G1Phase

G1 phase, or Gap 1 phase, is the first stage of the cell cycle that follows mitosis and cytokinesis and precedes DNA synthesis. During G1, cells grow in size, increase metabolic activity, synthesize RNA and proteins, and duplicate certain organelles in preparation for the upcoming S phase. The phase ends at the G1/S transition, also known as the restriction point, after which the cell is committed to DNA replication.

G1 is regulated by both extracellular signals and intracellular checkpoints. Growth factors, nutrients, and cell density influence progression, while internal mechanisms assess cellular size, energy status, and DNA integrity. Central regulators are cyclins and cyclin-dependent kinases (CDKs). Early in G1, cyclin D associates with CDK4/6, leading to phosphorylation of the retinoblastoma (Rb) protein. In late G1, cyclin E binds CDK2 and drives further phosphorylation of Rb, releasing E2F transcription factors that promote the transcription of genes required for S phase and DNA replication.

The duration of G1 varies among cell types and conditions; in many mammalian cells it comprises several hours of a roughly 24-hour cycle. Cells may lengthen G1 under poor growth conditions or enter a quiescent state, G0, if signaling is insufficient. Cells in G0 can re-enter G1 in response to appropriate stimuli.

Disruption of G1 control pathways can contribute to uncontrolled cell proliferation and cancer. G1 remains a focal point for understanding how cells coordinate growth, nutrient status, and the decision to commit to DNA replication.