GENETIC CONTROL 31 



diffraction data (Wilkins et al, 1953; Franklin and Gosling, 1953). The 

 rigidity and the shape of such a macromolecule fit with the hydrodynamic 

 properties of DNA solutions (Sadron, 1959). Denaturation is explained by 

 breakage of the purine-pyrimidine hydrogen bonds. 



Adenine H u -- — 'Q\ u~^J^ Thymine Guanine _.-H— N^ Cytosine 



N " W " C^H 0" \- M 





'Sugar\ N-'H 



/ 



H 

 Fig. 15. Hydrogen bonding of bases in DNA (Watson and Crick, 1953). 



An extremely interesting feature of the Watson-Crick model is that it 

 provides a ready made intuitive answer to the problem of replication of the 

 detailed arrangement of the nucleotides during duplication. The genetic 

 information is contained in each chain in two complementary sets of 

 symbols ; it can be visualized that during duplication the chains somehow 

 separate and that each serves as a template for building a copy of the other. 

 This remarkable hypothesis cannot be considered as established; it is at 

 least compatible with the experimental facts at the present time. 



The DNA macromolecule seems indeed admirably adapted to carrying 

 coded information. If the information resides in the sequence of the four 

 nucleotides, a linear chain of some ten thousand nucleotides can store an 

 enormous amount of information. The double strand system keeps the 

 information in a state which prepares it for duplication. DNA is chemically 

 stable at temperatures compatible with life and indeed more stable than 

 many essential cell constituents and it is metabolically inert. 



Part of the DNA only exists as double helices and part in a less orderly 

 structure. DNA fractionation indicates that a small fraction of DNA 

 might exist as single polynucleotide strands (Lucy and Butler, 1954; 

 Bendich et al, 1956). In bacteriophage X 174, DNA is not a double helix 

 but a single polynucleotide chain (Sinsheimer, 1959). Studies on this 

 particular system will probably help to clarify the duplication process, and 

 the mechanism of information transfer. 



5 . Structure of Virus RNA 



The chemical constitution of RNA is very similar to that of DNA. The 

 phosphate-carbohydrate backbone is identical to that of DNA, except for 

 the presence of a hydroxyl group instead of a hydrogen on the C2 of the 

 carbohydrate which is thus ribose instead of deoxyribose. The presence of 

 this hydroxyl group next to a phosphodiester makes the ribonucleic acid 

 backbone much more susceptible to acid and alkali hydrolysis than DNA. 



