148 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



is conceivable in a living system capable of orderly replication, 

 at any rate on the level of macromolecular events, is a question 

 which it would be out of place to discuss here. The entire com- 

 plex of randomness in biology would certainly deserve a more 

 profound examination than I am equipped to perform. In any 

 event, the conclusion that in no case investigated the pattern of 

 nucleotide distribution was that to be expected from a random 

 arrangement of constituents-^* ^^' ^^' ^^ may contribute to the as- 

 surance that in studying the nucleotide sequence in a deoxy- 

 ribonucleic acid we are, indeed, dealing with some sort of mean- 

 ingful message. If the array of constituents in a biologically 

 functional polynucleotide has been compared to a giant throw 

 of dice, we must conclude that it is a throw of loaded dice. 



Another consequence of our studies on deoxyribonucleic 

 acids of animal and plant original' ^'^-^^ is the conclusion that at 

 least 60% of the pyrimidines occur as oligonucleotide tracts 

 containing three or more pyrimidines in a row; and a correspond- 

 ing statement must, owing to the equality relationship, apply also 

 to the purines. 



b. Possibilities of cluster analysis 



It is quite clear that the various soUtary nucleotides and the small 

 bunched oligonucleotides, such as doublets or triplets, will vary 

 in frequency, i.e., in relative quantity, according to the plan 

 governing the nucleotide sequence of the particular deoxy- 

 ribonucleic acid; but they are most unhkely to exhibit uniqueness 

 of quality and will, therefore, not be sufficient to define a se- 

 quence unambiguously. Uniqueness may, however, be expected, at 

 least occasionally, when very large clusters can be included in 

 the survey. We have made a beginning with regard to clusters of 

 pyrimidine nucleotides^^. When a deoxyribonucleic acid is cleaved 

 by acid under moderate conditions (0.1 M H2SO4, 35 min, 100°) 

 and the fragments are fractionated on DEAE-cellulose by means 

 of a hthium chloride gradient, the phosphorylated pyrimidine 

 ohgonucleotides can be separated in order of increasing chain 

 length (compare Fig. 13). Further separation within the different 



