BIOLOGICAL ORDER 



which is the molecule, is made up of two firmly bound comple- 

 mentary parts. The molecule of DNA cannot grow and does not 

 grow. But by making use of complementarity it has solved the prob- 

 lem of molecular reproduction. 



By the template mechanism, a single strand of nucleic acid can 

 produce a molecule of protein. It can also produce a complementary 

 nucleic strand, which in turn can reproduce the original one. But 

 only a molecule made of two complementary structures can re- 

 produce directly its own structure. 



This is all right on paper, but how are things in the living system? 

 An enzyme has been isolated from a bacterium that polymerizes 

 desoxyribonucleotides. If these building blocks of DNA are added 

 in a test tube together with the enzyme, DNA is synthesized. The 

 necessary condition, however, is the presence of a DNA primer. 

 The original DNA, the primer, may be multiplied i?i vitro bv a 

 factor of 10. 



In DNA, as already mentioned, adenine is bound to thymine, 

 guanine to cytosine. The ratio adenine/thymine, like the ratio 

 guanine/cytosine, is equal to 1. But, in bacteria, the ratio 

 (adenine + thymine) /(guanine + cytosine) varies from 0.6 to 2.8 

 according to the species. 



What happens when these various types of DNA are taken as 

 primers? 



First: The ratio adenine/thymine and guanine/cytosine is always 1. 



Second: The synthesis does not take place if one of the bases is 

 missing. 



Third: Each base can be replaced by an analogue, provided its 

 hydrogen-bonding capacities are in agreement with the Watson- 

 Crick model. 



Fourth: The ratio (A -f- T)/(G + C) in the synthesized DNA is 

 the same as in the primer. If the "nonnatural" synthetic primer does 

 not contain guanine or cytosine, these bases are absent in the newly 

 formed DNA, despite being present in the test tube. 



Fifth: When the DNA primer is heated, the two strands separate, 

 and the rate of synthesis is markedly increased. 



Everything happens as if one of the strands would act as a 



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