98 



Proposed translation models— Although most of the models 

 that have been suggested for the first translation apparatus closely 

 resemble the contemporary process of reading a linear message with 

 triplet adapters, a number of other models have to be kept in mind 

 as possibilities. Examples include systems involving adapters reading 

 a single base rather than triplets, mechanisms with direct bonding of 

 amino acids to polynucleotide templates, and various feedback 

 linked systems. These tend to suffer from the problem of explaining 

 how the contemporary genetic system could have evolved from such 

 a different mechanism. On the other hand, some of these alternative 

 models tend to be more easily evaluated experimentally. For 

 instance, recent experiments have demonstrated that amino acids 

 attached to single nucleotides give an enhanced yield of peptide 

 formation when the nucleotides are lined up on a complementary 

 strand of polymer. There clearly remain many other novel mecha- 

 nisms to be discovered and suggested. 



It must be kept in mind that the problem may not yet be solva- 

 ble because some vital information may not have been discovered. 

 Additional knowledge of the three-dimensional structure of ribo- 

 somes and transfer RNA is beginning to shed more light on how the 

 complex translation apparatus operates in the contemporary cell. 

 Structural studies of nucleic acids can still yield some surprises. It 

 seems likely that increasingly detailed knowledge will suggest new 

 possibilities for prebiotic studies of the translation mechanisms. 



In spite of the caveats that we must acknowledge, the correla- 

 tions of properties and affinity data between amino acids and nucleo- 

 tides, while certainly leaving us far from final answers, at least sug- 

 gest that discernible patterns exist in the coding mechanism and give 

 hope that primitive translation can be elucidated when sufficient 

 data are available. 



MEMBRANES 



We can now ask what kinds of structures could coevolve with 

 a replicating system that might enhance the ability of the system to 

 incorporate the functions described above, and to evolve toward 

 the structure we accept as a living cell. A key component of all 



