The capacity of control and heated HeLa cells to process newly polymerized DNA at the nuclear matrix was measured. DNA which had been pulse-labeled with [3 H]thymidine was enriched by a factor of up to 6 at the cell's nuclear matrix. During continuous exposure to [3 H]thymidine at 37°C this enrichment for pulse-labeled DNA was reversed with a half-time of 7 min. We interpret this processing of newly replicated DNA to be a distribution of newly polymerized DNA throughout replicon-sized nuclear DNA domains. Both processing of newly polymerized DNA at the nuclear matrix and ligation of replicon clusters into the interphase cell chromosome were halted by incubation of cells at temperatures at or above 43°C. When HeLa cells were pulse-labeled during a 30-min incubation at 45°C and replaced at 37°C, the enrichment for3 H-labeled DNA at the nuclear matrix was reversed with an initial half-time of 4 h. The results indicate that exposure of cells to hyperthermic temperatures blocks ongoing nascent DNA processing at the nuclear matrix and results in a retardation of DNA processing in preheated cells replaced at 37°C.

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