Reciprocal induction of cell division by cells of complementary mating types in Tetrahymena

Chick embryo fibroblasts in monolayer culture were synchronized by contact inhibition and serum starvation. Nuclear DNA isolated from the [³H] thymidine pulse-labelled cells throughout the period of DNA synthesis (S phase) was analysed by hydroxylapatite chromatography after renaturation at different C₀t values. It is shown that repeated sequences having different frequencies of reassociation, replicate differently throughout the S period. In order to study the distribution of the repeated sequences, DNA isolated during the S period was fractionated according to its buoyant density. It is shown that only some of the highly reiterated sequences which are included in the high buoyant density DNA fractions, replicate equally well during the early and the late S periods. By contrast, reiterated sequences of the low buoyant density DNA fractions replicate mainly during the late S period.