Molecular Factors Affecting Cell Division

Cell division, or the mitotic phase of the cell cycle, is controlled by external chemical stimuli, such as growth factors and hormones, and internal factors, such as cell cycle regulators.

The cell cycle is regulated by a combination of proteins called cyclins and cyclin-dependent kinases or Cdks. Concentrations of different cyclins fluctuate with the different cell cycle phases.

As a cell nears the end of the G2 phase, the cyclin B level rises. At high concentrations, cyclin B forms a complex with Cdk1 called the M-phase promoting factor or MPF.

MPF activity is regulated by phosphorylation and dephosphorylation events that determine if the cell undergoes division or not.

Until the end of G2, Wee1 kinase phosphorylates tyrosine residues on MPF and keeps it inactive. This prevents cell cycle entry and halts the cell at the G2/M-phase.

Once Cdc25 dephosphorylates MPF, a Cdk-activating kinase phosphorylates a conserved threonine on MPF and activates it.

Now the cells undergo mitosis and produce two daughter cells.