Periodic mitotic events induced in the absence of DNA replication.

We have discovered and report here a means of separating a mitotic "subcycle" from the G1- and S-phase events of the mammalian cell cycle. Time-lapse videomicroscopy of Syrian hamster fibroblast (BHK) cells revealed that caffeine could induce multiple entries into mitosis while cells were blocked in DNA synthesis. As with normal mitoses, the abundance of mitosis-specific phosphoproteins was coupled with the condensation of chromatin. The BHK temperature-sensitive mutant tsBN2 also completed multiple entries into mitosis while arrested during DNA replication and raised to the restrictive temperature. Periodic mitotic events occurred even when BHK cells were exposed to low concentrations of serum or cycloheximide, conditions that prevent the cycling of BHK cells by blocking their entry into S phase. These results suggest that an oscillation governing the activation and inactivation of mitotic factors can be generated in mammalian cells and uncoupled from the G1 and DNA replication events of the normal cell cycle. This system will be useful for examining the molecular nature of mitotic factors.