90 



NUCLEIC ACIDS AND NUCLEOPROTEINS 



The "backbone", therefore, is represented by a regularly con- 

 structed polydeoxyribophosphate; and it is in the "superstruc- 

 ture" of purines and pyrimidines, linked glycosidically to V of 

 the sugar, that the differences between nucleic acids must reside 

 (Fig. 8). An alphabet of four or five letters may look meager, but 

 if the words which these four letters spell out are 25,000-letter 

 words, the result may be oppressively informative. Even if we 

 had a rigorously homogeneous nucleic acid and procedures for 

 its stepwise and orderly dismemberment, it would be hopeless to 

 undertake the decoding of its nucleotide sequence. By the time 

 the cryptographer had completed his task and written down the 

 entire arrangement, evolution would probably have overtaken 

 him and he could start over again. 



A strict sequence analysis is, therefore, not only unattractive, 

 but probably impossible, at any rate in DNA; an assay of the 

 distribution density of individual components, i.e., a survey of 

 tendencies in nucleotide sequence, could, however, be considered. 

 By survey I mean an attempt at crudely mapping the order in 

 which the mononucleotides are aligned in a given nucleic acid 

 chain. If this alignment does not take place at random, prefer- 

 ential arrangements discovered in the various preparations may 

 furnish a means for their differentiation in addition to what total 

 analysis may reveal. We have attempted to achieve this in three 

 independent lines of investigation, based on the vertical or the 

 horizontal degradation of the nucleic acids (p. 338 of Ref. 13) or 

 on both. 



D 



A n p A A ■ A 



Fig. 8. Schematic representation of a DNA segment. The purines are 



represented by black, the pyrimidines by white, symbols. There is an equal 



number of black and white squares and of black and white triangles. The 



dissymmetry ratio of squares to triangles is 1.5. 



