THE HEREDITARY ORDER: GENETIC INFORMATION 



This structure is now firmly established. The evidence derives 

 from X-rav studies as well as from physical chemical data (titration 

 curves, light scattering, viscosity, sedimentation, kinetics of break- 

 down by gamma rays and enzymes). 



Thus desoxyribonucleic acid is a double helix. Each helical chain 

 is made up of a chain of desoxyribose phosphate with one base 

 attached to each sugar. 



Ribonucleic acid. DNA is the genetic material of all animals, 

 plants, microorganisms, and of some viruses. But in a few viruses 

 the genetic material is ribonucleic acid. Ribonucleic acid, or RNA, 

 has the same fundamental structure as DNA: it is a long chain of 

 nucleotides. But whereas the sugar in DNA is desoxyribose, it is 

 ribose in RNA. Moreover, the pyrimidine base thymine present in 

 DNA is, in RNA, replaced by uracil. Finally, whereas DNx\ is a 

 double helix, RNA is a single helix. 



The two nucleic acids, DNA and RNA, both carry genetic 

 information. The substrate of genetic information can only be a 

 feature shared by these two nucleic acids. The common singularity 

 is obvioush" the linear arrangement of nucleic bases. 



The chemical identification of the genetic material by Avery, 

 MacLeod, and McCarthy has been the great discovery of modern 

 biology. It was soon followed by another great discovery, that of 

 the molecular structure of the desoxyribonucleic acid, the Watson- 

 Crick double helix. Both discoveries catalyzed the extraordinary 

 development of genetics, cell physiology, cell biochemistry, and 

 virology, which have now merged into a new integrative discipline: 

 molecular biology. 



Figure 5. The Watson-Crick Model of the DNA Double Helix. 



The two chains run in opposite directions. The backbones are constituted by 

 desoxyribose molecules united by phosphodiester bonds. The nucleic bases of 

 one chain are united to the bases of the other chain by hydrogen bonds. 



[21] 



