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The unraveling of the molecular mechanisms governing cell cycle progression in eukaryotes was remarkably rapid and was fueled by a powerful combination of genetic and biochemical approaches. In this section, we discuss several model systems and their contributions to the discovery of the molecular mechanisms of cell division. The three most important systems employed to study the cell cycle are the single-celled yeasts Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) and the oocytes and early embryos of the frog Xenopus laevis. We also discuss the fruit fly Drosophila melanogaster, which proved extremely powerful in the study of the interplay between cell division and development. The study of mammalian tissue culture cells led to the characterization of cell cycle control in mammals.

Studies of the cell cycle in many different experimental systems also led to two remarkable discoveries about the general control of the cell cycle. First, complex molecular processes such as initiation of DNA replication and entry into mitosis are all regulated and coordinated by a small number of master cell cycle regulatory proteins. Second, these master regulators and the proteins that control them are highly conserved, so that cell cycle studies in fungi, sea urchins, insects, frogs, and other species are directly applicable to all eukaryotic cells, including human cells.