SUMMARIZING REMARKS 151 



meaning only in reference to the possible mechanism of the 

 biosynthesis of the polynucleotide chain or to the existence of 

 such principles of pleromerism as those mentioned before. In 

 other respects, in polymer chains that are not constructed haphaz- 

 ardly, each monomeric constituent must have its preordained 

 place; a place that it must have entered, at the time of poly- 

 merization, through some form of a specific selection mechanism. 

 A replication mechanism of this nature is, indeed, offered by the 

 scheme^- postulating a specific pairing of a 6-aminopurine 

 (adenine) with a 6-ketopyrimidine (thymine) and of a 6-keto- 

 purine (guanine) with a 6-aminopyrimidine (cytosine). 



This postulate and the fact that 5-methylcytosine is pleromeric 

 with cytosine could have led to the expectation that these two 

 pyrimidines could replace each other at random in the deoxy- 

 ribonucleic acid chain, selection having regard only to the 

 presence of a 6-amino group in the pyrimidine. The amazingly 

 constant quantity of methylcytosine incorporated into the deoxy- 

 ribonucleic acids of wheat and rye germ (Table 30, and compare 

 also the tabulation given in a previous paper^^) should, however, 

 render such a conclusion most questionable. In what manner 

 could a guanine residue in the preformed chain exercise a 

 preference for pairing with cytosine, rather than with methyl- 

 cytosine, in the newly forming counterpart chain? There are 

 surely limits to the amount of foresight that we can attribute to 

 an ostensibly automatic process. 



Another observation, made repeatedly^- ^^' ^^, has to do with 

 the trend of the distribution of 5-methylcytosine in consecutive 

 fractions obtained by the fractionation of deoxyribonucleic acid 

 from calf thymus, wheat germ, and rye germ. In all instances, the 

 relative abundance of 5-methylcytosine with respect to cytosine 

 varied in such a manner that the percentage contribution of the 

 minor pyrimidine was higher in the earher fractions. In no case 

 could one have claimed that the two 6-aminopyrimidines replaced 

 each other at random within the polynucleotide chain. I have 

 discussed, in more detail, the significance of the finding of a 

 disproportionate distribution of cytosine and 5-methylcytosine in 



References p. 159 



