SYNTHESIS OF DNA IN SYNCHRONIZED CULTURES OF TETRAHYMENA PYRIFORMIS GL

The synthesis of DNA in heat-synchronized Tetrahymena pyriformis GL has been studied by deoxyribose assay experiments and by 3H-thymidine incorporation into phenol-extracted DNA, eluted from MAK columns, and into trichloroacetic acid-insoluble material. Excess DNA, synthesized during the eight, hourly heat shocks, suffices for two synchronized divisions of cells on a starvation medium in the absence of any detectable net DNA synthesis. On a growth medium, net DNA synthesis occurs to a variable extent during the first two synchronized cycles, depending upon the culture conditions. A doubling of total DNA content per cell does not occur. 3H-Thymidine incorporation experiments demonstrate a well-defined S-period following the first synchronous division in the growth medium. The DNA or a DNA-complex synthesized after the synchronizing heat treatment differs, at least in part, from the pre-existing DNA of the cell. © 1969.