chromosome aberrations in Tradescantia 735 



random distribution of boron and possibly of nitrogen in cells, with higher 

 relative concentrations in the nucleus and possibly in the chromosomes, it 

 is suggested that this may account in part for the observed efficiency 

 differences. In addition, recoil and transmutation effects may also be of 

 importance. It thus seems probable that the greater effectiveness of 

 particulate radiations originating internally from capture reactions results 

 from both the site and mode of origin of these radiations. 



RECAPITULATION 



Before considering the effect of modifying factors, it seems appropriate 

 to summarize briefly the general theory of aberration production by ion- 

 izing radiation as discussed in the previous sections. 



On this theory, as originally proposed by Sax and Lea and Catcheside, a 

 break in either a single (resting stage) or divided (prophase) chromosome 

 is due to the direct action on the chromosome of the ionization produced 

 by the passage through the chromosome of a single particle, such as an 

 electron, a proton, or an a particle. Such a break may then remain as 

 such giving rise to a terminal deletion, rejoin in the original position (the 

 process of restitution) and thus be undetected, or join with another 

 adjacent break in the same or in a different chromosome (the process of 

 reunion) to produce various types of aberrations. 



Comparative experiments with different radiations under various con- 

 ditions indicate that several ionizations (average ca. 17) are required to 

 produce a chromosome break; that the majority of breaks produced 

 undergo restitution; that restitution is, for most breaks, a relatively rapid 

 process (the average restitution time being ca. 4 minutes, although some 

 breaks may remain "open" for considerably longer periods); and that 

 reunion is not at random, since for an exchange to occur the two breaks 

 involved must be produced with an initial separation of not more than 

 ca. 1 ft. 



The quantitative relationship between a dose of radiation and the 

 yield of aberrations depends on both the type of aberration and the kind 

 of radiation. With X and y rays, certain types of breaks (simple, one-hit) 

 are linearly related to dose and are apparently produced by the relatively 

 concentrated ionization at the tail of a single electron track. Other 

 types, principally those involving breaks in separate chromosomes 

 (exchanges, two-hit types) increase as some power of the dose greater than 

 one. These aberrations apparently arise from the production of the two 

 separate breaks by the tails of two separate electron tracks. An intensity 

 effect with X rays exists for exchange breaks, as indicated by the fact that 

 the yield for exchanges, but not for simple breaks, is lower when the same 

 dose is delivered at a low as compared with a high intensity. If the dose 

 is given in a short time, all the initial breaks are present in the nucleus 

 simultaneously, reunion is favored, and the exchange yield is proportional 



