FIG. 1.

Cell cycle phase distribution of synchronized HeLa cells. HeLa cells were synchronized with amethopterin and thymidine as described in the text. At the indicated times, cells were fixed and analyzed by flow cytometry. DNA histograms represent asynchronous cells (A), amethopterin-blocked cells (16 h; B) and cells 9 h after release from the amethopterin block (C). Abscissa, relative fluorescence; ordinate, relative cell number. (D) Time course of mitotic figures of amethopterin-thymidine-synchronized HeLa cells starting 7 h after release from the block.