A complex system of cell cycle regulation has evolved in ____________ cells with both internal and external signals.
The ____________ signals ensure the stages follow one another in their normal sequence. They also make certain that each stage is properly completed before the next stage begins.
Research has identified proteins called ____________ , which fluctuate in number as the cell cycle continues, helping to regulate the timing of events.
The correct amount of these ____________ must be present in order for the cell cycle to proceed between the checkpoint stages of G₁, ____________ , and the M stage.
The ____________ signals tell the cell whether or not to divide. Much of the time mammalian cells enter the cell cycle only when stimulated by an ____________ factor.
Hormones called ____________ are one example of an external factor; these hormones deliver a signal to the plasma membrane of target cells.