BIOSYNTHESIS OF PROTEINS 



281 



is brought about by hydrogen bonds and the bases He in 

 planes perpendicular to the long axis of the molecule. 



The three-dimensional structure of RNA is still not com- 

 pletely clear ; maybe its molecules have branched chains. 

 A structure of this kind would allow 

 of an unlimited number of isomers, 

 the individual characteristics of which 

 would be determined by the relative 

 arrangement of the nucleotides in the 

 chain/" It is clear that these possi- 

 bilities must be very widely realised 

 in the world of living things and, in 

 fact, we find there a tremendous 

 variety of nucleic acids with specific 

 structures, just as we do with 

 proteins 



Fig. 25. Structural model of the 

 macromolecule of desoxyribonucleic 

 acid. Two spiral chains of desoxy- 

 ribose ; the horizontal lines represent 

 pairs of nitrogenous bases uniting the 

 chains by means of hydrogen bonds 

 (after Watson and Crick). 



There is a whole series of experimental data demonstrating 

 the species specificity of nucleic acids and showing that the 

 DNA and apparently also the RNA of different species have 

 different over-all compositions.^^* Furthermore, w^e may 

 speak of organ specificity or tissue specificity which means 

 that different organs within the same organism have different 

 nucleic acids, and, finally, there is organelle specificity of 

 nucleic acids. This applies specially to RNA, on account of 

 its localisation in the various formed elements of protoplasm. 

 In particular, much evidence has been given in the literature 

 showing that the RNA of the nucleus and that of the cyto- 

 plasmic granules are different from one another."^ 



The view is widely maintained in contemporary scientific 

 literature that the molecule of nucleic acid with its specific 



