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Cyclins and cyclin-dependent kinases (Cdks) are the primary regulatory proteins that control cell cycle progression. Cdks form complexes with cyclins to ensure all cell cycle stages are coordinated and correctly timed. These molecular factors affecting cell division operate at specific checkpoints to provide stop or advance signals, preventing the cell from progressing until earlier stages are completed without error.

At the G1 checkpoint, regulatory proteins verify that the cell has reached critical size, DNA is error-free, and sufficient nutrients and growth factors are present for DNA synthesis. If these conditions are not met, the cell switches to a resting G0 phase. Once conditions are satisfied, the cell proceeds to S phase for DNA replication.

At the G2 checkpoint, regulatory proteins check for errors in replicated DNA before the cell enters mitosis. If damage is detected, DNA repair mechanisms fix the errors before allowing progression. This checkpoint ensures that only cells with intact genetic material proceed to division, preventing the transmission of mutations to daughter cells.

The M phase checkpoint ensures sister chromatids are correctly aligned and attached to microtubules at the cell's equatorial plane. Once the regulatory protein verifies proper chromosome attachment, the cell advances to anaphase and completes division, producing two daughter cells with equal chromosome numbers.

If a cell fails a checkpoint and continues progressing through mitosis, it accumulates mutations that may lead to uncontrolled growth or tumor formation. At any point, if regulatory proteins detect irreparable DNA damage, the cell undergoes programmed death to prevent propagation of defective cells.

The cell cycle control system includes external triggers such as growth factors and nutrient availability that provide stop or advance signals at specific checkpoints. These external conditions determine whether a cell meets requirements to proceed through phases. If external signals are insufficient, the cell halts progression or enters the G0 resting phase.

Cell cycle regulation ensures cells progress through phases only when conditions are appropriate and DNA is intact. Proper checkpoint control prevents accumulation of mutations that cause uncontrolled growth. When regulation fails, cells may bypass checkpoints, accumulate genetic errors, and develop into tumors or cancerous cells.