interphaseG1

Interphase G1, or the G1 phase of the cell cycle, is the first stage of interphase that follows mitosis in many eukaryotic cells. During G1, cells grow in size, increase transcription and translation, and expand their cytoplasmic and organelle content in preparation for DNA replication. The phase also functions as a regulatory window where cells monitor environmental conditions and internal status to decide whether to commit to S phase.

A central feature of G1 is the decision point that determines progression to S phase, often called the restriction point in mammalian cells. Passage through this point depends on growth factors, nutrient availability, cell size, and DNA integrity. If conditions are unfavorable, cells may enter a quiescent state (G0) or slow proliferation; if conditions are favorable, they proceed to DNA replication in S phase.

Molecular regulation centers on cyclins and cyclin-dependent kinases. Cyclin D associates with CDK4/6 to initiate retinoblastoma (Rb) phosphorylation, followed by cyclin E/CDK2 activity that further phosphorylates Rb. This releases E2F transcription factors, which drive the expression of genes needed for DNA synthesis. The tumor suppressor p53 can induce p21 in response to DNA damage, delaying G1 progression to allow repair.

Toward the end of G1, origin licensing occurs, preparing replication origins for duplication in S phase. This involves loading the MCM helicase complex onto origins, a prerequisite for DNA replication and a mechanism to prevent re-replication.

Variability exists in G1 length among cell types, with some cells exiting the cycle to G0. In yeast, a Start checkpoint plays a similar regulatory role. Understanding G1 is key to studying cell growth control, development, and cancer biology.