Pulling the string: cell cycle regulation during Drosophila development.

The extensive cell proliferation which accompanies the development of multicellular organisms is co-ordinated with other developmental events. Cell cycle progression during embryogenesis is therefore controlled according to developmental stage and developmental fate. In Drosophila, entry into S phase is a constitutive, unregulated event until late in development. Entry into mitosis, however, is the first control point used for differential regulation, and the transcriptionally controlled expression of string (the Drosophila homologue of Schizosaccharomyces pombe cdc25+) directs the precise patterns of the embryonic cell divisions after the onset of morphogenesis. In contrast to string, cyclin proteins are produced in excess and their rate of accumulation does not regulate the time of mitosis.