120 FIRST STEPS TOWARDS A CHEMISTRY OF HEREDITY 



invariability within the species? This is apparently the case with 

 respect to the total deoxypentose nucleic acid of a species, and 

 also with respect to the total pentose nucleic acid of unicellular 

 organisms. Many examples can be found in the literature-^- ^^' ^^. 

 The two properties I have mentioned, namely, diversity in 

 composition with regard to different species and invariability of 

 composition within the same species, are not unique with the 

 nucleic acids; they are probably shared by all highly polymerized 

 cell constituents, although it may be possible to distinguish 

 between a direct, gene-controlled specificity (nucleic acids, 

 proteins) and a reflected specificity (polysaccharides, lipids) 

 which latter is mediated through specific enzymes that are them- 

 selves subject to control by the genes. But the nucleic acids 

 show, in addition, a third, most unusual characteristic which 

 distinguishes them from other cell-specific high polymers. I am 

 referring to the well-known compositional regularities^ ^ i^. These 

 are, in the case of deoxyribonucleic acids: (a) the molar sum of 

 the purine nucleotides equals that of the pyrimidine nucleotides; 



(b) the molar proportion of adenine equals that of thymine; 



(c) the molar proportion of guanine equals that of cytosine 

 (+ methylcytosine); (d) there is the same number of 6-amino 

 nucleotides (adenylic, cytidylic acids, etc.), as of 6-keto nucleo- 

 tides (guanylic, thymidylic acids). For most of the ribonucleic 

 acids only the last regularity holds: the molar sum of adenylic 

 and cytidylic acids equals that of guanylic and uridylic acids. 



This remarkable balance between the several nucleic acid 

 constituents represents a type of equipoise that I do not believe has 

 ever been encountered in other mixed polymers that do not contain 

 simple repeating units. That some of the principal beneficiaries 

 of these discoveries have acknowledged them with great reluctance 

 is a phenomenon not unusual in the history of the sciences. 



One of the greatest discoveries in biology that I have had the 

 good fortune to witness, namely, the recognition by Avery* and 



* In the two recent textbooks of genetics that I have among my books 

 the name of Avery does not appear. "Habent sua fata" not only the 

 "libelir but also what they leave out. 



