Effects of Radiation,Antibiotics and Alkylating Agents on Cell Division and Growth in the Ciliate Spathidium spathula*

SYNOPSIS. Spathidium spathula is very sensitive to division inhibition after X-irradiation. Five kr delivered to animals 1 hr into the cell cycle prolong the period until the next division to about 2 times the normal length. The next 2 cell cycles, however, are shorter than normal, and by the 4th division irradiated cells have recovered the normal division rate. During this division delay, scanning interference microscopy shows that growth in dry mass continues; at the 1st post-irradiation division the cells average 3 times the normal dry mass. After the 2nd post-irradiation division, dry mass is 1.5 times the normal amount. Dry mass measurements were not made beyond the 2nd division. Giant cells produced by X-rays have enlarged macronuclei, indicating that DNA synthesis is not inhibited by a dose of X-rays that blocks division. Mitomycin C and triethylene melamine, agents which attack or damage DNA, also produce division blockage and giantism in Spathidium. This suggests that damage to DNA in either the macronucleus, the micronucleus or other organelles may be much more effective in delaying cell division than cell growth.