What is the function of cyclin and cdks in cell cycle?
CDKs (cyclin dependent kinases) are, as the name implies, kinases (enzymes that phosphorylate other proteins) that are dependent of a family of proetins called kinases. Cdks do not have any phosphorylation enzymatic activity unless they are bound to cyclins. When they are active, as in the case during the beginning of mitosis, they phosphorylate proteins that control chromosome condensation, nuclear envelope breakdown and spindle assembly. As you can imgine therefore, their activities can be controlled by regulating cyclins, which is in fact the case. Cyclins are called so because their synthesis and degradation has a cyclic manner in the cell. When the cell is to exit mitosis, cyclins will be degraded after ubiquitination (addition of ubiquitins, which mark proteins for degradation by the proteosome) by enzymes called ubiquitin ligases. One of the most well studied of such ligases is the Anaphase Promoting Complex (APC), which ubiquitinates M-cyclins (those that promote mitoic events)
