G1fasen

G1-fasen (G1 phase) is the first stage of interphase, occurring after mitosis and before the S phase. During G1, the cell grows, increases its mass, synthesizes ribosomes, proteins, and other macromolecules, and assesses whether conditions are favorable for DNA replication. Organelles are replicated, and metabolic activity is adjusted to prepare for genome duplication.

Progression from G1 into S is regulated by growth signals and nutrient status. The retinoblastoma protein (Rb) and E2F transcription factors play central roles: growth-factor–dependent activation of cyclin D-CDK4/6 leads to phosphorylation of Rb, releasing E2F and promoting expression of S-phase genes. In late G1, cyclin E-CDK2 activity drives the commitment to DNA synthesis. The G1/S checkpoint monitors DNA integrity and environmental cues; DNA damage can activate p53 and p21, causing arrest in G1.

Cells may also enter a quiescent state, G0, from G1 when proliferation is not required or conditions are unfavorable. The length of G1 varies widely between cell types and conditions, from a few hours in cultured mammalian cells to longer durations in some tissues. The duration is often influenced by growth factors, nutrients, and the cell's needs.

Dysregulation of G1 control is a common feature of cancer, enabling unchecked progression to S phase. Understanding G1 regulation informs research on cell cycle, development, and oncology.