Answer;
G1-phase
The first checkpoint in the cell cycle where a cell will be caused to exit the cycle it this point is not passed is the G1-phase.
Explanation;
The interphase of the cell cycle contains three phases, G1 , S and G2 phases. It begins with G1 phase (G stands for gap). During this phase, the cell makes a variety of proteins that are needed for DNA replication. When the cell passes the G1 checkpoint it enters the S phase of the cell cycle, during which DNA is replicated, after the replication, the cell leaves S phase and enters G2, where the cell prepares for mitosis or meiosis.