AUG acts as the start codon (also coding for methionine), while UAA, UAG, and UGA are stop codons that signal termination of translation.
信使RNA（mRNA）将遗传信息的副本从细胞核带到细胞质。它是一种单链分子，碱基序列为A、U、C、G。遗传密码以不重叠的三联体形式读取，称为密码子。总共有64个可能的密码子（4³），但标准氨基酸只有20种，因此密码有简并性，即多个密码子可编码同一种氨基酸。密码子AUG作为起始密码子（同时编码甲硫氨酸），而UAA、UAG和UGA是终止密码子，发出翻译终止的信号。
4. tRNA and Anticodons | tRNA与反密码子
Transfer RNA (tRNA) molecules are small RNA chains (about 75–90 nucleotides) with a distinctive cloverleaf secondary structure. Each tRNA has an anticodon loop containing a triplet of bases called the anticodon, which is complementary to an mRNA codon. At the 3′ end, the tRNA carries the corresponding amino acid. The enzyme aminoacyl-tRNA synthetase attaches the correct amino acid to its specific tRNA in a process called aminoacylation, which requires ATP. There are different types of tRNA for each amino acid, ensuring the genetic code is accurately translated.
转运RNA（tRNA）是较小的RNA链（约75–90个核苷酸），具有独特的三叶草二级结构。每个tRNA都有一个反密码子环，其中含有与mRNA密码子互补的三个碱基，即反密码子。在3’端，tRNA携带相应的氨基酸。氨酰-tRNA合成酶通过需ATP的氨酰化过程，将正确的氨基酸附着到其特异的tRNA上。每种氨基酸都有不同类型的tRNA，从而确保遗传密码被准确翻译。
5. Activation of Amino Acids | 氨基酸的活化
Before translation can proceed, amino acids must be chemically activated and attached to their corresponding tRNA molecules. This is a two-step reaction catalysed by aminoacyl-tRNA synthetase. First, the amino acid reacts with ATP to form an aminoacyl-adenylate complex, releasing pyrophosphate. Then, the activated amino acid is transferred to the 3′ end of the specific tRNA, forming an aminoacyl-tRNA (charged tRNA). This charging process ensures that each tRNA is correctly paired with its cognate amino acid, maintaining the fidelity of protein synthesis.
在翻译进行前，氨基酸必须被化学活化并连接到相应的tRNA分子上。这是由氨酰-tRNA合成酶催化的两步反应。首先，氨基酸与ATP反应生成氨酰腺苷酸复合物，释放焦磷酸。然后，活化的氨基酸转移至特异tRNA的3’端，形成氨酰-tRNA（负载tRNA）。这种负载过程确保每个tRNA正确配对其同源氨基酸，从而维持蛋白质合成的忠实性。
6. Initiation of Translation | 翻译的起始
Translation initiation begins when the small ribosomal subunit binds to the 5′ cap of eukaryotic mRNA (or the Shine-Dalgarno sequence in prokaryotic mRNA). The small subunit scans the mRNA until it finds the start codon AUG. An initiator tRNA carrying methionine (Met-tRNAi) binds to the AUG codon in the P site via its anticodon. The large ribosomal subunit then joins the complex, completing the formation of the functional ribosome. Initiation factors (IFs) assist in the assembly and require GTP for energy.
翻译起始始于小核糖体亚基与真核mRNA的5’帽结合（或与原核mRNA的Shine-Dalgarno序列结合）。小亚基扫描mRNA，直至找到起始密码子AUG。携带甲硫氨酸的起始tRNA（Met-tRNAi）通过其反密码子在P位与AUG密码子结合。随后大核糖体亚基加入复合物，形成有功能的核糖体。起始因子（IFs）辅助组装并需要GTP提供能量。
7. Elongation: Peptide Bond Formation | 延伸：肽键形成
Elongation is the repetitive cycle of adding amino acids to the growing polypeptide chain. A charged tRNA with an anticodon complementary to the next mRNA codon enters the A site. The ribosome’s peptidyl transferase activity (catalysed by the large subunit rRNA) forms a peptide bond between the amino group of the incoming aminoacyl-tRNA and the carboxyl group of the growing chain attached to the tRNA in the P site. The polypeptide is thus transferred to the tRNA in the A site, lengthening by one amino acid.
延伸是反复将氨基酸添加到增长中的多肽链上的循环。具有与下一个mRNA密码子互补的反密码子的负载tRNA进入A位。核糖体的肽基转移酶活性（由大亚基rRNA催化）在进入的氨酰-tRNA的氨基与P位上连接着肽链的tRNA的羧基之间形成肽键。于是多肽转移到A位的tRNA上，长度增加一个氨基酸。
8. Translocation of the Ribosome | 核糖体的移位
After peptide bond formation, the ribosome undergoes translocation: it moves exactly one codon along the mRNA in the 5’→3′ direction. This movement requires elongation factor EF-G (in prokaryotes) and GTP. As the ribosome shifts, the uncharged tRNA in the P site moves to the E site and is released, while the peptidyl-tRNA in the A site moves to the P site, freeing the A site for the next incoming aminoacyl-tRNA. The cycle repeats, and the polypeptide chain grows sequentially.
肽键形成后，核糖体发生移位：其沿mRNA按5’→3’方向移动一个密码子的精确距离。该移动需要延伸因子EF-G（原核生物）和GTP。移位时，P位上不带氨基酸的tRNA移至E位并释放，而A位的肽基-tRNA则移动到P位，空出的A位准备迎接下一个氨酰-tRNA。循环反复进行，多肽链依次增长。
9. Termination of Translation | 翻译的终止
Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) in the A site. No normal tRNA carries an anticodon for these codons. Instead, release factors (RF proteins) bind to the A site, causing the peptidyl transferase to transfer the polypeptide chain to a water molecule, hydrolysing the bond between the completed polypeptide and the tRNA in the P site. The newly synthesised polypeptide is released, and the ribosomal subunits dissociate from the mRNA and from each other.
终止发生在核糖体在A位遇到终止密码子（UAA、UAG或UGA）时。没有普通tRNA携带与这些密码子互补的反密码子。取而代之的是释放因子（RF蛋白）结合到A位，促使肽基转移酶将多肽链转移给一个水分子，从而水解完整多肽与P位tRNA之间的键。新合成的多肽链被释放，核糖体亚基与mRNA彼此分离。
10. Post-Translational Modifications | 翻译后修饰
Once the polypeptide chain is released, it often undergoes folding assisted by chaperone proteins into its specific three-dimensional conformation. Additional modifications may include cleavage of signal sequences, formation of disulfide bridges, addition of carbohydrate groups (glycosylation), phosphorylation, or assembly into quaternary structures with other polypeptides. These modifications are crucial for the protein’s function, stability, and proper targeting within the cell or for secretion.
多肽链释放后，通常会在分子伴侣协助下折叠成其特异的三维构象。还可能发生其他修饰，包括切割信号序列、形成二硫键、添加糖基（糖基化）、磷酸化，或与其他多肽组装成四级结构。这些修饰对于蛋白质的功能、稳定性以及在细胞内的正确定位或分泌至关重要。
11. Key Exam Points for Translation | 翻译核心考点
For IGCSE CIE Biology, focus on the roles of mRNA, tRNA, ribosomes, and the one-way flow of genetic information from DNA to protein. Be able to define codons and anticodons and explain their complementary base pairing (A-U, C-G). Understand why the genetic code is described as universal, degenerate, and non-overlapping. Recognise that translation occurs in the cytoplasm and requires ATP and GTP. Know that proteins are made of amino acids linked by peptide bonds, and that the sequence of bases in DNA ultimately determines the sequence of amino acids in a protein.
就IGCSE CIE生物学而言，重点掌握mRNA、tRNA、核糖体的功能，以及遗传信息从DNA到蛋白质的单向流动。能够定义密码子和反密码子，并解释其互补碱基配对（A-U，C-G）。理解遗传密码为何具有通用性、简并性和不重叠性。认识到翻译发生在细胞质中，需要ATP和GTP。知道蛋白质由氨基酸通过肽键连接而成，而DNA中的碱基序列最终决定蛋白质中的氨基酸序列。
12. Comparison of Translation in Prokaryotes and Eukaryotes | 原核与真核生物翻译的比较
Although the core mechanism of translation is conserved, there are notable differences. In prokaryotes, transcription and translation can occur simultaneously because there is no nuclear membrane; ribosomes can bind to mRNA as it is being transcribed. In eukaryotes, mRNA is processed (capping, splicing, polyadenylation) and transported from the nucleus to the cytoplasm before translation. Prokaryotic ribosomes are 70S, whereas eukaryotic ribosomes are 80S. Initiation also differs: prokaryotes use Shine-Dalgarno sequences for ribosome binding, while eukaryotes rely on the 5′ cap. These differences are exploited by antibiotics that specifically inhibit bacterial protein synthesis.
尽管翻译的核心机制保守，但存在显著差异。原核生物由于没有核膜，转录与翻译可同时进行；核糖体可在mRNA转录时即与之结合。真核生物中，mRNA需经过加工（加帽、剪接、加尾）并从核运输到细胞质后才能翻译。原核核糖体为70S，真核核糖体为80S。起始方式也不同：原核生物利用Shine-Dalgarno序列进行核糖体结合，而真核生物依赖5’帽。这些差异被抗生素利用，特异性抑制细菌蛋白质合成。
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