Deoxyribonucleic acid replication in human diploid fibroblasts stimulated to proliferate

The elongation of replicating DNA chains was studied in resting (G0) cultures of WI-38 cells stimulated to proliferate. An alkaline sucrose gradient sedimentation method which avoids shear effects was employed. The earliest detected intermediate is a short (4.5 S) piece of DNA which is converted to 11 S and larger (50 to 60 S) pieces within 1 min. The 50 to 60 S pieces are gradually converted to two distinct size classes, 212 S and 275 S, which are in turn converted to a chromosomal aggregate of about 400 S. The overall rate of elongation of DNA is about 1.4 X 10(6) daltons/min.