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 […]