The Protein Text 91 



Normally the number of 6-keto (Gu + Ur) and 6-amino (Ad + Cy) groups in 

 RNA is equal (89). Virus RNA does not necessarily obey this rule, indicating 

 that it differs in this respect, at least, from all the others (Table VII). 



This hypothetical scheme is presented to show that the apparent contradic- 

 tions of the genetic and biochemical evidence do not make it logically necessary 

 to abandon a unitary view of RNA reproduction. 



The coding of protein information into RNA has attracted considerable 

 attention, but cannot as yet be considered as solved. Study of the protein text 

 indicates that any solution will have to meet several requirements. 



Firstly, since exactly twenty amino acids are incorporated into protein, 

 it is clear that at least three nucleotides are needed to determine an amino acid. 

 Gamow (134) has proposed that 20 is a 'magic' number, which is the result of 

 the existence of twenty possible sites of three nucleotides each. Four kinds of 

 items, taken three at a time, give twenty different combinations, if order is 

 disregarded. 



Crick, Griffith and Orgel (135) point out, however, that there is at least 

 one other way of deriving a 'magic' 20 number. They start by considering the 

 problem of what it is that delimits one amino acid-determining site from 

 another, the 'punctuation mark problem'. Assuming that three bases determine 

 a site, it is a problem why the 3n + U 3n + 2, 3« + 3 bases represent a site, 

 while 3« + 2, 3n + 3, 3« + 4 do not. They solve this problem by assuming that 

 only certain triplets of nucleotides correspond to an amino acid (sense sites), 

 while others do not (non-sense sites). The criterion separating these two types 

 of sites is the following. The set of sense sites are all triplets which, when 

 placed next to each other in any possible combination, give sense sites only 

 at positions 3/z + 1, 2?i + 2, 3n -j- 3, but not otherwise. For example, the triplet 

 AAA is a non-sense site, since when placed next to itself it gives the sequence 

 AAAAAA. The site is not unambiguously defined, as AAA occurs both at 

 the 1-3 position and at the 2-4 position. They find that there are exactly 

 twenty triplets (out of sixty-four) which satisfy the criterion of sense sites, as 

 follows : 



Other ways of selecting twenty sense sites are also possible. The sense sites, 

 these authors suggest, may correspond to amino acid-selecting sites of RNA. 



The 'punctuation mark problem' could, of course, also be solved if amino 

 acids were selected in a sequential manner starting from one end of the template. 



Secondly, besides the requirement that at least three nucleotides are required 

 to determine an amino acid site, the study of proteins indicates that these amino 

 acid determining sites are independent and share no nucleotides with their 

 neighbors. This conclusion follows from the absence of any intersymbol 

 correlations in the protein text, and also from the fact that a mutation (as 

 inferred from a study of homologous proteins) can result in a change at one 

 site only, leaving the rest of the sequence unchanged. The number of nucleotides 



