G 1 Events and Regulation of Cell Proliferation

Cells prepare for S phase during the G 1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G 1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G 1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G 0 ), with unduplicated DNA. Cells can be activated to reenter the cycle during G 1 . Similarly, cells in culture can be shifted between G 0 and G 1 . These switches in and out of G 1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.