The cell cycle is a crucial process that ensures proper cell division and function, and it is regulated by specific checkpoints that act like stop signs. These checkpoints are essential for preventing errors from accumulating and ensuring that the cell does not prematurely progress to the next phase. There are four major checkpoints in the cell cycle: G1 checkpoint, S checkpoint, G2 checkpoint, and M checkpoint, each serving a unique purpose in maintaining cellular integrity.

The G1 checkpoint occurs at the end of the G1 phase of interphase. Here, the cell assesses whether its DNA is free of damage before proceeding to the S phase, where DNA replication occurs. If any errors are detected, the cell will halt and repair the DNA to prevent the replication of faulty genetic material.

Following the G1 checkpoint, the S checkpoint takes place at the end of the S phase. This checkpoint ensures that DNA has been accurately replicated. The cell verifies the integrity of the newly synthesized DNA and attempts to correct any errors that may have arisen during replication.

Next is the G2 checkpoint, which occurs at the end of the G2 phase, just before mitosis (M phase). At this checkpoint, the cell checks for the presence of all necessary enzymes and proteins required for successful mitosis and cytokinesis. If any components are missing, the cell will pause to ensure everything is in place before proceeding.

Finally, the M checkpoint, also known as the metaphase checkpoint, occurs during metaphase of mitosis. The cell verifies that all chromosomes are properly aligned at the cell's equator and that spindle fibers are correctly attached. This alignment is critical for ensuring that each daughter cell receives the correct number of chromosomes during division.

These checkpoints are vital for regulating the cell cycle. If any checkpoint fails, it can lead to uncontrolled cell division, which is a hallmark of cancer. Understanding these checkpoints and their functions is essential for grasping how cells maintain their integrity and function properly throughout the cell cycle.