Hi, semilanceata. Welcome to EvC!
semilanceata writes:
Why shouldn't all codons have associated tRNA anti-codon complexes?
I'm not a cellular biologist or geneticist, but it seems to me that the answer could very well be simple. tRNA has to encoded by DNA. Each different tRNA in an organism would have to be encoded by a different gene, each of which would either arise independently or as a variation of the first tRNA gene. Either way, they would have arisen one by one.
Seeing how an "end" to a coding region is advantageous for condensing the genome into just a few chromosomes or plasmids, I see no reason why this condition would be selected against, thus it would be preserved and amplified in the succeeding generations. On the other hand, an organism that continues to produce new genes encoding tRNAs with "unused" anticodons, would reduce its ability to end a polypeptide and condense its genome, creating a greater inefficiency in transcription/translation.
{AbE: I can't confirm to you that this is correct: maybe Wounded King or other geneticist/molecular biologist could shed some light on this?}
Edited by Bluejay, : Addition
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