BIOLOGICAL ORDER 



In order to answer our question, an experiment is needed. It 

 consists of labeling the DNA of the parent organism and following 

 the distribution of the label in the offspring. The first experiment 

 of this kind was done by Cyrus Levinthal in 1956. Bacteriophage was 

 selected as the material. Its DNA was labeled with radioactive 

 phosphorus, and the concentration of hot phosphorus was measured 

 in the daughter bacteriophage. The conclusion was that DNA re- 

 produces by the semiconservative mechanism, that is, in agreement 

 with the original Watson-Crick hypotheses. A number of experi- 

 ments perform.ed since on various materials seem to agree with this 

 conclusion. 



Production of an RNA template froiM DNA. Three mecha- 

 nisms have been considered. 



1. The single-stranded RNA is formed within the deep groove 

 of the DNA double helix. The specific sequence of the RNA bases 

 is governed by the base pairs of the DNA duplex. Each of the bases 

 of the base pair of the duplex could, according to Stent, form an- 

 other pair of hydrogen bonds with a third base. If the bond between 

 the C of the sugar and the N of the base is to be always in the 

 same position, a given pair of the duplex bases would combine with 

 only one base of the future RNA. 



2. Each of the two strands of the DNA duplex forms an RNA 

 chain. As the two chains of the double DNA helix are complemen- 

 tary, the two RNA chains would be different. One could of course 

 be active, the other inactive. 



3. Only one of the strands of the DNA duplex produces an RNA 

 chain, the other being inactive. 



The problem of the mechanism by which the DNA produces the 

 RNA template for the synthesis of proteins, and more generally 

 an RNA molecule, is not yet solved. 



Reproduction of RNA. The same type of problem is posed 

 by RNA, which is the genetic material of a number of animal 

 viruses and of plant viruses. Here again, an enzyme has been dis- 

 covered which binds ribonucleotides and organizes them in long 

 chains. RNA is single-stranded. The formation of a complementary 

 strand by the template mechanism would end in two chains with 

 qualitatively different genetic information. But it could well be that 



[32] 



