CONCLUDING REMARKS 21 



add, but they also take away. Keeping in mind all these reser- 

 vations, we arrive at the following conclusions. The deoxy- 

 pentose nucleic acids from animal and microbial cells contain 

 varying proportions of the same four nitrogenous constituents, 

 namely, adenine, guanine, cytosine, thymine. Their composition 

 appears to be characteristic of the species, but not of the tissue, 

 from which they are derived. The presumption, therefore, is that 

 there exists an enormous number of structurally different nucleic 

 acids; a number, certainly much larger than the analytical 

 methods available to us at present can reveal. 



It cannot yet be decided, whether what we call the deoxy- 

 pentose nucleic acid of a given species is one chemical individual, 

 representative of the species as a whole, or whether it consists of 

 a mixture of closely related substances, in which case the con- 

 stancy of its composition merely is a statistical expression of the 

 unchanged state of the cell. The latter may be the case if, as 

 appears probable, the highly polymerized deoxypentose nucleic 

 acids form an essential part of the hereditary processes; but it 

 will be understood from what I said at the beginning that a 

 decision as to the identity of natural high polymers often still is 

 beyond the means at our disposal. This will be particularly true 

 of substances that differ from each other only in the sequence, 

 not in the proportion, of their constituents. The number of pos- 

 sible nucleic acids having the same analytical composition is 

 truly enormous. For example, the number of combinations ex- 

 hibiting the same molar proportions of individual purines and 

 pyrimidines as the deoxyribonucleic acid of the ox is more than 

 10^^, if the nucleic acid is assumed to consist of only 100 

 nucleotides; if it consists of 2,500 nucleotides, which probably 

 is much nearer the truth, then the number of possible "isomers" 

 is not far from lO^^^^^. 



Moreover, deoxypentose nucleic acids from different species 

 differ in their chemical composition, as I have shown before; and 

 I think there will be no objection to the statement that, as far 

 as chemical possibilities go, they could very well serve as one of 

 the agents, or possibly as the agent, concerned with the transmis- 



References p. 23 



