DIFFUSION MODEL 367 



intermediate products of ionization are much like those in water; that 

 they migrate and diffuse freely through the cellular medium; and that, 

 when they interact, such interactions are of the oxidation-reduction type. 

 These interactions may give rise to biochemical effects and some grossly 

 observable physical cytoplasmic effects of radiation. When intermedi- 

 ates migrate close to nuclear material, genetic and in some cases lethal 

 effects can occur. These effects are thus intermediate effects of radiation 

 because they do not result from direct action of ionization but rather are 

 due to chemical action of the intermediates, ^\^lile such intermediate 

 chemical reactions may occur quite frequently, in some cases biological 

 effects may occur as a primary result of ionization produced in the 

 immediate vicinity of the same cellular components. Such effects are 

 the "direct-hit" type. 



Table 1 indicates the parallelism of the mechanism of effect for extra- 

 genic and genie components of living cells. The two sets of effects may 



TABLE 1 



Mechanism of Extragenic and Genic Effects of Radiation 



Extragenic effects are the result of: Genic effects are the result of: 



1. Direct ionization or excitation of mole- 1. Direct hit. Ionization or excitation of 

 cules. The chance for this is usually essential sites. The chance for this is 

 small. usually small. 



2. Indirect mechanism; chemical inter- 2. Indirect action. Some of the ioniza- 

 action with ionization products. This tion products may diffuse to the 

 occurs frequently, but many mole- proper site through extragenic medium 

 cules have to be inactivated before and interact, leading to inactivation of 

 measurable biochemical deficiency genes. The inactivation of a single 

 occurs. Recovery may follow if essen- gene may lead to lethal or inheritable 

 tial genes are still intact. Lethal changes. Ionization products causing 

 effect may result if too many mole- genic effects may have originated from 

 cules become inactivated. ion pairs located at considerable 



distance from the essential sites. 



develop simultaneously, and with a given organism either type may 

 dominate the other. However, the comparative importance of these 

 effects is indicated by the following postulates: 



A. In genes essential to cell division, often only a single inactivation 

 needs to occur for inhibition of cell division. 



B. The mean dose at which a single genetic effect will occur depends 

 on extragenic composition of tissue. 



Having outlined the plausibility of similarity between radiation action 

 on genes and that on extragenic factors, we may now proceed to formulate 

 the diffusion model of radiation effects in simple mathematical terms. 



