Key Concepts of Section 19.1

• The eukaryotic cell cycle is divided into four phases: G₁ (the period between mitosis and the initiation of nuclear DNA replication), S (the period of nuclear DNA replication), G₂ (the period between the completion of nuclear DNA replication and mitosis), and M (mitosis).

• Cells commit to a new cell division at a specific point in G₁ known as START or the restriction point.

• Cyclin-CDK complexes, composed of a regulatory cyclin subunit and a catalytic cyclin-dependent kinase (CDK) subunit, drive the progression of a cell through the cell cycle.

• Cyclins activate CDKs and are present only in the cell cycle stage that they promote.

• CDK activities oscillate during the cell cycle. Positive and negative feedback loops drive these oscillations.

• Surveillance mechanisms, called checkpoint pathways, guarantee that each cell cycle step is completed correctly before the next one is initiated.