158 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



terminal sequence adenine-cytosine-cytosine is required; a se- 

 quence that can apparently be enforced by a specific enzymic 

 mechanism. 



The manner in which a growing deoxypolynucleotide chain 

 could guarantee the quality of its continuation is not yet a topic 

 encouraging speculation*. Our present ideas about the enzymes 

 taking part in the biosynthesis of specific polymers wiU probably 

 have to undergo a stringent revision before the directed synthesis 

 of comphcated sequences will begin to be understood. For this, 

 more than a biological form of Scotch tape is required. Certain 

 sequential restrictions could, incidentally, be explained if, under 

 circumstances, runs of more than one nucleotide were the im- 

 mediate precursors of the polymer chain. We have, for instance, 

 discussed the evidence, in rye germ nucleic acid, of the two 

 dinucleotides cytosine-guanine and methylcytosine-guanine being 

 able to substitute each other at random^''^. 



In considering the problem of the nucleotide sequence in 

 deoxyribonucleic acids we have barely turned the comer. There 

 is a long road before us; and we shall not see its end. 



* "Whereof one cannot speak, thereof one must be silent." Wittgenstein, 

 Tractatus Logico-Philosophicus. 



