Image-based quantitative determination of DNA damage signal reveals a threshold for G2 checkpoint activation in response to ionizing radiation

Table 1 chromatid breaks induced by various doses of x-rays

Treatment	No. metaphases analyzed	No. metaphases with aberrations	No. of chromatid breaks
Control (+DMSO)
0 Gy	55	1	1
0.02 Gy	59	7	8
0.04 Gy	61	3	3
0.08 Gy	75	11	16
0.1 Gy	70	15	21
0.2 Gy	50	14	18
0.4 Gy	55	25	53
+KU55933
0 Gy	54	0	0
0.02 Gy	56	9	9
0.04 Gy	64	5	5
0.08 Gy	61	11	15
0.1 Gy	53	19	25
0.2 Gy	50	31	42
0.4 Gy	51	42	106

Cells were treated with KU55933 (10 μM) 30 minutes before irradiation until 30 minutes before metaphase harvest. Immediately after irradiation, colcemid (0.1 μM) was added to the medium, and cells were incubated for 2 hours. Then, metaphases were collected and chromosome samples were prepared as described in Methods.