(1) During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA. (2) S or synthesis phase marks the period during which DNA synthesis or replication takes place. (3) During the G2 phase, proteins are synthesized in preparation for mitosis while cell growth continues. (4) Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage ( G0 ) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so.