The Chemical Basis of Heredity Determinants 323 



substances. A set of "genes" could be, for instance, a set of specific 

 reactions going on, or a specific distribution of independent molecules; 

 such suggestions were, indeed, made in the past. To dispose of these 

 notions one has only to turn to the simplest living entities, the crystal- 

 line viruses. Each of these consists of a single molecule of nucleopro- 

 tein; there are no reactions going on, no distribution of independent 

 molecules because only one is present. Since it is a matter of common 

 sense (or definition) that each living entity carries its own heredity, 

 one has to conclude that the only heredity determinants of these viruses 

 are indeed chemical substances: either nucleoproteins as such or their 

 components, nucleic acids * and/or proteins. One could argue that 

 perhaps a different principle is involved in higher organisms; however, 

 as will be shown below, the evidence there points in the same direction. 



The next more complex living entities studied in this respect are 

 the bacterial viruses. The viruses of Escherichia coli consist mainly, 

 but not exclusively, of nucleoprotein, about 40% of which may be 

 nucleic acid (here deoxyribonucleic acid or DNA). As shown by 

 Hershey and Chase, 1 upon infection practically only the DNA of the 

 virus reaches the inside of the host and is allowed to reproduce (deter- 

 mine) the new virus particle; the DNA, then, must be the only heredity 

 determinant of these species. 



In bacteria, the discovery of the transforming phenomenon (Grif- 

 fith -) and of the nature of the transforming principle (Avery, Mac- 

 Leod, and McCarty 3 ) made it clear that there, too, the DNA alone 

 is capable of acting as a heredity determinant; however, the question 

 as to whether the DNA is the sole or only one of many heredity deter- 

 minants still remains debatable. The subject of transforming phe- 

 nomena will be discussed in detail below. 



The situation in higher organisms is, naturally, mure complex, and 

 evidence of the above-mentioned degree of validity has not yet been 

 presented; however, there, too, all the existing evidence, though in- 

 direct, points to DNA as a heredity determinant. 



The spermatozoa, which carry the entire heredity of the male, may 

 contain over 90% of deoxyribonucleoprotein (referred to dry weight) ; 

 the high deoxyribonucleoprotein content of somatic chromosomes, un- 

 doubtedly concerned with the orderly transmission of heredity, is well 

 realized. But, although the composition and gross structure of DNA 

 seem to remain unchanged when the spermatozoon (and ovum) 



* The nucleic acid involved here is the ribonucleic acid (RNA) ; this RNA, 

 however, seems to consist of giant molecules resembling more, in this respect, 

 the cellular DNA than RNA. 



