BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 437 



Another cytochemical correlation, different in its nature, provides 

 supporting evidence. In 1942, Mirsky and Pollister- identified the fibrous 

 protein extractable from tissues with strong salt solutions as DNA-protein, 

 and showed that its extraction from cytological preparations occasioned 

 the loss of definition and stainability of the nuclear material. 



Cytochemical evidence then, relative to DNA function, has taught us 

 that DNA is a characteristic constitutent of the chromosomes and all of 

 the cellular DNA is probably associated with these bodies. 



h. Localization of Genetic Determinants within the Cell 



In another boundary science, parallels were being drawn; in this case 

 between cytological phenomena and genetic processes. Even for the early 

 observers, the behavior of the sperm nucleus in fertilization of the egg was 

 a reasonable indication that the nucleus was probably the essential con- 

 tribution from the sperm and therefore the source of the inheritable factors 

 of the paternal line. The properties of denucleated cells or parthogenetically 

 induced ova were sufficiently in keeping with this conception to support it. 



Of course, it is not reasonable to assume, a priori, that the relatively 

 easily observable chromatin bodies are more important than some invisi- 

 ble elements in the functions of the nucleus. It was not until the classic 

 papers of Morgan and his co-workers^'' pointed out the impressive paral- 

 lels between the organized chromosome movements and the movements of 

 the genetic determinants deduced from Mendelian genetics, that the 

 chromosome was finally accepted as a "gene carrier." Among the most 

 significant parallels in this demonstration were: (1) the number of homol- 

 ogous chromosomes carried was the same as the number of genetic units 

 or groups, especially with regard to the sex-linked groups; (2) correspon- 

 dence in the cycles of reduction, duplication, or pairing of these respective 

 cytological and genetic entities in the reproduction of various species; 

 and (3) the genetic effects of the chromosome redistribution when a pair 

 failed to divide (nondisjunction).^ Subsequently, other important correla- 

 tions have been shown, particularly the genetic results accompanying 

 translocations of chromosome fragments or breakage of the chromosomes 

 either by X-rays^ or by occasional spontaneous processes. It is clear that 

 the Mendelian genes are associated in some manner with the chromosomes 

 or equivalent structures. 



Cytochemical data, then, has located DNA in the same cellular struc- 



^ A. E. Mirsky and A. W. Pollister, Proc. Natl. Acad. Set. U.S. 28, 344 (1942). 

 3 T. H. Morgan, J. Exptl. Zool. 11, 365 (1911). 

 * A. H. Sturtevant, J. Exptl. Zool. 14, 43 (1913). 

 6 C. B. Bridges, Genetics 1, 1, 107 (1916). 

 « H. J. MuUer, Science 66, 84 (1927). 



