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Cell Cycle Regulators
division happens only if the repairs can be made. Badly damaged DNA make p53 products, which trigger apoptosis instead of cell division.
There are regulators that will be discussed that drive the cell through the process of mitosis. The sister chromatids become separated and move to two ends of the cell. Some organelles get divided as well and cytokinesis takes place in order to completely divide the cell. The main checkpoint in mitosis is called the “metaphase checkpoint”. It checks for misaligned chromosomes and makes sure that microtubules are properly attached. Anaphase does not happen until the metaphase checkpoint occurs.
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CELL CYCLE REGULATORS
There are specific cell cycle regulators, most commonly the cyclins. There are four types of cyclins in humans: G1 cyclins, G1/S cyclins, S cyclins, and M cyclins. They are active during the named phases of the cell cycle. Most cyclins are at low levels during the cell cycle but rise to a peak at specific parts of the cell cycle.
The cyclin needs to activate or deactivate target proteins within the cell. They operate with cyclin-dependent kinases to drive the cell cycle. A typical cyclin-dependent kinase is not active but it takes the binding of a cyclin in order to make the kinase functional. As kinases, these enzymes phosphorylate target proteins. In the case of G1/S cyclins and CDK, the activated proteins are DNA polymerases.
Cyclins and CDKs or cyclin-dependent kinases are considered evolutionarily conserved because they are the same across many different organisms. Yeast has just one CDK, while humans have several different CDKs active in the cell cycle.
Maturation-promoting factor or MPF is a protein that participates in cell cycle transitions. It triggers a cell to enter the M phase of the cell cycle. M cyclin remains at a low level until mitosis is necessary. It binds to a cyclin-dependent kinase in order to make protein complexes that help to trigger the M phase. The MPF complex adds phosphates to proteins in the nuclear envelope so that it can be broken down and promotes chromosome condensation.
