Regulation of mitosis in response to incomplete DNA replication. T.T. Su , M. Garner. Dept MCD Biol, Univ Colorado, Boulder, CO.80309-0347. tin.su@colorado.edu.

   We are interested in how cell division is regulated in the presence of damaged or incompletely replicated DNA. We found previously that in response to damaged DNA, both the entry into and the exit from mitosis are delayed in Drosophila embryos. Inhibitory phosphorylation on Cdk1 is important for delaying the entry into mitosis, whereas stabilization of Cyclin A is important for delaying the exit (Su et al., 2000; Su & Jaklevic, Curr. Biol., in press). We report here that when DNA synthesis is blocked cells also delay both the entry into mitosis and the exit from mitosis. However, staining for Cyclins A & B, tubulins and spindle checkpoint proteins suggests that the block in exit from mitosis in response to incomplete DNA replication is mechanistically different from the block in exit from mitosis that occurs after DNA damage. The former does not appear to involve stabilization of Cyclin A, but instead may be mediated by the spindle checkpoint.