Chromosome aberrations and the mitotic cycle in trillium root tips after x-irradiation

The mitotic parameters of root tips of five species of the Trilliaceae were determined. A small X-ray dose (50 R) causes a delay to the division of nuclei synthesising DNA at the time of irradiation, but has no effect on those which have completed DNA replication. Elongation of S by cold treatment of nuclei containing tritium label has the same effect.The pattern of aberrations following a dose of 50 R is related to the mitotic cycle. The change from chromosome- to chromatid-type aberrations is correlated with the onset of DNA replication. Sub-chromatid effects indicate that the processes of mitosis are initiated before cytological prophase is distinguishable.The distribution of abberations within and between chromosomes in the interphase nucleus is discussed.Finally it is concluded that the present observations are compatible with Revell's³¹ exchange hypothesis for chromatid-type aberration formation, and it is shown that the apparent differences between Trillium and other material are explicable in terms of its longer mitotic cycle.