10 : 7/ Cellular Events Produced by Ionizing Radiations 201 



pregnant women under almost all circumstances; even dental use of 

 X rays contributes a significant dose to the developing embryo. 



Background radioactivity is due in part to cosmic rays, which are still 

 beyond human control, and, in part, to radioactive elements in the air, 

 soil, water, food, and our bodies. Since 1950, the total background 

 radiation has risen detectably as a result of testing nuclear weapons. 

 These tests release radioactive fission products into the upper atmos- 

 phere. Then radioactive atoms fall out over the surface of the earth, 

 both near the original test site and farther away. The fall-out in 1961 

 had not reached such proportions that it greatly increased the back- 

 ground radiation, but any increase, no matter how small, can be expected 

 to increase the mutation rate. It is not proposed to debate here 

 whether the supposed benefits of testing atomic and nuclear weapons 

 outweigh the best estimates of the genetic cost. It is important to 

 emphasize that genetic damage to survivors would be a major long- 

 term result of any war involving nuclear weapons. 



The greatest immediate biological danger from fall-out appears to be 

 the production of radioactive isotopes which are incorporated into the 

 organism, particularly C 14 and Sr 90 . These had reached limits in 1961 

 in some parts of the world where the rate of carcinogenesis (production of 

 new cancers) might be detectably increased by these isotopes. This type 

 of damage would also be multiplied manyfold for the survivors of a 

 nuclear war. 



7. Summary 



Many types of ionizing radiation produce similar effects in all living 

 cells. The different types of radiation, their measurement in terms of 

 dosage and target theory, and their action are discussed in this chapter. 

 The cellular effects may be divided into two types: visible and genetic. 

 The former consist primarily of changes in the pattern of cell division 

 called mitosis, although other effects, particularly the death of lympho- 

 cytes, are also observed. Direct microbeam experiments show that 

 some mitotic effects involve the direct action of the ionizing radiation 

 on or near the chromosomes, whereas other effects result from irradiation 

 anywhere within the cell. These studies have confirmed the role of 

 the chromosomes in carrying genetic information and have emphasized 

 the physical action of the centromere during mitosis. 



Genetic effects produced by ionizing radiations and ultraviolet light 

 include increasing the frequency of crossover and the production of 

 mutations. The former is the predominant effect with ultraviolet 

 irradiation, and the latter occurs with all the types of irradiation dis- 

 cussed in this chapter. The mutations consist primarily of lethal 



