the periodic variations obtained by correlating the relative positions of purines and pyrimidines ( and of the four bases thymine , cytosine , adenine , and guanine ) in a wide variety of genomes of wholly or partly known sequence suggest that there may be enough of an earlier comma-free coding system ( i.e. , only readable in one frame ) still present to permit determination of the reading frame and approximate extent of the present protein coding stretches .
C0034140
purines
C0001407
adenine