I C. L. SADRON 



rather special means pertaining to the field of macromolecular physical 

 chemistry. Combining the two types of data one can have in principle a 

 complete and clear description of the macromolecule. 



Of course, this distinction of two classes of parameters is somewhat 

 schematic and, in practice, the cut will not be sharp between them. In 

 general, the knowledge of data belonging to one group — the first one for 

 instance — is quite necessary to establish the data of the second one. 



The case of DNA will be a very clear example of this fact. According to 

 the situation that we have described, we shall in this paper, admit that a 

 certain number of facts concerning the composition and structure of the 

 DNA macromolecules are well established and we shall devote all our at- 

 tention to the second group of parameters, that is, weight, shape, and di- 

 mensions, that we shall call morphological parameters. 



Now it is clear that in a solution as it has been prepared by the biochem- 

 ist, we have no evidence that the macromolecules are all alike. Heteroge- 

 neity may exist for the two groups of parameters. Since we have chosen 

 to treat the second only, we shall have to consider that the solution we 

 have to deal with is in principle polydispersed relatively to the weight, 

 shape, and dimensions of the particles — or macromolecules — that it con- 

 tains. 



We then have to consider not only one molecular weight, one shape, and 

 one dimension, but we have to find out the respective laws of distribution 

 of these three parameters for the assembly of the macromolecules contained 

 in a given sample of DNA. This problem, which we have set forth in its 

 more general terms, is one of the most difficult in macromolecular physical 

 chemistry, and we shall see that we are still far from a complete and satis- 

 factory answer. 



II. A Brief Survey of the General Principles of the Determination 

 of Weight, Shape, and Dimensions of Large 

 Molecules in Solution 



Before coming to the special case of DNA it may be useful to give, in a 

 preliminary chapter, a short summary of the general methods which are 

 used in the determination of the morphological parameters of large mole- 

 cules in solution. 1 We shall as far as possible avoid any detail that does not 

 apply here, and we shall keep as close as possible to the information strictly 

 necessary for a through understanding of the results obtained in the case 

 of DNA. 



The methods of investigation can be usefully divided into two groups: 

 the so called "hydrodynamical" methods and the "optical" methods. 



1 For more details see, for example, C. Sadron, Prog, in Biophys. 3, 237 (1953). 



