DEOXYPENTOSE NUCLEIC ACIDS 33 



nucleotide sequence and periodicity in a nucleic acid chain. Any 

 simplified assumption with respect to periodicity has been dis- 

 proved by the studies on the course of action of deoxyribonu- 

 clease on the DNA of calf thymus-^ and of wheat germ. The 

 composition of both the dialyzable degradation products and the 

 dialysis residues, the "cores", exhibited continuous and charac- 

 teristic changes with respect to the distribution of purines and 

 pyrimidines. One must conclude that the sequence is highly 

 aperiodic and that it is not inconceivable that the same cellular 

 DNA could give rise to many different nucleoproteins, depending 

 upon the shape and configuration of the particular protein. 



It must, moreover, be understood that the recognition of 

 periodicity, i.e., the presence of recurring units, will be particu- 

 larly difficult in a macromolecule of the type of DNA. If, for 

 instance, in a chain composed of 3000 nucleotides a particular 

 sequence of 100 consecutive nucleotides were repeated 30 times, 

 this periodicity could not be recognized, unless we had a method 

 producing cleavage only at the points where these repeating units 

 are joined. In other words, the perception of periodicity would 

 require the proper distance for a bird's-eye view which will not 

 be easy to attain. 



Another approach to the problem of sequence analysis in 

 DNA may be seen in the study of its controlled chemical degra- 

 dation. That the purines can be detached from a nucleic acid with 

 much greater ease than can the pyrimidines has long been known; 

 but the resulting end product, thymic acid, was rather non- 

 descript. In collaboration with M. E. Hodes and C. Tamm it has 

 been possible to develop procedures, soon to be published, in 

 which all purines could be cleaved from a DNA preparation, 

 leaving behind a non-dialyzable product in a yield of about 94% 

 that retained all the pyrimidines of the original DNA in unchanged 

 proportions. We have designated preparations of this type as 

 apurinic acid. These compounds may prove of interest for struc- 

 tural studies on DNA, since the position in the DNA chain of the 

 initially present purine nucleotides is now marked by reactive 

 aldehydo groups. The properties of a typical preparation of 



References p. 37 



