Description:
<p>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.</p>