VII 



Ejfects of X-rays and Atomic Radiations 



on Living Cells 



H 



The great permanence of the gene shows that the material it is made 

 of must be well shielded from external influences, and at one time it 

 was thought that the genes were so well protected that they could not 

 be modified by any external agency applied to the organism. 



However, it was discovered by H. J. Miiller in 1928 that the fre- 

 quency with which gene mutations occurred could be definitely 

 increased by exposing the germ cells to X-rays and similar pene- 

 trating radiations, which are produced by radioactive substances. 

 The characteristic property of all these radiations is their ability to 

 penetrate into tissue and produce chemical changes within it. The 

 dangerous nature of X-rays was discovered very early by the pioneer 

 workers, who often suffered serious and frequently fatal X-ray burns. 

 The widespread use of radioactive substances and, even more, the 

 possible exposure of large populations to the effects of radiations 

 from atomic bombs, has greatly increased the importance and in- 

 terest of these effects and in recent years an enormous amount of 

 scientific research has been devoted to them. 



The radiations concerned are called ionizing radiations because 

 they are sufficiently powerful to disrupt the molecules of matter 

 through which they pass, with the formation of electrically charged 

 fragments or ions. They usually bring this about by knocking one or 

 more electrons out of the neutral molecules they pass through. The 

 intensity of the radiation is measured by the number of ions primarily 

 formed in the material through which the radiation passes. It is ex- 

 pressed in terms of a unit, the roentgen, which is the quantity of radia- 

 tion which brings about 10^" (a million million) ionizations in a cubic 

 centimetre of the substance. This may seem to be a very large num- 

 ber, but it is small compared with the number of molecules present in 

 the same volume. For example, one cubic centimetre of water con- 



