Jukes T H
University of California, Berkeley Space Sciences Laboratory, Oakland 94608, USA.
Adv Space Res. 1984;4(12):177-82. doi: 10.1016/0273-1177(84)90560-x.
Anticodons are trinucleotides in transfer RNA (tRNA) molecules. The latter carry amino acids for insertion into the polypeptide sequences of proteins during the translation of messenger RNA (mRNA) molecules. Messenger RNA molecules are transcribed from genes. Evolution of tRNA molecules has resulted in a set of anticodons for the 20 amino acids that are used in protein synthesis. This set of anticodons is slightly different in mitochondrial codes from the set that used in the nuclear "universal" code. Theories for the evolution of the code include frozen accident, doublet expansion, repeating triplets and coevolutionary distribution. The number of codons has always been fixed at 64 by mathematical rules, but because an anticodon may pair with more than one codon, the number of anticodons is only 54 in the universal code, is smaller in mitochondrial codes, and was probably even smaller in archetypal primitive codes. Evidence of anticodon evolution can be seen by comparing mitochondrial codes with the universal code. Codes used by very primitive organisms that are now extinct might have specified fewer amino acids than are now used.
反密码子是转运核糖核酸（tRNA）分子中的三核苷酸。tRNA分子携带氨基酸，以便在信使核糖核酸（mRNA）分子翻译过程中插入蛋白质的多肽序列。mRNA分子由基因转录而来。tRNA分子的进化产生了一套用于蛋白质合成的20种氨基酸的反密码子。这套反密码子在线粒体密码中与核“通用”密码中使用的那套略有不同。密码进化的理论包括冻结偶然、双联体扩展、重复三联体和共同进化分布。根据数学规则，密码子的数量一直固定为64个，但由于一个反密码子可能与多个密码子配对，因此在通用密码中反密码子的数量只有54个，在线粒体密码中更少，在原始原型密码中可能甚至更少。通过将线粒体密码与通用密码进行比较，可以看出反密码子进化的证据。现已灭绝的非常原始的生物体所使用的密码可能指定的氨基酸比现在使用的要少。
