FIG. 2.

Cdc2 tyrosine phosphorylation is required for HU-induced cell cycle arrest. (A) A synchronous culture of cdc2-Y15F (PR714) cells pretreated with HU was prepared by centrifugal elutriation as described in the legend to Fig. 1. Immediately after elutriation the HU was washed out of the culture labeled HU−. The subsequent cell cycle progression of the two cultures was monitored by determining what percentage of the cells had gone through mitosis. (B) The samples of the HU+ and HU− cultures were taken at various times for FACS analysis. At 100 min, just before the cultures entered mitosis, most cells in the HU− culture had finished replication, while the HU+ cells were still blocked with 1C DNA content. At 240 min, the cells in the HU+ culture had undergone mitosis without replicating. Most cells had been unable to complete cytokinesis and still appeared at 1C, while others had divided into two aneuploid daughters that contained less than 1C DNA. As the cells grew larger over time, the background increased, causing the 1C peaks to drift to the right.