The cell cycle regulation directs how a cell proceeds from one phase to the next and begins mitosis. The cell cycle control system includes intracellular regulatory molecules and external triggers. They provide "stop" or "advance" signals and operate at specific cell cycle stages termed checkpoints to ensure that a particular process is completed before the cell advances to the next phase.
Cyclins and cyclin-dependent kinases (Cdks) are the primary cell cycle regulators and function at the cell cycle's G1, G2, and M phases. The regulators at the G1 checkpoint verify that appropriate conditions are available for DNA synthesis, such as cell size and growth factors. If a cell does not meet these requirements, they are prevented from entering the S phase. The cell either halts the cell cycle or moves to the G0 stage until its needs are met. The regulators at the G2 checkpoint verify that the replicated DNA is free of any damage or error; otherwise, the DNA repair mechanism repairs the damage and then allows the cell to progress to the next stage. The regulators of the M phase checkpoint ensure that the sister chromatids are correctly aligned and attached to the microtubules in the equatorial plane of metaphase. This allows the cell to advance to the anaphase stage of cell division and produce two daughter cells with an equal number of chromosomes.
If a cell fails any of the above checkpoints and continues progressing through mitosis, it accumulates mutations that may ultimately lead to uncontrolled growth or tumor formation.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 3.5: Section Title Cell Growth and Division
