The previous section described the multiple mechanisms that control the different cyclin-CDK complexes. In this and the following two sections, we examine each cell cycle stage carefully and discuss how it is induced and controlled. We examine how cells initiate DNA replication and mitosis and how chromosomes are segregated. We focus on how cyclin-CDK complexes and other key cell cycle regulators affect each cell cycle phase, and we examine the mechanisms that coordinate their activities.

This section investigates how cells decide whether or not to undergo cell division and how DNA replication is initiated. The process of cell cycle entry is well understood in budding yeast, and it was in this organism that the molecular mechanisms underlying this cell cycle transition were initially elucidated. We therefore begin by examining the molecular events governing cell cycle entry in budding yeast. We then investigate the striking similarities between the pathways that govern cell cycle entry in yeast and in metazoan cells, and we discuss the realization that many genes involved in this decision are frequently found mutated in cancer. Next we see how the decision to enter the cell cycle is influenced by extracellular events and learn about the signaling mechanisms that convey these environmental cues to the cell cycle machinery. Finally, we discuss the molecular mechanisms that govern initiation of DNA replication. We see why degradation of an S phase CKI is essential for this process, discover how CDKs ensure that DNA replication occurs only during S phase and then only once, and see how proteins that associate with DNA during DNA replication lay the foundation for accurate chromosome segregation during mitosis.