sparrow: cytogenetic effects of ionizing radiations 83 



B. Production of Aberrations 

 1. General 



Although chromosome breakage normally appears in dividing 

 cells shortly after exposure to ionizing radiation, in certain cases 

 breakage does not occur for a relatively long period after treatment. 

 For instance, the irradiation of meiotic first metaphase (or of later 

 stages during meiosis) produces no apparent breakage at meiosis, 

 but breakage and aberrations do appear during the subsequent micro- 

 spore division (157). It is obvious that some kind of lesion must have 

 been produced at the time of exposure, but that it did not develop 

 to the point that it could be recognized as a break until a later stage 

 or even until the nucleus had passed through an interphase. Such 

 lesions or incipient damage are usually referred to as "potential 

 breaks." Irradiation of certain stages, such as diplotene, may produce 

 both immediate and delayed breakage. As one can readily imagine, 

 this delayed expression of radiation damage has led to considerable 

 confusion since chromosomes carrying potential breaks could be con- 

 sidered normal if observations were not continued Ions* enough to 

 determine that delayed breakage also occurs in the next cell division. 



Chromosome or chromatid breaks may either restitute in the 

 original position, rejoin in a new combination, or remain open. While 

 much effort has been devoted to all aspects of this problem, it is dim- 

 cult to generalize concerning the time required for each of these 

 events to occur or to define explicitly how and why restitution or 

 reunion occurs. There is evidence that, in certain cases, breakage and 

 reunion occur in a very few minutes, whereas in other cases, breakage 

 does not occur for a very long period after exposure to radiation, 

 and, under certain conditions, broken ends may remain open for long 

 periods before union or reunion occurs (23). It is generally assumed 

 that the initial pattern of chromosome breakage is completely random 

 within a given karyotype, but there are several nonrandom cases 

 reported (39, 183). These are generally thought to result from sec- 

 ondary factors such as (a) differences in the freedom of movement of 

 different regions of the chromosome, (b) the presence or absence of 

 heterochromatin, (c) the position of heterochromatin within the 

 karyotype, (d) the position of the nucleolus or nucleolar-organizing 

 regions, and /or (e) the position of the centromere relative to any 

 given locus. 



