Triple-Stranded Polynucleotide Helix Containing Only Purine Bases

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Science  06 Jul 1973:
Vol. 181, Issue 4094, pp. 68-69
DOI: 10.1126/science.181.4094.68


The structure of the complex involving one polyadenylic acid and two polyinosinic acid chains has been determined by x-ray diffraction. The three coaxial, helical chains have conformations like conventional RNA double helices despite the absence of purine-pyrimidine pairing. Formation of hypoxanthine pairs in codon-anticodon interactions therefore requires only trivial changes in the conformation of a standard nucleotide. Evolution of the contemporary genetic code involving purine-pyrimidine complementarity from a primeval code with only adenine-hypoxanthine pairing would have been possible without major discontinuities in molecular geometry.