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  • Title: Genetic code evolution started with the incorporation of glycine, followed by other small hydrophilic amino acids.
    Author: Bernhardt HS, Patrick WM.
    Journal: J Mol Evol; 2014 Jun; 78(6):307-9. PubMed ID: 24916657.
    Abstract:
    We propose that glycine was the first amino acid to be incorporated into the genetic code, followed by serine, aspartic and/or glutamic acid-small hydrophilic amino acids that all have codons in the bottom right-hand corner of the standard genetic code table. Because primordial ribosomal synthesis is presumed to have been rudimentary, this stage would have been characterized by the synthesis of short, water-soluble peptides, the first of which would have comprised polyglycine. Evolution of the code is proposed to have occurred by the duplication and mutation of tRNA sequences, which produced a radiation of codon assignment outwards from the bottom right-hand corner. As a result of this expansion, we propose a trend from small hydrophilic to hydrophobic amino acids, with selection for longer polypeptides requiring a hydrophobic core for folding and stability driving the incorporation of hydrophobic amino acids into the code.
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