336 Essays in Biochemistry 



type or the genotype of the cells ; 32 for on a medium free of 5-bromo- 

 uracil or 5-iodouracil, the substitution is reversed. Thus it appears 

 that certain drastic changes in the DNA molecule may be without 

 consequences, but, on the other hand, the maintenance of the original 

 pattern of electrical changes or H-bonds may be essential for un- 

 changed activity. As a working hypothesis one could even postulate 

 that a mutation is a heritable change in such a pattern of electrical 

 charges. 34 



DNA with bases deaminated or partially absent has never been 

 found in nature. The change of pattern must therefore occur through 

 some other mechanism such as change of sequence or proportion of 

 nucleotides or change of length of the DNA molecule. At present any 

 such process can be visualized only as a rare fault occurring during 

 DNA reproduction. Such a process, then, might be the chemical basis 

 of mutation. 



Unstable DNA 



The premise that mutation occurs only during DNA reproduction 

 might appear inconsistent with the phenomenon of delayed mutations 

 since in certain cases mutations have been found to occur long after 

 the originally working mutagenic agent had been removed. It has 

 been suggested 35 that in such cases, or perhaps in every case, the role 

 of the mutagenic agent is to bring the gene into an unstable state 

 from which it can either return to the previous stable state or change 

 into a stable mutant gene. This secondary change might no longer 

 require the presence of the mutagenic agent. 



It is of interest to note that the process of unstabilization of heredity 

 determinants can also be demonstrated on the transforming principle 

 in vitro. When the transforming principle of H. influenzae is subjected 

 in vitro to the sublethal action of heat, H+ ion, deoxyribonuclease, 

 ultraviolet irradiation, or nitrogen mustard, the surviving (active) 

 molecules become very unstable to heat or even to storage in the cold, 

 under conditions entirely harmless for intact molecules. 36 Study of 

 the kinetics of inactivation of the transforming principle by heat indi- 

 cated that at least two reactions are involved: the unstabilization and 

 the actual inactivation. 



When the transforming principle, made very unstable by heat treat- 

 ment or by mustard treatment, was used for the transformation experi- 

 ment, it was found to reproduce as completely stable. Thus, the injury 

 to DNA (unstabilization) induced in vitro was not retained on repro- 

 duction; the change was therefore not a "mutation in vitro." However, 



