title: Triple-stranded polynucleotide helix containing only purine bases.
abstract: 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.
mesh_terms: D000227:Adenine Nucleotides; D001483:Base Sequence; D007292:Inosine Nucleotides; D008961:Models, Structural; D008968:Molecular Conformation; D011119:Polynucleotides; D014961:X-Ray Diffraction
chemical_list: D000227:Adenine Nucleotides; D007292:Inosine Nucleotides; D011119:Polynucleotides