M-Cdk Drives Transition Into Mitosis

A cell’s transition into mitosis is characterized by the activation of M-Cdk complexes, consisting of the protein kinase Cdk1—or cyclin-dependent kinase 1—bound to M cyclin.

M-Cdk complexes form when M cyclin accumulates. In most cells, M cyclin levels peak during G₂—the gap phase following the chromosomal duplication of S phase—and early mitosis.

The M-Cdk complex is phosphorylated at an active site by CDK-activating kinase, or CAK. However, the complex remains inactive, because it is also phosphorylated at two inhibitory sites by the protein kinase Wee1.

M-Cdk is activated largely by the protein phosphatase Cdc25. Cdc25 removes the phosphates that inhibit M-Cdk and suppresses the inhibitory activity of Wee1.

M-Cdk drives the transition into mitosis by activating factors necessary for early mitotic processes.

In prophase, M-Cdk activity spurs the shortening and compaction of chromosomes, known as chromosome condensation. During prophase, M-Cdk also initiates the formation of the mitotic spindle—which separates chromosomes into two daughter cells.

During prometaphase in animal cells, M-Cdk helps degrade the nuclear envelope, allowing the nucleus to break apart.

In metaphase, M-Cdk mediates the attachment of sister chromatids to opposite poles of the spindle.

M-Cdk promotes the multiphase reorganization of the Golgi apparatus, which is important for correct spindle formation and segregation of the organelle. In addition, throughout mitosis,  M-Cdk is involved in the reorganization of the actin cytoskeleton, which helps determine spindle orientation and the axis of cell division, among other functions.