Evolution of the genetic code. Emergence of stop codons

In my previous study [2] I wrote that the UG base doublet, unlike GU, CU and UC, acquired its meaning after the emergence of adenine. Thus, in my opinion, this base doublet is the first version of a stop codon. In the table of the universal genetic code, this base doublet is a stop codon; moreover, this base doublet encodes cysteine and tryptophan. What could cause the emergence of non-encoding codons in the course of evolution of the genetic code? I would venture to suppose that the reason was quite simple – relative instability of complementary interaction of the codon with the respective anticodon or, more exactly, with the part complementary to the base doublet. The UG base doublet could be essentially different from the GU pair in its interaction with the complementary dinucleotide. As we are considering the stage at which there was no adenine yet, we should investigate the interaction of the UG and GC dinucleotides. There are reliable data that can provide a basis for substantiating my hypothesis prior to experiments and calculations. Let us examine the table of the anticodon-codon correspondence at the third position [3] (Table 1). Table 1. Ambiguity of nucleotide coupling at the third position of the codon and anticodon Uracil can complementarily interact with adenine and guanine, but at the first and second positions of the codon there are always classic uracil-adenine pairs, indicating greater energy efficiency of this interaction. If there were no energy benefits, there would be frequent amino acid replacements in proteins, due to a foreign anticodon joining the codon. Thus, we can make the conclusion that the uracil-guanine pair was less stable than the uracil-adenine one. This alone reduces the stability of complementary interactions of all base doublets that originated at the stage of uracil incorporation into the genetic code.