THE MECHANISM OF 5-AMINOURACIL-INDUCED SYNCHRONY OF CELL DIVISION IN VI CIA FABA ROOT MERISTEMS

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Abstract
Cessation of mitosis was brought about in Vicia faba roots incubated for 24 hours in the thymine analogue, 5-aminouracil. Recovery of mitotic activity began 8 hours after removal from 5-aminouracil and reached a peak at 15 hours. If colchicine was added 4 hours before the peak of mitoses, up to 80% of all cells accumulated in mitotic division stages. By use of single and double labeling techniques, it was shown that synchrony of cell division resulted from depression in the rate of DNA synthesis by 5-aminouracil, which brought about an accumulation of cells in the S phase of the cell cycle. Treatment with 5-aminouracil may have also caused a delay in the rate of exit of cells from the G2 period. It appeared to have no effect on the duration of the G1 period. When roots were removed from 5-aminouracil, DNA synthesis resumed in all cells in the S phase. Although thymidine antagnoized the effects of 5-aminouracil, an exogenous supply of it was not necessary for the resumption of DNA synthesis, as shown by incorporation studies with tritiated deoxy-cytidine.