Purpose: To analyse the kinetics of chromatid break induction, rejoining, and misrejoining after γ-irradiation in G2 phase human cells using premature chromosome condensation induced by calyculin A. Materials and methods: Human fibroblast AG1522 cells were irradiated with γ-rays and chromosomes were then prematurely condensed by calyculin A. The number of chromatid breaks and chromatid exchanges in G2 chromosomes were scored, and fitted curves were calculated. Results: Calyculin A induced premature chromosome condensation in cells immediately after irradiation. Kinetics of rejoining of chromatid breaks demonstrated two exponential components with rapid and slow time constants. Within 5 min after irradiation, the number of chromatid breaks fell rapidly to about one-half, then gradually decreased. Chromatid exchanges were formed very quickly, reaching a plateau within 20 min from exposure. Conclusions: Chemically induced premature chromosome condensation technique allows a simple, rapid and precise analysis of chromatid breakage and rejoining. The rapid kinetic component was particularly well characterized.
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