80 Information Storage and Neural Control 



adenine (A) going with thymine (T) and guanine (G) going with 

 cytosine (C). The phosphate groups of the DNA chains are 

 accessible to hydrogen or hydroxyl ions and to dyes and are, 

 therefore, on the outside, whereas the bases occur opposite one 

 another on the inside. From x-ray diffraction studies, it has been 

 deduced that there is a succession of flat nucleotides spaced 

 3.36 A apart and standing out perpendicular to the fiber axis. 

 The structure is relatively rigid and serves as a template for either 

 its own replication or for the replication of "Informational" RNA. 

 Plausible mechanisms for DNA replication and for spontaneous 

 mutations were embodied in the proposals of Watson and Crick 

 (77). These mechanisms are strongly supported by a large number 

 of experiments. According to the proposal for DNA replication, 

 a twin stranded DNA molecule partially unwinds, and each base 

 attracts a complementary free nucleotide already available for 

 polymerization within the cell. These free nucleotides, whose 

 phosphate groups already possess the free energy necessary for 

 polyesterification, then link up with one another, after being held 

 in place by the parental template chains, to form a new poly- 

 nucleotide molecule of the required nucleotide sequence. Thus, 

 each DNA strand serves as a template for the synthesis of a com- 

 plementary strand. The replication process can be schematically 

 represented as follows: 



-A-C-T-G-->-A-C-T-G- 



-A-c-T-G-/* : : : : 



'••• -T-G-A-C- 



-T-G-A-C-\ 



-T-G-A-C-->-T-G-A-C- 

 Parental DNA '.'.'. 



Duplex DNA Chains .... 



After Unwinding -A-C-T-G- 



New DNA 

 Duplexes 



It is a corollary of the Watson-Crick hypothesis that a change of 

 one or a few nucleotides in the DNA sequence will be mutagenic. 

 Mechanisms for spontaneous mutation and for experimentally 



