📚 Appendix 2: DNA and RNA Triplet Codes | 附录2：DNA与RNA三联体密码
The genetic code is the set of rules by which the information stored in DNA and RNA sequences is translated into proteins. It is based on triplets of nucleotide bases, known as codons in mRNA, that specify particular amino acids. Understanding the triplet code is fundamental to molecular biology and is a core topic in A-Level Biology under the Cambridge International syllabus. This article explores the relationship between DNA triplets, mRNA codons, tRNA anticodons, and the features of the universal genetic code.
遗传密码是一套规则，将储存在DNA与RNA序列中的信息翻译成蛋白质。它基于三个核苷酸碱基组成的序列，在mRNA中称为密码子，指定特定的氨基酸。理解三联体密码是分子生物学的基础，也是剑桥国际A-Level生物课程的核心主题。本文探讨DNA三联体、mRNA密码子、tRNA反密码子之间的关系以及通用遗传密码的特征。
1. Introduction to the Genetic Code | 遗传密码简介
The genetic code consists of 64 possible triplet combinations of the four bases (A, U, G, C in RNA; A, T, G, C in DNA). Each triplet, or codon, codes for one of the 20 standard amino acids or serves as a stop signal during protein synthesis. The code is read in a non-overlapping, linear manner along the mRNA from a fixed start point, ensuring the correct amino acid sequence of a polypeptide.
遗传密码由四种碱基（RNA中为A、U、G、C；DNA中为A、T、G、C）组成的64个可能的三联体组合构成。每一个三联体，或称密码子，编码20种标准氨基酸之一，或在蛋白质合成中充当终止信号。密码子以非重叠、线性的方式沿mRNA从一个固定的起始点读取，保证了多肽正确的氨基酸序列。
2. DNA Triplets and the Template vs. Coding Strand | DNA三联体与模板链和编码链
In the DNA double helix, the two strands are the template strand and the coding strand. The template strand is read by RNA polymerase during transcription in the 3' to 5' direction, producing a complementary mRNA. A DNA triplet on the template strand is complementary to the mRNA codon. The coding strand has the same sequence as the mRNA (with thymine instead of uracil) and is not directly used for transcription.
在DNA双螺旋中，两条链分别是模板链和编码链。转录时，RNA聚合酶沿模板链以3'到5'方向读取，生成互补的mRNA。模板链上的DNA三联体与mRNA密码子互补。编码链的序列与mRNA相同（胸腺嘧啶取代尿嘧啶），并不直接用于转录。
3. Transcription: From DNA Triplets to mRNA Codons | 转录：从DNA三联体到mRNA密码子
During transcription, the enzyme RNA polymerase binds to the promoter and unwinds the DNA. It moves along the template strand, adding RNA nucleotides complementary to the DNA triplets according to base-pairing rules (A–U, T–A, C–G, G–C). The resulting pre-mRNA in eukaryotes undergoes processing before the mature mRNA is exported to the cytoplasm, carrying a sequence of codons ready for translation.
转录过程中，RNA聚合酶与启动子结合并解开DNA双链。它沿模板链移动，根据碱基互补配对规则（A–U、T–A、C–G、G–C）添加与DNA三联体互补的RNA核苷酸。真核生物中产生的前体mRNA经过加工，成熟的mRNA被运送到细胞质，携带待翻译的密码子序列。
4. The Codon Table: mRNA Triplets and Amino Acids | 密码子表：mRNA三联体与氨基酸
The standard genetic code is often displayed as a table of 64 codons. Each codon corresponds to an amino acid or a stop signal. For example, the codon AUG codes for methionine and also serves as the start codon. Reading the table requires the first, second, and third positions of the codon. The table highlights the degenerate nature of the code.
标准遗传密码通常以64个密码子的表格形式展示。每个密码子对应一种氨基酸或终止信号。例如，密码子AUG编码甲硫氨酸，同时也作为起始密码子。阅读密码子表需要第一、第二和第三位碱基。该表突出了密码的简并性。
Table 1: Selected mRNA Codons and Corresponding Amino Acids
| Codon | Amino Acid | Abbreviation |
|---|---|---|
| AUG | Methionine (Start) | Met |
| UUU, UUC | Phenylalanine | Phe |
| UUA, UUG, CUU, CUC, CUA, CUG | Leucine | Leu |
| GGU, GGC, GGA, GGG | Glycine | Gly |
| UAA, UAG, UGA | Stop | — |
5. Start and Stop Codons | 起始密码子和终止密码子
Translation begins at the start codon AUG, which codes for methionine. The ribosome recognizes this codon via the initiator tRNA. The end of the coding sequence is marked by one of three stop codons: UAA, UAG, or UGA. These do not code for any amino acid and instead signal release factors to terminate polypeptide synthesis and detach the ribosome.
翻译从起始密码子AUG开始，它编码甲硫氨酸。核糖体通过起始tRNA识别该密码子。编码序列的末端由三个终止密码子之一标记：UAA、UAG或UGA。它们不编码任何氨基酸，而是通过释放因子发出信号，终止多肽合成并使核糖体解离。
6. tRNA and Anticodons: Decoding the Message | tRNA与反密码子：解码信息
Each tRNA molecule has an anticodon triplet at one end and an amino acid attachment site at the other. The anticodon is complementary to the mRNA codon, allowing correct insertion of the amino acid into the growing polypeptide. For example, the tRNA for methionine has the anticodon UAC (3'-UAC-5'), which pairs with the codon AUG (5'-AUG-3').
每个tRNA分子一端带有一个反密码子三联体，另一端带有氨基酸附着位点。反密码子与mRNA密码子互补，使正确的氨基酸可以插入到不断延长的多肽链中。例如，甲硫氨酸的tRNA具有反密码子UAC（3'-UAC-5'），与密码子AUG（5'-AUG-3'）配对。
7. Wobble Hypothesis and Codon-Anticodon Pairing | 摆动假说与密码子-反密码子配对
The wobble hypothesis explains how a single tRNA can recognize more than one codon. The pairing between the third base of the codon and the first base of the anticodon is less stringent. For instance, inosine (I) in the anticodon can pair with U, C, or A in the codon. This reduces the number of tRNAs required and accounts for some degeneracy in the code.
摆动假说解释了为何单个tRNA能够识别多个密码子。密码子的第三位碱基与反密码子第一位碱基之间的配对不那么严格。例如，反密码子中的肌苷（I）可以与密码子中的U、C或A配对。这减少了所需tRNA的数量，也解释了密码子简并性的部分原因。
8. Features of the Genetic Code: Degeneracy and Universality | 遗传密码的特征：简并性与通用性
The code is degenerate because most amino acids are specified by more than one codon. For example, leucine has six codons, while methionine and tryptophan each have one. The code is also nearly universal: the same codons specify the same amino acids in almost all organisms, providing strong evidence for a common evolutionary origin. Exceptions do appear in mitochondria and some protists.
遗传密码具有简并性，因为大多数氨基酸由多个密码子指定。例如，亮氨酸有六个密码子，甲硫氨酸和色氨酸则只有一个。密码子还具有近乎通用性：在几乎所有生物中，相同的密码子指定相同的氨基酸，这为共同进化起源提供了有力证据。例外情况出现在线粒体和某些原生生物中。
9. Experimental Evidence: Nirenberg and Khorana | 实验证据：尼伦伯格与科拉纳
In the 1960s, Marshall Nirenberg and Heinrich Matthaei demonstrated that a synthetic mRNA containing only uracil (poly-U) directed the synthesis of polyphenylalanine, establishing that UUU codes for phenylalanine. Har Gobind Khorana used repeating dinucleotide and trinucleotide sequences to decipher the remaining codons. These experiments confirmed the triplet, non-overlapping nature of the code.
20世纪60年代，马歇尔·尼伦伯格和海因里希·马特伊证明，仅含尿嘧啶的合成mRNA（poly-U）指导了聚苯丙氨酸的合成，确立了UUU编码苯丙氨酸。哈尔·戈宾德·科拉纳利用重复的二核苷酸和三核苷酸序列破译了其余密码子。这些实验证实了密码子的三联体、非重叠性质。
10. Differences in Organelles and Exceptions | 细胞器中的差异与例外
While the nuclear genetic code is almost universal, mitochondrial genomes often show variations. For instance, in human mitochondria, UGA codes for tryptophan instead of a stop signal, and AUA codes for methionine rather than isoleucine. These deviations are important for understanding organelle genetics and are noted in Cambridge specifications.
虽然细胞核遗传密码几乎通用，但线粒体基因组常呈现变异。例如，在人类线粒体中，UGA编码色氨酸而非终止信号，AUA编码甲硫氨酸而非异亮氨酸。这些偏差对于理解细胞器遗传学很重要，在剑桥考试大纲中有所提及。
11. Applying the Triplet Code: Practice Examples | 三联体密码的应用：练习示例
To reinforce understanding, let us translate a simple mRNA sequence: 5'-AUG GGC UUU UAA-3' e resulting peptide is Met-Gly-Phe. Be aware that the DNA template strand would have the complementary sequence 3'-TAC CCG AAA ATT-5'.
为加深理解，我们来翻译一条简短的mRNA序列：5'-AUG GGC UUU UAA-3'。查阅密码子表，AUG=Met（起始），GGC=Gly，UUU=Phe，UAA=终止。生成的多肽为Met-Gly-Phe。请注意，DNA模板链的互补序列应为3'-TAC CCG AAA ATT-5'。
12. Summary of DNA and RNA Triplet Codes | DNA与RNA三联体密码总结
The flow of genetic information from DNA to protein relies on a triplet-based code. DNA triplets on the template strand are transcribed into mRNA codons, which are translated through tRNA anticodons. The code is degenerate, generally universal, and includes specific start and stop signals. Experiments by Nirenberg, Khorana, and others revealed the codon assignments. A clear grasp of these concepts is essential for A-Level Biology students, particularly when tackling questions on gene expression and protein synthesis.
遗传信息从DNA传递到蛋白质依赖于基于三联体的密码。模板链上的DNA三联体转录为mRNA密码子，后者通过tRNA反密码子进行翻译。遗传密码具有简并性，通常为通用密码，并包含特定的起始和终止信号。尼伦伯格、科拉纳等人的实验揭示了密码的分配。对A-Level生物学生而言，清晰地掌握这些概念至关重要，尤其是在解答基因表达和蛋白质合成相关问题时。
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