Sensitivity of the cell division cycle to α-amanitin in Allium root tips

The effect of α-amanitin on the cell cycle in Allium cepa meristematic cells was studied: the G1 and G2 periods are prolonged respectively to 1.9 and 1.7 times the normal duration; the S-period is lengthened very little; and the prophase of mitosis is increased to twice the normal duration. It is postulated that real differences in the activity of the non-nucleolar RNA poly-merase might exist in the course of the cell division cycle and that they would account for the higher sensitivities shown by G1, G2 and prophase. On the other hand, the interphase nucleolus responds by segregation in the first few hours of α-amanitin treatment, but recovers its normal structure in continued presence of the drug; and nucleolar reorganization is inhibited in the first few hours in recently formed cells, but the process is subsequently speeded up to attain the same value 4 h after the treatment was begun as in untreated cells.