H(3)-THYMIDINE DERIVATIVE POOLS IN RELATION TO MACRONUCLEAR DNA SYNTHESIS IN TETRAHYMENA PYRIFORMIS

The formation of a soluble H(3)-thymidine derivative pool has been examined in Tetrahymena pyriformis as a function of macronuclear DNA synthesis during the cell life cycle. An autoradiographic technique which allows the detection of water-soluble materials within a cell has shown that these cells do not take up and retain exogenous H(3)-thymidine during G1 or G2. Uptake of H(3)-thymidine is restricted to the S period of the cell cycle. Additional autoradiographic experiments show, however, that a soluble pool of H(3)-thymidine derivatives persists from the end of one DNA synthesis period to the beginning of the next synthesis period in the subsequent cell cycle. Since this persisting pool cannot be labeled with H(3)-thymidine, the pool does not turn over during non-S periods.