29. DEOXYRIBONUCLEIC ACIDS AS MACROMOLECULES 15 



Then knowing the asymptote to the c/K curve, it is possible to calculate 

 the values of M» and of L/M. 



In the case of a large zigzag chain the slope of the asymptote is still 

 (h/ir)(Nb/M) and it is evident that its intercept with the axis of ordinates 

 is still negative. 



If it has been possible for the same solution to measure M n and M w , 

 respectively, from c/K at large and small values of 0, then the comparison 

 between M n and M w can give an estimation of the degree of polydispersity 

 of the solution. If it is found that M w = M„ , there is a great probability — 

 though no absolute certitude — that all the particles are identical in weight, 

 shape, and dimensions. 



III. DNA Particles in Dilute Solutions 



As we have already mentioned, the discussion and the interpretation of 

 the data collected from optical or hydrodynamical investigations on macro- 

 molecular solutions are helped to a great extent by all kinds of information 

 that one can obtain from other sources. As far as we are concerned, we shall 

 largely use the simplest notions that chemical analysis as well as X-ray 

 diffraction have made familiar. 



It is well known that DNA macromolecules are formed by chains of the 

 type 



B O B O B 



I I I II 



— S— O— P— O— S— O— P— 0— s — 



h h 



where S is the deoxyribose molecule and B one of the four bases adenine, 

 guanine, cytosine, and thymine. 



X-ray diffraction shows that fibers in the B form — that is to say prepared 

 with the highest degree of moisture — contain bundles of long and thin par- 

 ticles, each of them being formed by the association of two simple chain 

 molecules — or strands — each corresponding to the schematic formula that 

 we have given above. 



These two strands are stuck one along the other by means of H bonds 

 between the opposite bases in such a way that there exist only two kinds 

 of pairs: A-T or C-G. The two strands are wound around each other in the 

 familiar shape of the Watson-Crick double helix which once more will be 

 represented in Fig. 6. 



The cylinder on which the two helices are wound has a diameter of 20 A., 

 the distance between the A-T and C-G couples is 3.4 A. This means that the 

 molar weight of the double helix is about 20,400 gm. for a length of 100 A. 



Owing to this structure, the DNA particle should be rather stiff. Though 

 as everybody knows, one of the most urgent problems is to determine the 



