EFFECTS OF ULTRAVIOLET LIGHT ON MITOSIS IN PHYSARUM POLYCEPHALUM

Plasmodia of the slime mold, Physarum polycephalum, were irradiated with ultraviolet light at different times in the mitotic cycle. Mitosis was delayed most if irradiation took place during the preceding mitosis or during early interphase (= S-phase). From then on the sensitivity decreased gradually to almost zero at early prophase when irradiated with 2900 ergs/mm2. With a dose of 14,500 ergs/mm2, the sensitivity remained at an approximately constant level during the latter half of the intermitotic period. Irradiation with 14,500 ergs/mm2 during early prophase, as late as 13 min before metaphase, caused reversion to interphase and approximately the same mitotic delay as irradiation at other times during the latter part of the intermitotic period. The mitotic cycles following the first postirradiation mitosis were shorter than the corresponding mitotic cycles of unirradiated control halves. At the first and second postirradiation mitosis the RNA/DNA and the protein/DNA ratios were above the levels measured for the corresponding mitoses in the control halves. After the second postirradiation mitosis these values gradually approached the levels found in the control halves. © 1968.