Cell Cycle Phases and Regulation

The eukaryotic cell cycle is divided into four ordered phases — G1, S, G2, and M — and progression through them is driven and timed by the periodic activity of cyclin-dependent kinases. This topic covers what happens in each phase, how cells commit irreversibly to division at the restriction point, and how cyclin synthesis and destruction set the rhythm of the cycle.

Definition

Cell cycle phases are the successive intervals — G1, S (DNA synthesis), G2, and M (mitosis and cytokinesis) — through which a dividing cell passes, with their timing and order governed by the oscillating activity of cyclin-dependent kinases bound to phase-specific cyclins.

Scope

The entry describes interphase (G1, S, G2) and M phase, the molecular control machinery of cyclins and cyclin-dependent kinases, the G0 resting state, and the restriction (start) point at which cells commit to division. It treats phase structure and its regulation as a cell-biology topic, not as clinical instruction.

Key concepts

• G1 phase (cell growth)
• S phase (DNA replication)
• G2 phase (preparation for mitosis)
• M phase (mitosis and cytokinesis)
• G0 quiescent state
• Restriction point / Start
• Cyclins and cyclin-dependent kinases (CDKs)
• CDK inhibitors and CDK activation
• Maturation-promoting factor (MPF)

Mechanisms

Each phase is governed by the activity of cyclin-dependent kinases (CDKs), enzymes that are catalytically active only when bound to a regulatory cyclin. Cyclins are synthesised and then abruptly degraded in a phase-specific manner — first shown for the sea urchin cyclin that is destroyed at each cleavage division (Evans and colleagues, 1983) — so CDK activity rises and falls in waves. Different cyclin-CDK complexes trigger different transitions: G1/S complexes commit the cell to replicate its DNA at the restriction point, S-phase complexes license and fire replication origins, and M-phase CDK1 (the universal mitotic kinase identified by Nurse) drives entry into mitosis. Morgan's synthesis frames CDKs as the engines and clocks that both power and time these transitions, while their deregulation, reviewed by Vermeulen and colleagues, is a hallmark of cancer.

Clinical relevance

Phase structure and CDK regulation provide the conceptual basis for describing cell proliferation, the rationale behind cell-cycle-targeted anticancer agents, and the meaning of proliferation markers in tissue analysis. This entry is a reference description of normal and deregulated cycle control and does not provide diagnostic or treatment guidance.

History

The phase model (G1, S, G2, M) was established by mid-twentieth-century labelling studies of DNA synthesis. The molecular basis followed from two converging lines: genetic identification in yeast of the universal mitotic kinase (Nurse) and the biochemical discovery of cyclins as proteins whose periodic destruction times the cycle (Evans, Hunt, and colleagues, 1983). The 2001 Nobel Prize recognised this body of work.

Key figures

• Paul Nurse
• Timothy Hunt
• David Morgan
• Arthur Pardee

Seminal works

• nurse-1990
• evans-1983
• morgan-1997

Frequently asked questions

What is the restriction point?

The restriction point (called Start in yeast) is a control point late in G1 after which a cell is committed to completing the cycle independently of external growth signals; before it, cells can divert into the resting G0 state.

What do cyclins and CDKs do?

Cyclin-dependent kinases are enzymes that phosphorylate target proteins to drive cell-cycle transitions, but they are active only when bound to a cyclin; because cyclins are made and destroyed periodically, CDK activity oscillates and sets the timing of each phase.