Human cyclin A is required for mitosis until mid prophase

We have used microinjection and time-lapse video microscopy to study the ro le of cyclin A in mitosis. We have injected purified, active cyclin A/cycli n-dependent kinase 2 (CDK2) into synchronized cells at specific points in t he cell cycle and assayed its effect on cell division. We find that cyclin A/CDK2 will drive G2 phase cells into mitosis within 30 min of microinjecti on, up to 4 h before control cells enter mitosis. Often this premature mito sis is abnormal; the chromosomes do not completely condense and daughter ce lls fuse. Remarkably, microinjecting cyclin A/CDK2 into S phase cells has n o effect on progress through the following G2 phase or mitosis. In compleme ntary experiments we have microinjected the amino terminus of p21(Cip1/Waf1 /Sdi1) (p21N) into cells to inhibit cyclin A/CDK2 activity. We find that p2 1N will prevent S phase or G2 phase cells from entering mitosis, and will c ause early prophase cells to return to interphase. These results suggest th at cyclin A/CDK2 is a rate-limiting component required for entry into mitos is, and for progress through mitosis until late prophase. They also suggest that cyclin A/CDK2 may be the target of the recently described prophase ch eckpoint.