Studies on the Frequencies of Chromatid Aberrations Induced by X Rays at Different times of the Cell Cycle of Vicia Faba.

T has been known for some time that there are differences in the radiationI induced chromosomal aberrations observed at metaphase, according to the stage of the cell cycle the cells were in at the time of irradiation (SWANSON 1957). The observed differences are not only quantitative (changes in frequency per cell), but are also qualitative in that, depending on the stage and the chromosomal stmctural unit involved, three different classes of aberrations can be induced. In the pre-DNA-synthesis stage (GI) of interphase, chromosome aberrations are induced. These involve the single, pre-split chromosome. During the DNA-synthesis (S) and post-DNA-synthesis (G,) periods of interphase, chromatid aberrations are induced. These aberrations involve the double, post-split chromosomes. During prophase subchromatid aberrations that presumably involve half-chromatids are formed. In addition to these, two different groups of aberrations (one-break and two-break) are observed; these vary with respect to the kinetics of their induction. With sparsely ionizing radiations, one-break aberrations result from single ionizing clusters and show a linear increase with dose, whereas two-break aberrations involve the interaction of breaks that are produced by two independent ionizations (or clusters) and increase approximately as the square of the dose (see reviews by WOLFF 1961; EVANS 1962). In the GI cells the chromosome has been found to exhibit constant sensitivity to radiation-induced breakage (see SAX 1938), whereas as the cell approaches metaphase the sensitivity changes and the frequency of chromatid aberrations increases (see SWANSON and SCHWARTZ 1953). Very close to metaphase the sensitivity drops (WOLFF and LUIPPOLD 1960). EVANS and SAVAGE (1963) and EVANS and SCOTT (1964) have attempted to find the relationship between the changes in sensitivity that occur when chromatid aberrations are induced and the stage of the cell cycle. They have shown that at the onset of S the sensitivity of the chromosome to breakage doubles and then increases slightly through S until G,, when another increase in sensitivity is observed. Since the aberrations induced during S and subsequently scored at metaphase are chromatid aberrations, they have suggested that the apparent gradual increase in sensitivity through S is probably caused by an increase in the number of doubled chromosomes produced as S progresses, and not to an actual increase in the sensitivity of the chromosome material during this period.