If cells sense such conditions after the restriction point, they complete the currentround of the cell cycle and exit to G
0 during the subsequent G
1 phase. The G
2 phase is shorter than G
1, but it, too, consists of important mechanisms that control the completion and fidelity of DNA replication and that prepare the cell for entry into mitosis. Whereas some conditions cause cells to enter the G
0 phase, others trigger
apoptosis. One such signal that may trigger apoptosis is if a cell's DNA has undergone significant damage.
After the restriction point, at the transition from the G2 to the M phase, another checkpoint occurs. Mitosis is prevented if DNA damage has occurred or if genomic replication is not complete. The final key checkpoint occurs at the end of mitosis, when the cycle stops if chromosomes are not properly attached to the mitotic spindle.
Proteins That Regulate the Cycle
The mammalian cell cycle control system is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins.
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