chromosome aberrations in Tradescantia 723 



age or of restitution and reunion during the nuclear cycle have been sug- 

 gested (Sparrow, 1951). Differences in primary breakage may reflect 

 variations in the chemical composition of the chromosomes during 

 mitosis and meiosis, for example, in their desoxyribonucleic acid (DNA) 

 content, as suggested by Darlington and La Cour (1945). [However, 

 quantitative measurements by Ris (1947) and by Sparrow, Moses, and 

 Steele (1950) indicate that there is no significant change in the amounts 

 of either desoxyribonucleic acid or pentose nucleic acid during meiosis 

 and mitosis.] It is also possible that changes in the cellular environ- 

 ment (e.g., changes in oxygen tension during mitosis and meiosis) may 

 be responsible for modifying the relative efficiency of ionizing radiations 

 in breaking chromosomes. Among the factors that have been suggested 

 as possibly influencing reunion and restitution are the type of coiling 

 present in the chromosomes and the degree of chromosome duplication 

 (plus the relative proximity of the duplicated sister strands) at the time 

 of irradiation (Sax and Swanson, 1941; Bishop, 1950). 



QUANTITATIVE RADIATION RESULTS 



THE RELATION OF ABERRATION YIELD TO DOSE 



The initial development of a quantitative theory of chromosome aber- 

 ration production in Tradescantia microspores by radiation resulted 

 primarily from studies of the relation between the yield of various aberra- 

 tions with dosage and intensity of X rays and neutrons. The early 

 experiments of Sax (1938; 1940) with X rays demonstrated two types of 

 relationship: (1) a linear relation with dose for certain aberration types, 

 e.g., for isochromatid breaks, and (2) a nonlinear (geometric-exponential) 

 relation (the exponent of the dose being greater than one) for other aberra- 

 tions, such as chromatid and chromosome exchanges. In the first experi- 

 ments of Sax, in which the radiation dose was administered at a constant 

 intensity, the exponent of the dosage curve for exchanges was approxi- 

 mately 1.5. In later experiments, when the time of irradiation was kept 

 constant and the intensity varied, the exponent of the dosage curve for 

 exchanges was approximately 2 (Fig. 10-2). 



These results were interpreted as indicating that with X rays certain 

 aberration types (one-hit) are produced by single events, whereas, other 

 aberrations (two-hit types) are produced by two separate events (which 

 must be related in time and space, as will be discussed later). The 

 nature of the event producing the break, whether related to a single 

 ionization or to the passage of a single ionizing particle, could not be 

 determined from the initial X-ray experiments alone. 



Subsequently, experiments with fast neutrons (Giles, 1940b) showed 

 that with this radiation the frequencies of all aberration types studied, 



