Differential Rejoining as a Factor in Apparent Sensitivity of Chromosomes to X-Ray Breakage

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Abstract
Buds of Trillium erectum were irradiated with 25, 50, 100 and 200 r of X-rays at meiotic 1st metaphase (stage of high sensitivity), and with 50, 100, 200, 400, 800, and 1000 r at early post-meiotic interphase (stage of low sensitivity). In each case scoring for acentric fragments (representing breaks) and for dicentrics and rings (representing rejoins) was done at microspore anaphase. The nos. of dicentrics and rings increased with increasing fragment frequency for both stages irradiated. Irradiation at metaphase produced 3.5 dicentrics for every ring compared with 2.7 following irradiation at interphase. Rejoining, calculated as a function of fragmentation occurred 1.6 times more frequently after interphase than after metaphase irradiation, there being 6.1 fragments per rejoin (dicentric and ring) for the latter compared with 3.8 for the former. The difference in apparent sensitivity of the two stages of cell division is concluded to be due in part to a differential rejoining of broken ends, as well as to a difference in the no. of breaks initially induced. The relationship between initial breakage and the amount and kind of rejoining is variable, since rejoining is affected by stage of division irradiated and by environmental factors. The inverse relationship between breakage sensitivity and tendency for rejoins is discussed in the light of mutation rate and gene size.