MECHANISM OF RADIOBIOLOGICAL ACTION 337 



deed, in our specific case under discussion, most of the energy absorbed 

 by the cell must ultimately go into irrelevant processes, since the energy 

 passed on to only one or a few genes must be a very small fraction of the 

 total dissipated throughout the cell, even though we know, as noted 

 earlier, that this total energy has a small absolute value. 



It will be noticed that the relevant processes are visualized as in com- 

 petition with the irrelevant ones. They may also encounter two other 

 types of competition. The first is from reverse 'processes, as indicated for 

 imaginary processes C and M in Fig. 1. The second is from restitution 

 (not indicated in Fig. 1), by which is meant a change from any given 

 state back to an earlier one by any process (es) other than strict re- 

 versals of relevant processes. 



Among the various relevant states, I have introduced for convenience 

 the notion of a decisive state (n), by which is meant that, once this state 

 has been attained in any individual cell maintained under a given set of 

 conditions, the remaining relevant states and processes, including the 

 end effect w, are inevitable. In other words, after the decisive state, the 

 relevant processes have no competition from irrelevant processes, from 

 reverse processes, or from restitution. In view of the likelihood of tfiese 

 competitive processes, it may well be that in many radiobiological ac- 

 tions the decisive state is attained only with the end effect and is iden- 

 tical therewith. This, however, does not impair the value of the general 

 concept. 



For any radiobiological action, the decisive state in any individual 

 cell must consist of a critical number n (one or more) of decisive entities 

 (individual structures of molecular, or greater, size). Each of these de- 

 cisive entities must have a precursor, that is a molecule or group of mole- 

 cules which has a definite role in normal cell structure, activities, or 

 both.* In our special case, a decisive entity is an inactivated gene, and 

 its precursor is the normal gene. If the cell is haploid, n = 1; if the cell 

 is diploid and homozygous, n = 1 or 2, depending on whether or not one 

 gene can suffice for the formation of enough enzyme for normal cell* 

 operations. The process type M, by which a decisive entity is produced, 

 is termed a decisive process. To produce a single decisive entity, indi- 

 vidual processes of type M must occur m times (m > 1). In our special 

 case, M is the reaction of peroxide molecules with the relevant gene. If 



* It is evident that the molecules (or groups of molecules) involved in the prede- 

 cisive processes are not, in the non-irradiated cell, essential to the normal biological 

 role of the precursor or to the structure or function whose alteration constitutes the 

 end effect. The precursor is almost certainly a solute; it seems inconceivable, on 

 numerical grounds, that the decisive entities could be water molecules so altered as 

 to render them unfit for the normal biological roles of water. 



