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is that clays could not only adsorb and catalyze reactions between 

 organic molecules but that they could, like DNA, replicate. If we 

 now suppose that, as in the case of DNA, the possibility exists of an 

 error of replication or mutation, the replicating clays would evolve! 

 At present, however, the self-replicating systems we know best involve 

 molecules that resemble proteins and nucleic acids. The remainder of 

 this section, therefore, is concerned with studies of these molecules. 



The Contemporary System 



A genetic apparatus is an essential requirement for all living 

 things on Earth. It is by means of the genetic material that living 

 organisms are able to store, express, and upon reproduction, transmit 

 to their progeny the information for all of the capabilities which 

 they possess. Cellular life forms usually store genetic information in 

 double-strand DNA polymers (fig. V-3), though some viruses make 

 use of RNA instead. As mentioned earlier, the information is coded 

 in the sequence of the nucleotides in such a way that each of the 

 64 possible trinucleotides codes for one of the 20 acids to be incor- 

 porated into a protein (table V-l), or codes for a stop signal to 

 terminate protein synthesis. The two strands of the nucleic acid are 

 held together by relatively weak (hydrogen) bonds, made specific by 

 a unique and essential feature of the conformation of the four 

 nucleotides — adenylic acid hydrogen-bonds specifically with thymi- 

 dylic acid, while guanylic acid pairs only with that of cytidylic acid 

 (fig. V-4). These interactions are referred to as the Watson-Crick 

 pairing rules. Partly because of these unique pairing specificities, 

 either strand can serve as a template for the synthesis of the other, 

 given free energy, in the enzyme-catalyzed process known as replica- 

 tion. In this fashion, one double strand can yield two new double- 

 stranded molecules, one for each of the progeny after cell division. 



The expression of genetic information requires that the infor- 

 mation in the DNA be converted into protein. This is accomplished 

 in two steps. First, in a process called transcription, one strand is 

 copied by an enzyme, yielding a complementary strand of RNA 

 (messenger RNA). This new strand then serves as the template for 

 the synthesis of protein. The process of protein synthesis is termed 

 translation because the nucleic acid "language" is now translated into 



