Figure 2.

Molecular processes for M-phase control in eukaryotic cells. (a) Cdc2 protein kinase monomers combine with cyclin subunits to form dimers. Subunits of kinase Cdc2 can be phosphorylated and desphosphorylated at an activatory threonine (Thr) residue or/and an inhibitory tyrosine (Tyr) residue. All cyclin subunits can be degraded by an ubiquitin pathway. (b) Active MPF stimulates its own production, which is positive feedback. (c) But active MPF also stimulates the destruction of cyclin, which is negative feedback. Reproduced with permission from the Company of Biologistd Ltd. [218].