436 ROLLIN D. HOTCHKISS 



often explicitly, from relatively early times. It was almost inevitable that 

 chromatin should speculatively be assigned such a function as soon as the 

 processes in which it undergoes such dramatic change — mitosis, meiosis, 

 and fertilization — came to be considered as genetic ones. This proposition, 

 expressed during the last half century progressively more precisely and 

 significantly, is now supported by certain evidences of cause and effect 

 which point up the general importance of the supposed relationship. Let 

 us first consider the circumstantial evidence supplied by the study of 

 DNA (deoxypentosenucleic acid) distribution in tissues — a subject already 

 treated in Chapters 18 and 19 — then proceed to discuss the evidences of a 

 dynamic functional relationship of DNA to genetic processes. 



1. Organization of DNA in Genetic Elements of Tissues 



Three lines of investigation — chemical, cytological, and genetic — had 

 to merge to furnish the information we can now marshal to support the 

 genetic significance of DNA. Chemists for many years had been learning 

 the properties of this material as it was recovered from separated nuclei, 

 from sperm heads, and from a variety of tissues. On another side, its 

 histological analog, chromatin, was being recognized in situ in nuclei, and 

 the intricate behavior of the chromosomes at mitosis was being observed. 

 Meanwhile, too, the conception of the genes, units transmitting heritable 

 properties from cell to cell, was being developed. It was the integration at 

 the borderlines between the three fields of chemistry, cytology, and genetics 

 which in retrospect seems to have provided the greatest or most difficult 

 steps in the development which we are discussing. 



a. Localization of DNA within the Cell 



In 1922, Feulgen made the advance which permitted a correlation 

 between the chemical and cytological information which had accumulated.' 

 He had shown, ten years before, that the instability of DNA in warm 

 acid would result in the liberation of aldehyde groups giving color with 

 Schiff's reagent. This characteristic and specific reaction, demonstrable 

 with isolated purified material, furnished precision to cytological studies. 

 The acidic components of the chromatin, which bind basic dyes, could 

 now be identified as DNA nucleoproteins, rather than PNA (ribonucleic 

 acid, pentosenucleic acid), phosphoprotein, etc. Not only is DNA responsi- 

 ble for the characteristic chromogenicity of chromatin, but subsequent 

 work, with the histological, enzymic, and photometric methods since 

 developed, has revealed that virtually all of the DNA of the cell is lo- 

 calized in the nuclear chromatin. These investigations have been covered 

 in Chapters 17, 18, and 19. 



' R. Feulgen and H. Rossenbeck, Z. physiol. Chem. 135, 203 (1924). 



