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one important function of many catalysts). In what follows, we will 

 examine first the systems that contain only proteins, then those that 

 contain only nucleic acids, and finally the combined systems. 



Proteins first— The nonordered polymerization of amino 

 acids to form analogs of proteins (called peptides if the chains are 

 short) has been demonstrated in the laboratory under a wide variety 

 of experimental conditions. The energy needed for this reaction can 

 be provided in a number of ways, one of which is heat. Polyamino 

 acids (sometimes called proteinoids) have been produced by heating 

 mixtures of amino acids to about 180°C. These temperatures are 

 greater than the boiling point of water; however, in a few experi- 

 ments similar products were found at temperatures below the boiling 

 point of water, after very long periods. Clays also catalyze the reac- 

 when alternately wetted and dried. The formation of peptides on 

 clay is further catalyzed by a simple peptide, i.e., histidyl-histidine. 

 This suggests that the first enzyme-like molecules may have been 

 very simple peptides. 



Another approach is to provide the energy needed for synthesis 

 of peptide bonds through the use- of other energy-rich chemical 

 species called condensing agents. Still another approach is to use 

 amino acids which have been "activated" prior to reaction. These 

 experiments are convincing evidence that amino acids readily enter 

 into combinations with one another, but these experiments do not 

 address the problem of reproducible organization into specific 

 sequences which would participate in self-replication. 



The main difficulty with the "protein first" hypothesis is that 

 it does not seem possible to formulate a plausible scheme for protein 

 self-replication based on known properties of amino acids and pep- 

 tides. One type of proposal postulates a complementary pairing of 

 amino acids on two chains, analogous to base-pairing in nucleic acids. 

 Complementariness might depend on size, charge, hydrogen 

 bonding, or some combination of these properties. Such schemes 

 are certainly possible in principle, but there is no experimental evi- 

 dence for them. 



Another group of proposals suggests that a family of peptides 

 forms a cycle in which each member of the cycle catalyzes the 

 synthesis of one or more other members of the cycle. It has been 



