Cyclins and CDKs Cell Cycle Regulation

A cell can switch from G0, or cell cycle arrest, to G1 once cells have attained a critical size. For multicellular organisms, growth factors and mitogens, which are substances encouraging cell division, need to be present. For unicellular organisms, nutrients must be adequate in surroundings. Once G1 is initiated, Cyclin D is synthesized and drives the G1/S phase transition. In eukaryotes, Cyclin D binds with CDK 4 and CDK 6. These two complexes can partially phosphorylate retinoblastoma tumour suppressor protein, or Rb for short. Rb is bound to E2F, a transcription factor. When the CD-CDK4 and CD-CDK6 complexes partially phosphorylate Rb, it loosens its grip on E2F. E2F can then activate transcription of the cyclin E gene. Cyclin E binds CDK2, and this complex fully phosphorylates Rb, completing its inactivation. Cyclin E also phosphorylates p27Kip1, an inhibitor of Cyclin D. Phosphorylation of p27Kip1 tags it for degradation. Degradation of this protein promotes expression of cyclin A. E2F also promo