Which phase of the cell cycle is referred to as the quiescent phase?
Quiescent phase G0 represents reversible exit from cell cycle distinct from terminal differentiation and senescence, providing flexible pause aligning proliferation with physiological needs. Cells enter G0 from early G1 when mitogens withdraw, contact inhibition activates Hippo signaling with YAP nuclear exclusion, or nutrient deprivation suppresses mTORC1. Molecularly, cyclin D transcription declines, p27Kip1 and p130 Rb2 pocket protein accumulate, assembling DREAM complex with E2F4-DP1 repressing over 800 cell cycle genes including cyclin A, cyclin B, MCM helicases, and PCNA. Ribosomal RNA transcription by RNA polymerase I drops, reducing ribosome biogenesis and metabolic rate. Chromatin acquires repressive marks but retains plasticity. Upon re-addition of growth factors, Ras-ERK induces Myc and cyclin D, Skp2 mediated degradation reduces p27, DREAM disassembles, and cells re-enter G1. Lymphocytes, fibroblasts, hepatocytes, and stem cells exemplify G0. This reversible quiescence preserves proliferative capacity, prevents stem cell exhaustion, and matches tissue renewal to injury or demand, essential for longevity.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: G0 and Quiescence Regulation.