730 



RADIATION BIOLOGY 



much of the theory of aberration production as outlined previously. 

 The experiments of Lane have, however, been repeated by Sax (1952) — 

 the only difference being that a higher radiation intensity was used — and 

 on the basis of a much more extensive analysis in terms of numbers of 

 cells scored, no evidence of an inhibition or recovery effect is found, even 

 with an interval between doses as long as 12 hours (Table 10-1). Thus 



Table 10-1. Effect of X-ray Dose Fractionation on Yield of Chromosome 



Interchanges, 180 r/Minute 

 (Data from Sax and Luippold, 1952) 



the rejection by Lane, on the basis of his experiments, of the theory of 

 relatively rapid reunion and restitution, appears not to be warranted. 



THE SPACE FACTOR IN REUNION 



Sax (1940) pointed out that the observed ratio of certain types of 

 chromosome inter- and intrachanges (i.e., dicentric-centric rings = 3.3 : 1) 

 was not as expected (10 : 1) on the basis of random reunion of broken ends, 

 intrachanges being favored. These data are interpreted as indicating 

 that space limitations influence the reunion process, and that most 

 breaks must be relatively close together in order for reunion to occur. 

 The further conclusion is drawn that the limitations to reunion imposed 

 by this factor of proximity must mean that most of the breaks induced by 

 radiations do not lead to the production of aberrations. Rather, the 

 two broken ends simply reunite in the original position (the process of 

 restitution) and no structural alteration is visible at succeeding meta- 

 phases or anaphases. 



Lea (1946, 1947) has developed several quantitative methods for 

 estimating the average distance over which reunion occurs. One of 

 these depends on the use of biological and physical data on exchanges 

 induced by fast neutrons and X rays. It is possible to calculate the 

 number of ionizing particles, either protons or photoelectrons, which will 

 pass through a given Tradescantia microspore nucleus irradiated with 

 equivalent physical doses of neutrons of X rays. Since neutron-induced 

 exchanges are primarily one hit, the data on protons can be used to set 

 an upper limit to the distance from a given break at which a second break 

 normally taking part in an exchange must occur. Since X-ray-induced 



