Description:
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Cells prepare for S phase during the G
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phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G
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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
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that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G
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), with unduplicated DNA. Cells can be activated to reenter the cycle during G
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. Similarly, cells in culture can be shifted between G
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and G
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. These switches in and out of G
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are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.
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