Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell cycle: one near the end of G1, a second at the G2/M transition, and the third during metaphase. Positive regulator molecules allow the cell cycle to advance to the next stage. Negative regulator molecules monitor cellular conditions and can halt the cycle until specific requirements are met.
cell cycle checkpoint
mechanism that monitors the preparedness of a eukaryotic cell to advance through the various cell cycle stages
one of a group of proteins that act in conjunction with cyclin-dependent kinases to help regulate the cell cycle by phosphorylating key proteins; the concentrations of cyclins fluctuate throughout the cell cycle
one of a group of protein kinases that helps to regulate the cell cycle when bound to cyclin; it functions to phosphorylate other proteins that are either activated or inactivated by phosphorylation
cell cycle regulatory protein that inhibits the cell cycle; its levels are controlled by p53
retinoblastoma protein (Rb)
regulatory molecule that exhibits negative effects on the cell cycle by interacting with a transcription factor (E2F)