THE SUMMING UP 387 



stration of in vitro DNA synthesis by replication from pre-existing DNA 

 provides the chemical basis for the genetic autonomy of this class of 

 molecules. 



We have come to associate heredity with particular macromolecular 

 configurations. As a general definition, we have proposed that heredity 

 consists of the conservation of specificity during replication, and, in 

 this sense, a hereditary determinant must replicate with conservation of 

 specificity. The possibility that other chemical constituents besides DNA 

 carry genetic information has not been excluded. The genetic material 

 of some viruses is RNA, but the chemistry of RNA replication is not 

 known. If there are structural elements of the cell, such as membranes 

 or organelles, which contain genetic information, it may not be possible 

 to study them by classical genetic analysis or by transfer from cell to 

 cell; in that event, the demonstration that they replicate would provide 

 evidence of genetic autonomy. 



Thus, the stage is being reached at which chemical and biological 

 tests may be used not only to supplement each other but also as alter- 

 native methods for the demonstration of the same point. 



The identification of genes has depended upon their ability to mutate. 

 Mutant genes have provided the material for genetic analysis, and 

 studies of the mutation process have provided an avenue of approach to 

 the identification of genetic material. For example, the treatment of 

 RNA and DNA with nitrous acid in vitro, followed by biological assay 

 of the treated nucleic acids (by infection and transformation respec- 

 tively), has shown that the deamination of a single nitrogen base may 

 result in mutation. In attempts to decipher the genetic code, the known 

 specific action of particular mutagens will be of great value. 



What classes of hereditary factors have been overlooked because they 

 do not mutate? Nonchromosomal genes almost fall into this category. 

 As yet, no systematic method has been developed for the recovery of 

 nonchromosomal mutants, although the use of acridine dyes and strep- 

 tomycin appears promising. All sorts of phenotypic traits have been 

 found to be under genetic control, but the methodology of genetics 

 provides no assurance that most of the determinants of cell heredity 

 have been recognized. 



The great advances of the last few years make possible the rephrasing 

 of problems open to experimental attack in this area of genetics: the 

 detailed analysis of gene chemistry and of the mutation process, the sys- 

 tematic search for new classes of hereditary determinants, and the appli- 

 cation of mutation analysis to the study of gene replication. For bio- 

 chemists there is the further challenge to discover replicating molecules 

 other than DNA. 



