440 p. MITCHELL 



polynucleotide chains in the double helix of deoxyribonucleic acid may occur 

 by the polymerization of the appropriate nucleotides on to either member of 

 the helix, the copying process being due to hydrogen bonding and close packing 

 of the nucleotides into the complementary polynucleotide chain. They also 

 suggested that this process may be the basis of genetic replication. If, as pro- 

 ponents of template hypotheses have suggested or implied [lo, ii], the order 

 of polymerization of amino acids to form structural and catalytically active protein 

 may be determined by the base pairs in the deoxyribonucleic acid chain, we would 

 seem to have the requirements for a system capable of exhibiting both the open- 

 system characteristics described by Bertalanffy [8] and the chemical memory 

 necessary to give the properties of stability to the individual and continuity to 

 the hneage. According to Dixon [12], 'For life to exist, we must have a certain 

 minimum number of enzymes linked together in a system which can both bring 

 about energy-yielding reactions and also transfer and use this energy for other 

 purposes'. I suggest that the minimum requirement for an organism capable of 

 stepwise evolution therefore consists essentially of a group of paired semi- 

 periodic solids which determine the order of polymerization in another group of 

 unpaired semiperiodic solids of different backbone structure which, by their 

 catalytic and carrier activities, make available the required concentrations of 

 the monomeric precursors of the genetic and catalytic polymers. Seeing that the 

 choice of molecular shapes and types of bonding is quite restricted I suggest 

 that the occurrence of proteins and nucleic acids as the main types of semi- 

 jjeriodic solids found in all living organisms that have been examined may be 

 governed by the fact that other types of materially and energetically inexpensive 

 semiperiodic substances do not display the complementariness required to give 

 efficient repUcation, The polypeptide and the polyribosephosphate or polyde- 

 oxyribosephosphate backbones are relatively simple and are composed of sub- 

 stances that may well have been amongst the first organic materials available. 



The minimum requirement for an organism capable of stepwise evolution 

 described above includes the system that makes available the required concen- 

 trations of the monomeric precursors of the genetic and catalytic polymers. In 

 this context, Haldane's assertion [3] that 'The critical event which may best be 

 called the origin of life was the enclosure of several different self-reproducing 

 polymers within a semipermeable membrane' is very relevant, for we have not 

 yet considered the spatial requirements of the above system. As Haldane has 

 implied, the genetic and catalytic machinery could not function unless the small- 

 molecular-weight substances which act as substrates and precursors were re- 

 tained within a membrane. It is not sufficient, however, to shut the chemical 

 machinery off from environmental pressures which supply the raw materials 

 and the environment by an inert membrane, for this would isolate the organism 

 from the environmental pressures which supply the raw materials and the free 

 energy required for the processes of synthesis. Moreover, the materials of the 

 membrane must be synthesized or accumulated from the environment, and 

 must be organized in a stable arrangement between the environment and the 

 inner aqueous medium of the organism. There are two questions that we must 

 consider: first, the mechanism of formation and stabilization of natural mem- 



