MACROMOLECULES AND NATURAL SELECTION 7 



to interrupt the regular fold. This fact, and the existence of 20 types 

 of side-chain rather than only four, allows the polypeptide chain to 

 take up an enormous variety of different folds, depending at least in 

 part on the amino-acid sequence. (Whether the fold depends on any- 

 thing else remains to be seen.) Thus upon a rather simple chemical 

 ground plan it is possible to build many different well-defined struc- 

 tures, and it is this that gives the proteins, as a class, their enormous 

 versatility. 



Can nucleic acid take up defined but complicated folds? Unfortu- 

 nately we do not know, though we suspect not. What we do know is 

 that the functions of nucleic acid appear rather limited (though this 

 may, of course, merely reflect our ignorance), whereas the functions of 

 proteins are extremely various and very delicately adjusted to any 

 particular job. When we know just how DNA and RNA control the 

 synthesis of proteins, as we believe they do, we shall be in a better 

 position to assess the limitations imposed by the folding of polynucleo- 

 tides. 



It is easy to see, then, that with nucleic acid alone or with protein 

 alone we would have great difficulty in meeting efficiently the demands 

 of natural selection. With protein alone, the replication mechanism 

 would have to be more complicated, since we cannot form a two-chain 

 complementary structure— at least not, as far as we know, with the 

 present side-chains. With nucleic acid alone, we could probably repli- 

 cate quite nicely, but it would be difficult to use the nucleic acids as the 

 basis for a vast family of enzymes. Life as we know it appears to be a 

 symbiosis between these two very different families of polymers. Each 

 —protein and nucleic acid— contributes its own particular capabilities, 

 and the kinds of ways they can fold turn out to be a rather significant 

 part of these capabilities. 



When our knowledge of the biosynthesis and functions of these 

 polymers is much more complete, and when we know the manner in 

 which the nucleic acids and the proteins are linked symbiotically ( the 

 problems of protein synthesis, "coding," etc. ) , it is reasonable to expect 

 that other key features of biology will be explainable by basic molecu- 

 lar properties. For example, I have said nothing about the necessity 

 for "particulate" rather than "blending" inheritance. Again, it seems 

 highly likely that the general absence of the inheritance of acquired 

 characteristics can be explained by the irreccrsibility of RXA and 

 protein synthesis, produced by the large flow of free energy into the 

 process needed to minimize mistakes. At bottom, it is the simplicity 

 and universality of the basic operations of biochemistr>' that encourage 

 one to look for the explanation of the general features of biology in 

 simple molecular properties. 



