The cell cycle control system features regulatory proteins that halt the cell cycle at G1 , G2, and mitotic phases. They help prevent the initiation of the next stage until the earlier stages are completed without any error. In addition, these key regulator proteins, called the cyclin-dependent protein kinases or Cdks, form complexes with cyclins and ensure all the stages are coordinated and correctly timed. At the G1 checkpoint, the regulatory protein checks if the cell has reached the critical size, the DNA is error-free, and sufficient nutrients and growth factors are present to enter the S phase for DNA synthesis. If the cell does not receive the necessary signal, it switches to a resting state or G0 phase. Otherwise, the DNA is replicated, and the cell encounters the G2 checkpoint, where the regulatory proteins check for errors in the DNA before entering mitosis. During mitosis, after the regulatory protein verifies that the chromosomes are attached to the spindle and are accurately aligned, the cell proceeds to division. At any point in the cell cycle, if the regulatory protein detects irreparable damage, cell death occurs.