Concept 37 -...

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Concept 12.3 The eukaryotic cell cycle is regulated by a molecular control system. The timing and rates of cell division in different parts of an animal or plant are crucial for  normal growth, development, and maintenance. The frequency of cell division varies with cell type. Some human cells divide frequently throughout life (skin cells). Others human cells have the ability to divide but keep it in reserve (liver cells). Mature nerve and muscle cells do not appear to divide at all after maturity. Investigation of the molecular mechanisms regulating these differences provides important  insights into the operation of normal cells and may also explain how cancer cells escape controls. Cytoplasmic signals drive the cell cycle. The cell cycle appears to be driven by specific chemical signals present in the cytoplasm. Some of the initial evidence for this hypothesis came from experiments in which cultured  mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei. Fusion of an S phase cell and a G phase cell induces the G nucleus to start S phase. This process suggests that chemicals present in the S phase nucleus stimulated the fused cell.
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Concept 37 -...

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