96 



It does not explain why only D- instead of L-nucleic acids are impor- 

 tant in biology. 



Proteins and nucleic acids together- Since neither proteins 

 alone nor nucleic acids alone seem likely to be able to account for all 

 of the genetic properties needed for self-replication, the alternative is 

 to consider the development of a combined system of both proteins 

 and nucleic acids. This requires a coupling between the two kinds of 

 molecules in the form of at least a primitive genetic code. 



While speculations as to how genetic coding might operate even 

 predated elucidation of the structure of DNA in 1953 by Watson and 

 Crick, an understanding of the essential nature of the genetic mate- 

 rial greatly stimulated the desire to understand how it is expressed. 

 As information accumulated in the 1950s and 1960s about the 

 molecular mechanisms of transcription, translation, and the coding 

 process, another question began to emerge. Why, for example, is 

 UUU a code for phenylalanine? There has perhaps been more specu- 

 lation about the basis for the origin of the genetic code than any 

 other aspect of molecular biology, and to enumerate and discuss 

 them all is impossible in a short review. The theories fall into two 

 groups: (a) Were the genetic code assignments based on some rela- 

 tionship (perhaps affinities) between amino acids and nucleotides, or 

 (b) were they the result of random processes? The idea that the code 

 is based on chance evolutionary processes, implies that we are not 

 likely ever to understand the basis for the origin, so that experimen- 

 talists have necessarily been concerned with the first theory. Evi- 

 dence for a physicochemical basis for the code, however, has not 

 been abundant. Experimentalists have shown that mononucleotides 

 have differential affinities for polybasic amino acids, but these affini- 

 ties relate more to the self-associative properties of the mononucleo- 

 tides than to code-related specificities. However, recent work has 

 shown a preferential affinity of certain polyamino acids, polylysine 

 for A-T rich DNA, and polyarginine for G-C rich DNA. In a similar 

 fashion, it has been shown that the aromatic amino acids (trypto- 

 phan, phenylalanine, and histidine) have different affinities for poly- 

 adenylic acid. This information was important in showing that, at the 

 monomer level, selectivities do exist. 



Investigations have demonstrated that there are variations in 

 affinities of nucleotides for amino acids affixed to a column material, 



