📚 Protein Synthesis: Transcription and Translation | 蛋白质合成：转录与翻译
Proteins are essential biomolecules that perform a vast array of functions within living organisms, from catalysing metabolic reactions as enzymes to providing structural support in cells and tissues. The instructions for building these proteins are encoded within the DNA of every cell. Protein synthesis is the process by which cells use the genetic code to assemble amino acids into specific polypeptide chains, which then fold into functional proteins. Understanding this process is fundamental to grasping how genotype determines phenotype.
蛋白质是生命中不可或缺的生物大分子，执行着从酶催化代谢反应到提供细胞和组织结构支持的各种功能。构建这些蛋白质的指令编码在每个细胞的DNA中。蛋白质合成就是细胞利用遗传密码将氨基酸组装成特定多肽链，进而折叠成功能性蛋白的过程。理解这一过程是掌握基因型如何决定表现型的基础。
1. Overview of Protein Synthesis | 蛋白质合成概述
Protein synthesis involves two major stages: transcription and translation. In transcription, a segment of DNA is copied into a complementary messenger RNA (mRNA) molecule. This mRNA carries the genetic instructions from the nucleus to the ribosomes in the cytoplasm. During translation, the ribosome reads the mRNA sequence and, with the help of transfer RNA (tRNA) molecules, assembles the corresponding amino acids into a polypeptide chain. This flow of genetic information from DNA to RNA to protein is often referred to as the central dogma of molecular biology.
蛋白质合成包含两个主要阶段：转录和翻译。在转录中，一段DNA被复制成互补的信使RNA（mRNA）分子。该mRNA将遗传指令从细胞核携带到细胞质中的核糖体。在翻译过程中，核糖体读取mRNA序列，并在转运RNA（tRNA）分子的帮助下，将相应的氨基酸组装成多肽链。这种遗传信息从DNA流向RNA再到蛋白质的过程常被称为分子生物学的中心法则。
2. The Genetic Code | 遗传密码
The genetic code is a set of rules by which a sequence of nucleotide bases in DNA or RNA determines the sequence of amino acids in a protein. Each sequence of three bases, called a codon, codes for one specific amino acid. For example, the DNA triplet TAC codes for the amino acid methionine, which often signals the start of a protein. The genetic code is degenerate, meaning that more than one codon can encode the same amino acid. It is also universal, being almost identical across all organisms.
遗传密码是一套规则，DNA或RNA中的核苷酸碱基序列决定了蛋白质中的氨基酸序列。每三个碱基组成的序列称为密码子，编码一个特定的氨基酸。例如，DNA三联体TAC编码甲硫氨酸，它通常标志着蛋白质合成的开始。遗传密码具有简并性，即多个密码子可以编码同一种氨基酸。它还具有通用性，几乎所有生物都使用几乎相同的密码子表。
3. Transcription – Making mRNA | 转录——制造mRNA
Transcription takes place in the nucleus. The enzyme RNA polymerase binds to a specific region of the DNA called the promoter, which signals the start of a gene. The DNA double helix unwinds, and one strand acts as the template. RNA polymerase moves along the template strand, adding complementary RNA nucleotides: adenine (A) pairs with uracil (U) in RNA (instead of thymine), thymine (T) pairs with adenine (A), cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C). This produces a single-stranded pre-mRNA molecule with a sequence complementary to the template DNA. Once the entire gene is transcribed, RNA polymerase detaches, and the pre-mRNA is processed (in eukaryotes) to become mature mRNA by removing introns and adding a 5' cap and poly-A tail. However, for IGCSE, the focus is on the synthesis of mRNA.
转录发生在细胞核内。RNA聚合酶与DNA上称为启动子的特定区域结合，该区域标志着基因的起始。DNA双螺旋解旋，其中一条链作为模板。RNA聚合酶沿着模板链移动，添加互补的RNA核苷酸：腺嘌呤（A）与RNA中的尿嘧啶（U）配对（代替胸腺嘧啶），胸腺嘧啶（T）与腺嘌呤（A）配对，胞嘧啶（C）与鸟嘌呤（G）配对，鸟嘌呤（G）与胞嘧啶（C）配对。这样就产生了一条与模板DNA互补的单链前体mRNA分子。当整个基因转录完毕后，RNA聚合酶脱离，前体mRNA经过加工（在真核细胞中）成为成熟mRNA，包括切除内含子、添加5'帽和poly-A尾。但在IGCSE中，重点是mRNA的合成。
4. The Role of mRNA | mRNA的作用
Messenger RNA (mRNA) acts as a transient copy of the genetic information needed to make a specific protein. It carries the code from the DNA in the nucleus to the ribosomes, the protein-making machinery in the cytoplasm. The mRNA molecule is a linear sequence of codons, each consisting of three bases. Importantly, mRNA uses uracil (U) instead of thymine (T). The sequence of codons directly dictates the order of amino acids in the polypeptide. Because mRNA is single-stranded and relatively short-lived, it can be rapidly produced and degraded, allowing the cell to control protein synthesis efficiently.
信使RNA（mRNA）充当制备特定蛋白质所需遗传信息的临时拷贝。它将密码从细胞核中的DNA携带到细胞质中的蛋白质合成机器——核糖体。mRNA分子是一条包含多个密码子的线性序列，每个密码子由三个碱基组成。重要的是，mRNA使用尿嘧啶（U）而不是胸腺嘧啶（T）。密码子的序列直接决定了多肽中氨基酸的顺序。由于mRNA是单链且寿命相对较短，它可以被迅速生成和降解，使细胞能够高效地控制蛋白质合成。
5. Translation – Building the Polypeptide | 翻译——构建多肽链
Translation occurs at ribosomes in the cytoplasm. A ribosome attaches to the mRNA at the start codon (AUG in mRNA, which codes for methionine). The ribosome then moves along the mRNA, reading one codon at a time. For each codon, a specific transfer RNA (tRNA) molecule carrying the matching anticodon and its attached amino acid binds temporarily to the ribosome. The ribosome catalyses the formation of peptide bonds between adjacent amino acids, creating a growing polypeptide chain. The process continues until the ribosome encounters a stop codon (UAA, UAG, or UGA), which signals the end of translation. The completed polypeptide is then released.
翻译在细胞质的核糖体上进行。核糖体附着在mRNA的起始密码子（mRNA中的AUG，编码甲硫氨酸）上。然后核糖体沿着mRNA移动，每次读取一个密码子。对于每个密码子，一个特定的转运RNA（tRNA）分子携带匹配的反密码子和相应的氨基酸，暂时与核糖体结合。核糖体催化相邻氨基酸之间肽键的形成，生成不断延伸的多肽链。该过程持续进行，直到核糖体遇到终止密码子（UAA、UAG或UGA），这标志着翻译结束。此时，完整的多肽链被释放出来。
6. The Role of tRNA | tRNA的作用
Transfer RNA (tRNA) molecules serve as adaptors between the mRNA codons and the amino acids they specify. Each tRNA has a specific anticodon sequence of three bases that is complementary to an mRNA codon. At the opposite end, the tRNA carries a binding site for a specific amino acid. Enzymes called aminoacyl-tRNA synthetases attach the correct amino acid to its corresponding tRNA. During translation, the tRNA anticodon pairs with the mRNA codon inside the ribosome, ensuring that the correct amino acid is added to the growing polypeptide. This precise pairing safeguards the fidelity of protein synthesis.
转运RNA（tRNA）分子充当mRNA密码子与其所指定氨基酸之间的衔接子。每个tRNA都有一个特定的反密码子序列，由三个碱基组成，与mRNA密码子互补。在另一端，tRNA带有一个特定氨基酸的结合位点。称为氨酰-tRNA合成酶的酶将正确的氨基酸连接到其对应的tRNA上。在翻译过程中，tRNA的反密码子在核糖体内与mRNA密码子配对，确保将正确的氨基酸添加到不断延伸的多肽链中。这种精确的配对保证了蛋白质合成的准确性。
7. Ribosomes – The Site of Protein Synthesis | 核糖体——蛋白质合成的场所
Ribosomes are complex molecular machines composed of ribosomal RNA (rRNA) and proteins. They consist of two subunits, large and small, which assemble around the mRNA during translation. The ribosome has three binding sites for tRNA: the A (aminoacyl) site, where incoming tRNA carrying an amino acid enters; the P (peptidyl) site, where the growing polypeptide chain is held; and the E (exit) site, where empty tRNA molecules leave. The ribosome moves along the mRNA in a stepwise manner, catalysing peptide bond formation and ensuring the fidelity of translation.
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两个亚单位组成，在翻译过程中围绕mRNA组装。核糖体具有三个tRNA结合位点：A位（氨酰位），携带氨基酸的tRNA由此进入；P位（肽基位），此处持有不断增长的多肽链；E位（出口位），空载tRNA由此离开。核糖体沿mRNA逐步移动，催化肽键形成并确保翻译的准确性。
8. Codons and Anticodons | 密码子与反密码子
The genetic code is read in triplets. On the mRNA, each triplet is called a codon. For example, the mRNA codon AUG codes for methionine and also acts as the start signal. Other codons such as UUU code for phenylalanine, and GGU codes for glycine. A tRNA molecule has a complementary triplet called an anticodon. For instance, the tRNA for methionine has the anticodon UAC (complementary to AUG). During translation, the ribosome orchestrates the pairing of mRNA codons with tRNA anticodons, allowing amino acids to be positioned precisely for peptide bond formation. The table below shows some example codons and their corresponding amino acids.
遗传密码以三联体形式被读取。在mRNA上，每个三联体称为一个密码子。例如，mRNA密码子AUG编码甲硫氨酸，同时也作为起始信号。其他密码子如UUU编码苯丙氨酸，GGU编码甘氨酸。tRNA分子具有一个互补的三联体，称为反密码子。例如，甲硫氨酸的tRNA具有反密码子UAC（与AUG互补）。在翻译过程中，核糖体协调mRNA密码子与tRNA反密码子的配对，使氨基酸得以精确定位以形成肽键。下表显示了一些示例密码子及其对应的氨基酸。
| mRNA Codon | Amino Acid | tRNA Anticodon |
|---|---|---|
| AUG | Methionine (Start) | UAC |
| UUU | Phenylalanine | AAA |
| GGU | Glycine | CCA |
| UCA | Serine | AGU |
9. The Stages of Translation in Detail | 翻译阶段的细节
Translation can be divided into three phases: initiation, elongation, and termination. Initiation begins when the small ribosomal subunit binds to the mRNA near the start codon, and the initiator tRNA carrying methionine attaches to the start codon. The large ribosomal subunit then joins to form the functional ribosome. During elongation, the ribosome moves along the mRNA, with new tRNA molecules entering the A site and peptide bonds forming between the amino acids. The ribosome translocates, shifting the tRNA from A to P to E sites, and the cycle repeats. Termination occurs when a stop codon (UAA, UAG, UGA) enters the A site. Release factors bind, causing the ribosome to disassemble and release the completed polypeptide.
翻译可分为三个阶段：起始、延伸和终止。起始始于小核糖体亚基与mRNA靠近起始密码子的区域结合，携带甲硫氨酸的起始tRNA附着于起始密码子。然后大核糖体亚基加入形成功能核糖体。在延伸阶段，核糖体沿mRNA移动，新的tRNA分子进入A位，氨基酸之间形成肽键。核糖体移位，将tRNA从A位移至P位再到E位，周而复始。终止发生在终止密码子（UAA、UAG、UGA）进入A位时。释放因子结合，导致核糖体解体并释放完整的多肽链。
10. From Polypeptide to Protein | 从多肽到蛋白质
The primary structure of a protein is the linear sequence of amino acids in the polypeptide chain, which is determined directly by the mRNA codon sequence. Once the polypeptide is released, it folds into a specific three-dimensional shape, forming secondary structures like alpha helices and beta sheets, and further tertiary and quaternary structures. This folding is driven by hydrogen bonds, ionic interactions, hydrophobic effects, and disulfide bridges. The final shape determines the protein's function. Post-translational modifications, such as the addition of chemical groups, can also occur to activate the protein. Thus, a correctly synthesised polypeptide is just the first step toward a functional protein.
蛋白质的一级结构是多肽链中氨基酸的线性序列，直接由mRNA密码子序列决定。多肽链释放后，会折叠成特定的三维形状，形成α螺旋和β折叠等二级结构，以及更高级的三级和四级结构。这种折叠由氢键、离子相互作用、疏水效应和二硫键驱动。最终形状决定了蛋白质的功能。翻译后修饰，如添加化学基团，也可能发生以使蛋白质活化。因此，正确合成的多肽只是通往功能蛋白质的第一步。
11. Summary and Importance | 总结与重要性
Protein synthesis is a highly regulated, multi-step process that converts genetic information into functional
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