DEOXYPENTOSE NUCLEIC ACIDS 27 



lengths of ultraviolet light has yielded curves closely resembling, 

 but not entirely identical with, the UV absorption spectrum of 

 nucleic acids. This work, mainly due to Stadler and Uber, Hol- 

 laender and Emmons, and Knapp and Schreiber, has been 

 reviewed by Lea^^. 



A certain degree of constancy, within the same species, of the 

 DNA concentration per diploid nucleus, and of roughly one-half 

 this amount per haploid sperm nucleus, has been discovered by 

 Boivin and his collaborators*- ^^' ^^ and confirmed in other labo- 

 ratories^^- ^^. 



DNA is in its composition identical in different tissues of the 

 same species^^. Moreover, in the few cases where comparison 

 was possible, no chemical differences have been observed be- 

 tween the composition of the DNA from the sperm cells and 

 from differentiated tissues of the same species, in contrast to the 

 very different composition of nuclear proteins in such instances. 



2. DEOXYPENTOSE NUCLEIC ACIDS 



All DNA preparations that have been studied in detail have 



several features in common. They are asymmetrical molecules of 



high molecular weight (around 10^), yielding extremely viscous 



solutions. They appear to contain the same deoxy sugar, namely 



2-deoxyribose. They contain two purines, adenine and guanine, 



and two or three pyrimidines, viz. thymine and cytosine, and in 



several cases^^ also 5-methylcytosine. The deoxyribonucleotides 



released enzymically from calf thymus DNA appear to be the 

 5'-phosphates-i'^2^ 



The conclusions to which our work has led us have been sum- 

 marized recentlyi^' 2^, and I shall limit myself here only to the 

 main points. 



a. DNA is in its composition characteristic of the species from 

 which it is derived. This can in many, but not in all, cases be 

 demonstrated by determining the ratios in which the individual 

 purines and pyrimidines occur. There will, however, be very 

 many borderhne cases in which such differences in composition 



References p. 37 



