Protozoan cell cycle control.

Many genes belonging to the cyclin-dependent kinase (cdk) family have been isolated from protozoans. While their role in cell cycle has yet to be proven unequivocally, at least one cdk can complement the cdc2ts/cdc28ts mutants in yeasts. Among the interesting questions relating to cdks in protozoa are: whether one cdk acts throughout the whole cell cycle and whether cyclin partnership is absolutely required. In protozoa, cell cycle control is closely associated with developmental control. Many life cycle differentiation phases can only occur during a specific window in the cell cycle. Because of different DNA biosynthetic pathways, some protozoa among the earliest eukaryotic lineages are unresponsive to common inhibitors of DNA synthesis like hydroxyurea. However, many protozoa do have different checkpoint controls in relation to their response to cell cycle inhibitors.