The cell cycle is the series of events a cell goes through to create a copy of itself. The cell grows and replicates the RNA (ribonucleic acid) in the interphase stage and divides into two new cells during mitosis. An internal regulator is a special protein that permits the cell cycle to continue only when certain conditions have been met inside the cell.
Cyclin is the special protein within a cell that regulates the cell cycle. The amount of cyclin rises and falls with each stage of the cycle. Cyclin-dependent kinases (CDKs) are proteins that ensure a cell will not enter each cell stage until certain conditions have been met. While Cyclin is the regulatory unit, the CDK is the entity that propels cell division forward. Cyclins bind to CDKs and are activated during specific checkpoints of the cell cycle.
Several regulatory proteins are poised during the cell cycle to ensure that a cell will not enter mitosis until the chromosomes have been properly replicated. These proteins are located at the checkpoints between each stage of the cycle. The major checkpoints, during the cell cycle, are at G1 (Gap 1), G2 (Gap 2) and M (Mitosis). For example, when Cyclin A binds to CDK2, DNA (deoxyribonucleic acid) synthesis will be allowed to continue. The G2 checkpoint will activate if the DNA is damaged or incomplete. If this occurs, mitosis is postponed to allow the DNA to be repaired; otherwise, the cycle is terminated.
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Cells get bigger, produce RNA and synthesize protein in the Gap 1 phase. The G1 checkpoint is in place to ensure that the cells are big enough and conditions are suitable. If the cells are big enough and the proper amount of RNA and proteins is synthesized, then DNA synthesis can occur in the S (synthesis) phase.
Another regulatory protein, called kinetochore, is located at the M phase checkpoint. Kinetochores secrete a special protein called anaphase promoting complex (APC) that prevents anaphase from continuing until every kinetochore is attached to the spindles. This will ensure the new daughter cells will get the proper amount of chromosomes.