The goal of each cell division is to generate two daughter cells of identical genetic makeup. To achieve this, cell cycle events must occur in the proper order. DNA replication must always precede chromosome segregation. Today we know that the activity of the key proteins that promote cell cycle progression, the CDKs, fluctuates during the cell cycle. For example, CDKs that promote S phase are active during S phase but are inactive during mitosis. CDKs that promote mitosis are active only during mitosis. These oscillations in CDK activity are a fundamental aspect of eukaryotic cell cycle control, and we have gained some understanding as to how these oscillations are generated. Oscillations are generated by positive feedback mechanisms, whereby specific CDKs promote their own activation. These positive feedback loops are coupled to subsequent negative feedback mechanisms by which, indirectly or with a built-in delay, CDKs promote their own inactivation. Their oscillations not only propel the cell cycle forward but also create abrupt transitions between different cell cycle states, which is essential to bring about distinct cell cycle states.

Overlaid on the cell cycle oscillator machinery is a system of surveillance mechanisms that further ensures that the next cell cycle event is not activated before the preceding one has been completed or before errors that occurred during the preceding step have been corrected. These surveillance mechanisms are called checkpoint pathways, and their job is to ensure the accuracy of the chromosome replication and segregation processes. The system that ensures that chromosomes are segregated accurately is so efficient that a mis-segregation event occurs only once in 10⁴–10⁵ divisions! These multiple layers of control on the cell cycle control machinery ensure that the cell cycle is robust and error free.