Abstract
Several preparations of the apyrimidinic acid of calf-thymus DNA were subjected to hydrolysis by alkali under a variety of conditions. The purine isostichs of the general structure d(Punpn+1), and of length 1 to 8 were separated by chromatography on DEAE-cellulose and their relative frequencies determined. The results were compared with those obtained with the pyrimidine isostichs released similarly from apurinic acid. Although apyrimidinic acid poses several special problems, the results obtained with the two partial degradation products of DNA were in satisfactory agreement, especially when balances based on ultraviolet-light absorbance were compared.The various purine oligonucleotides comprising isostichs 1 to 5 were separated into groups of sequence isomers having the same total composition, and their relative frequencies were estimated and compared with the corresponding pyrimidine fractions. The results are in agreement with the suggestion that the base-pairing complementarities characteristic of the intact DNA also apply to those individual purine and pyrimidine sequences that can be studied. The findings also support the previous demonstration that the two DNA strands of calf-thymus DNA are antiparallel.
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