Q.State the reason for arresting of cells in G1 phase.

Cells get arrested in G1, entering a quiescent G0 phase, when they lack the nutrients/growth-factor signals needed to proceed, or once they have terminally differentiated and no longer need to divide — controlled at the G1 checkpoint (restriction point).

The cell cycle is tightly regulated at several checkpoints, and the most important one for deciding whether a cell will continue dividing is the G1 checkpoint (also called the restriction point, R-point):

1. Cell size and nutrient/energy status — before committing to DNA replication (an expensive, essentially irreversible process once started), the cell 'checks' whether it has grown enough and has sufficient nutrients and energy reserves. If conditions are unfavourable, the cell does not proceed to S phase.
2. Growth factor availability — many cells require external growth factor signals to pass the G1 checkpoint; in their absence, the cell halts in G1.
3. DNA integrity — if DNA damage is detected, the checkpoint prevents the cell from replicating damaged DNA, holding it in G1 until repair occurs (or triggering apoptosis if damage is severe).
4. Terminal differentiation — many specialised, fully differentiated cells (e.g. neurons, cardiac muscle cells) permanently exit the cell cycle after G1, since they no longer need to divide; they enter a stable, non-dividing resting state. …