Initiation of DNA synthesis in the prematurely condensed chromosomes of G1 cells

Abstract

The objective of this study was to investigate whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells. HeLa cells synchronized in early G1, mid-G1, late G1, and G2 and human diploid fibroblasts synchronized in Go and G, phases were separately fused by use of UV-inactivated Sendai virus with mitotic HeLa cells. After cell fusion and premature chromosome condensation, the fused cells were incubated in culture medium containing Colcemid (0.05 lAg/ml) and [3H]thymidine ([3H]ThdR) (0.5 ACi/ml; sp act, 6.7 Ci/mM). At 0, 2, 4, and 6 h after fusion, cell samples were taken to determine the initiation of DNA synthesis in the prematurely condensed chromosomes (PCC) on the basis of their morphology and labeling index. The results of this study indicate that PCC from Go, G, and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. In addition, the G,-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their "pulverized" appearance and incorporation of [3H]ThdR. Further, the initiation of DNA synthesis in G,-PCC occurred significantly earlier than in the mononucleate G, cells. Neither pulverization nor incorporation of label was observed in the PCC of Go and G cells. These findings suggest that chromosome decondensation, although not controlling the timing of a cell’s entry into S phase, is an important step for the initiation of DNA synthesis. These data also suggest that the entry of a G, cell into S phase may be regulated by cell cycle phase-specific changes in the permeability of the nuclear envelope to the inducers of DNA synthesis present in the cytoplasm. © 1980, Rockefeller University Press., All rights reserved.