X-ray-induced chromosomal aberrations in Vicia faba: Changes in response during the cell cycle

Abstract

Primary roots of Vicia faba were pulse labelled with [ 3H]thymidine followed immediately by 0, 50, 100 or 150 rads of X-rays and fixed from 0–48 h post-treatment. Autoradiographs of root meristem cells were prepared and mitotic cells scored for the presence or absence of labelling and chromosomal aberrations (including achromatic lesions or gaps). As the X-ray dose was increased, the duration of the postsynthetic ( G 2) phase increased. After 50 rads DNA synthesising ( S) cells were partially synchronised, whilst the 100 and 150 rad doses caused a protraction of the S phase. There was a pronounced double peak of chromatid aberrations in the highly sensitive G 2 phase after 100 or 150 rads, the yield falling throughout S and a low frequency of chromosome-type aberration was induced in G 1 cells. A positive correlation was established between the degree of incompleteness of chromatid interchanges and the radiation dose for cells in G 2 and at the time of peak frequency of aberrations, but not for cells irradiated whilst in the S phase. The ratio of true chromatid breaks to incomplete isochromatid breaks differed from that predicted by the exchange hypothesis of Revell 25 and differed significantly from one mitotic stage to another. The partitioning of the mitotic cycle into its various phases enabled the relationship between aberration yield and dose to be determined for all aberration types induced in the different phases of interphase. Dose response kinetics for a given aberration type were found to be strongly influenced by stage in cell development at the time of irradiation. It was suggested that these kinetic differences between stages were due to differences in the time available for repair and to changes in the organization of chromatid structure during replication. The data are interpreted as being at variance with the classical breakage-rejoining hypothesis of aberration formation and, to some extent, with Revell's 25 exchange hypothesis. It is concluded however that all aberrations (excluding gaps) are a consequence of exchange following a process of mis-repair of primary lesions that are not chromatid breaks ( Evans 13).