29. DEOXYRIBONUCLEIC ACIDS AS MACROMOLECULES 37 



molecules. If one postulates that there is neither branching (Fig. 46) nor 

 formation of bundles (Fig. 4c), a zigzag line (Fig. 4a) makes it very reason- 

 able to assume that at the bending points, A, there is a kind of dislocation or 

 heterogeneity of the double helix permitting an abrupt bending. It is then 

 necessary to imagine that at such points the local structure is different. It is 

 appealing to assume that at these points the chain is particularly easily 

 broken by physical or chemical agents. In this case, the effect of ultrasonic 

 or X-ray irradiation as observed, respectively, by Doty 2 and Butler, 24 would 

 be to cut the zigzag into rod-shaped AA segments (those segments between 

 two successive points, A) of a length, as we have seen, larger than 300 A. 

 (molecular weight 60,000). 



It could be suggested that these weak points, where the chain is able to 

 bend, correspond to segments of a single strand. Such a structure has al- 

 ready been proposed for other reasons by Dekker and Schachman but 

 with a number of weak points per molecule which is too large to be com- 

 patible with the necessary minimum length (300 A.) of each double-helix 

 segment. Another model, with ladder-shaped zones has been suggested by 

 Ambrose (Fig. 196). 



However, it has to be kept in mind that such considerations are strictly 

 speculative, and that so far no direct experiment allows us to say that they 

 are more than assumptions which may be useful in stimulating new investi- 

 gations. 



(e) It is evident that owing to the many uncertainties in our conclusions 

 there is no possibility of knowing with precision whether or not the charac- 

 ters of DNA particles depend on the organs or the animal species from which 

 they have been extracted. At present, no significant differences can be seen 

 between different species such as bacteria, fishes, birds, or mammalians. If 

 this point were proved, this could perhaps lead to some interesting conclu- 

 sions. 19 For this reason, and for many others, it may be believed that, de- 

 spite the meager results obtained till now regarding the determination of the 

 morphological characters of DNA particles in solution, it is worthwhile not 

 to stop efforts which probably will, in the near future, give valuable results. 



Acknowledgment 



I want to thank J. Pouyet for having kindly helped me in the redaction of this 

 paper. 



29 P. Doty, B. B. McGill, and S. A. Rice, Proc. Natl. Acad. Set. U. S. 44, 432 (1958). 



30 C. A. Dekker and H. K. Schachman, Proc. Natl. Acad. Set. 40, 894 (1954). 



