[962 I. II. C. C RICK 



At the present tunc therefore, the genetic code appears to have the fol- 

 lowing general properties: 



(1) Most if not all codons consist of three (adjacent) bases. 



(2) Adjacent codons do not overlap. 



(3) The message is read m the correct groups of three by starting at some 

 fixed point. 



(4) The code sequence 111 the y^cne is co-linear with the amino acid se- 

 quence, the polypeptide chain being synthesized sequentially from the 

 ammo end. 



(5) In general more than one triplet codes each amino acid. 



(6) It is not certain that some triplets may not code more than one amino 

 acid, i.e. they may be ambiguous. 



(7) Triplets which code for the same amino acid are probably rather sim- 

 ilar. 



(S) It is not known whether there is any general rule which groups such 

 codons together, or whether the grouping is mainly the result of histor- 

 ical accident. 



(9) The number of triplets which do not code an amino acid is probably 

 small. 



(10) Certain codes proposed earlier, such as comma-less codes, two- or 

 three-letter codes, the combination code, and various transposible 

 codes arc all unlikely to be correct. 



(11) The code in different organisms is probably similar. It may be the same 

 in all organisms but this is not yet known. 



Finally one should add that in spite of the great complexity of protein syn- 

 thesis and in spite of the considerable technical difficulties in synthesizing 

 polynucleotides with defined sequences it is not unreasonable to hope that 

 all these points will be clarified in the near future, and that the genetic code 

 will be completely established on a sound experimental basis within a few 

 years. 



s-142 



