【摘要】 遗传密码子的设定表现出令人困惑的多态性特点 :不同氨基酸拥有的密码子的数目 ,除 5个外 ,从 1个到 6个都有 .这种特点显示出密码子无论在翻译行为还是进化轨迹上 ,都存在诸多的异质性 .因此 ,简并性一词的收敛含义 ,并不能表征这种多态性的进化内涵 .没有同义密码子的AUG(Met)和UGG (Trp)并无简并现象 .其余的密码子则可分为两大类 :一类是 ,4个同义密码子为 1组 ,具有相同的第 1、2位碱基 ,并遵循“3中读 2”的读出规则 .同组的 4个同义密码子 ,不过是来自同一个双字母原始密码子 (XYN)的孑遗物 ,从这个意义上讲 ,也不宜视为简并现象 ;另一类则主要是 ,2个同义密码子为一组 ,并遵循“3中读 3”读出规则 .它们是由编码 2个氨基酸的双义原始密码子 ,第 3位的未定碱基N进一步设定形成 .至于有 6个同义密码子的 ,特别令人困感不解的组别 ,实际上是 4 + 2个 ,这启示它们可能源于上述两大类 .遗传密码子多态性的起源 ,可能始于最初阶段 ,氨基酸同某类寡核苷酸的起始二联体的相互作用 ,而完成于所有的双义原始密码子的第 3位碱基的分化 .这种进化轨迹被传统的简并性一词所模糊 ,并导致鉴定各有关理论可信性的坚实依据和令不同观点取得共识的基础被掩盖起来 .这可能就是在遗传密码子起源领域里 ,长期存在着众更多还原

【Abstract】 Genetic codons display puzzling polymorphism in their assignment: the number of codons assigned to each amino acid ranges from 1 to 6 but 5. The characteristic reflects tremendous heterogeneity of genetic codons existing in either translation behavior or evolutionary track, and the convergent term “degeneracy" fails to define the evolutionary essence of such polymorphism. Two codons, AUG (Met) and UGG (Trp), having no synonymous codon are not degenerative at all. The other codons are classified into two major classes: one class includes codon groups each having 4 synonymous codons with identical first two bases. They might have evolved from a two\|letter primitive codon ( XYN ) encoding specifically for a single amino acid, and following 2 out of 3 reading rule, which are simply the relics from the primitive codons still in use, and in this sense, also are not degenerative. The other class mainly includes groups each including 2 synonymous codons and obeying 3 out of 3 reading rule. They might have come from the ambiguous primitive codons each for two amino acids via the assignment of the unassigned 3 rd bases N to make two specific modern codons. As for the peculiar codon groups each having 6, or actually 4+2, synonymous codons might imply heterologous origins from the above two groups. The origin of polymorphism of the assignment of genetic codons is believed to initiate from the interaction between amino acids and specific terminal doublets of certain nucleotides at the earliest stage, and finalize via the differentiation of the 3 rd bases of ambiguous procodons mediated by cognate aminoacyl tRNA synthetases. These evolutionary traces have been dimmed by the traditional term “degeneracy". As a result, the substantial criterion for the judgment of the plausibility of any hypothesis concerned and the consensus basis for different concepts in this field have been covered. This may explain the long term controversies among various theories in the field of the origin of genetic codons.更多还原