Subphases of DNA replication in Drosophila cells

Exponentially growing Drosophila S2 cells in suspension culture were synchr onized at low- and high-resolution centrifugal elutriation, and DNA synthes is was measured by [H-3]-thymidine incorporation throughout the S phase. At low resolution, one repair peak at the C-1/G(0) border and two replication peaks known as early and late S subphases were observed. At high resolutio n, six chronologic compartments were distinguished. The distribution of the se peaks indicated one repair peak at 2.05 C value, one minor replication p eak at 2.43C, and four major subphases of replication corresponding to 2,64 C, 2.89C, 3.32C, and 3.60C, representing 6.7%, 3.4%, 15.3%, 20.4%, 32.1%, a nd 22.0% of the synthetic activity, respectively. The five major peaks of c ell growth with 2.32C, 2.56C, 2.85C, 3.18C, and 3.58C values consistently p receded those of replication subphases.