Proto-anticodon RNA

Proto-anti-codon RNAs (pacRNAs) are auto-aminoacylating ribozymes, which are thought to have evolved from any nucleic acid molecular system that was capable of forming a hairpin followed by an exposed "anti-codon" sequence and an adjacent step-loop acceptor stem. pacRNAs are postulated as ancestral precursors of proto-tRNAs but are distinguished from tRNAs in having evolved prior to the evolution of protein translation. The "anti-codon" binding pocket is called an anti-codon because the specific Erives model of pacRNAs predicts many of the anti-codon sequences of the genetic code. The pacRNA model is also thought to provide a key explanation for the origins of homochirality, or at least of complementary homochiralities between amino acids and nucleotide sequences.
Aminoacylated RNA world
pacRNAs are thought to represent the key molecular entities during an extensive aminoacylated RNA world. The pacRNA model holds that early RNA world was predominantly an aminoacylated RNA world.
List of things explained by the pacRNA model
The pacRNA model simultaneously explains many previously stated questions in biological evolution:
* evolutionary limited use of the 20 proteinogenic amino acids out of many potential amino acids;
* universal homochiralities of L-amino acids and nucleotide sugars (D-ribose);
* evolutionary origin of the genetic code (specific codon patterns) ;
** evolutionary constraint or origin of synonymous third nucleotide position of codons;
** evolutionary origin of stop codons.

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