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

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Abstract
Whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells was investigated. HeLa [human cervical carcinoma] cells synchronized in early G1, mid-G1, late G1 and G2 and human diploid fibroblasts synchronized in G0 and G1 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 .mu.g/ml) and [3H]thymidine ([3H]ThdR) (0.5 .mu.Ci/ml; specific activity 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. PCC from G0, G1 and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. The G1-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their pulverized appearance and incorporation of [3H]ThdR. The initiation of DNA synthesis in G1-PCC occurred significantly earlier than in the mononucleate G1 cells. Neither pulverization nor incorporation of label was observed in the PCC of G0 and G2 cells. 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. The entry of a G1 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.