Figure 1.

 Scheme of experimental procedures. A,B,and C show time lines representing the course of G₁ and S phase under different experimental conditions. (A) Entry into and passage through a normal S phase. Before entry into S phase, cells transmit the α-factor arrest point and cdc7 arrest point, indicated by arrows. The rectangular box represents S phase. Replication origin activation events are drawn as shaded circles, with lighter shading indicating early-activated replication origins and darker shading for later origins. (B) Scheme of the experimental approach of Hartwell (1973) employing a cdc7-4 mutant. Vertical offset in lines represents temperature shift. Broken circles and question mark indicate replication origins that may not have been active under the particular experimental conditions. Although the temperature shift is drawn as having taken place two-fifths of the way through S phase, it should be noted that this is arbitrary; it was not possible to determine the time in S phase at which cells were shifted. (C) Illustration of experimental approach taken in this study, employing a cdc7-1 mutant. Broken circles as in B. See text for further explanation.