232 FINE-STRUCTURE OF PROTOPLASM II 



Ionization consists in the formation of ions from neutral molecules 

 by the action of irradiated energy. The molecules in question are, as 

 it were, struck by the energy quanta of the radiation and are thereby 

 modified. That is why the occurrence of a single ionization is called 

 a hit. The relation between mutation rate and dose of rays indicates 

 that a mutation is the result of such a hit. It can also be demonstrated 

 (Timofeeff-Ressovsky, 1940) that the interdependence of dose and 

 rate would not produce a straight line if several hits were needed to 

 bring about one mutation. The conclusion to be drawn from the bio- 

 physical analysis of chromosome irradiation is, therefore, that the 

 artificial mutation of genes is the elementary result of a single hit. 

 There are, it is true, other possible physical explanations, besides the 

 target theory, which may account for the effects observed (Minder 

 and Liechti, 1945). 



The approximate number of atoms in a cubic centimetre of organic sub- 

 stance being known, and also the number of single ionizations which one 

 r unit is able to evoke, it is possible to calculate how many atoms are needed 

 for one of them to be hit to produce the mutation in question, this by means 

 of the experimentally ascertained mutation constant, which indicates the 

 degree of probability to incite a mutation by a given dose of radiation. The 

 volume occupied by these atoms altogether is called the target area. It varies 

 with different mutations of genes within the x-chromosome of Drosophila; 

 nevertheless an average can be calculated, according to which the susceptible 

 volume amounts to 3.20-10-20 cm^., from which it follows that the radius 

 of the target area (assumed to be spherical) is 1.97 m/x (Timofeeff- 

 Ressovsky, 1940). 



There is an alternative method by which the target area can be computed. 

 If very strong ionizing rays are used, of very great density, such as neutron 

 rays, for example, more than one ionization may take place in one target area, 

 only one of which, however, effects mutation. The other ionizations are 

 inoperative and the mutation rate must consequently be smaller than was to 

 be expected from the irradiated dose of rays in r units. Indeed, in the case 

 of the >r-chromosome of Drosophila, the mutation rate actually is 1.6 times 

 smaller for neutron rays than for X-rays, with the same dose of rays. The 

 radius of the spherical target area can now be deduced from this factor to- 

 gether with the known density of ionization for neutron rays; Lea (1940) 

 finds 1.89 m/i,. Seeing that this figure so nearly agrees with the value found 

 by Timofeeff-Ressovsky, it may be taken as fairly certain that the order 

 of magnitude of the target area is roughly 4 m/x diameter. 



The target area is not to be identified with the gene, since it only gives us 

 the size of a sensitive area within which something has to happen favouring 

 the probability of a mutation. The gene may therefore be larger than the 



