Control Of Cell Division

Recall that a cell spends most of its time in interphase, the time between cell divisions. What triggers a cell to leave interphase and begin dividing? In eukaryotes, proteins regulate the progress of cell division at certain checkpoints. This system of checkpoints can be thought of as a kind of "traffic signal" for the cell. Certain feedback signals from the cell can trigger the proteins to initiate the next phase of the cell cycle, much as a green light signals traffic to move forward. Other feedback signals from the cell can trigger the proteins to halt the cycle, just as a red light signals traffic to stop.

Control occurs at three main checkpoints. These checkpoints are illustrated in Figure 8-10 on the next page. 1.Cell growth (Gj) checkpoint. Proteins at this checkpoint control whether the cell will divide. If the cell is healthy and has grown to a suitable size during the G1 phase, proteins will initiate DNA synthesis (the S phase). The cell copies its DNA during this phase. If conditions are not favorable for DNA synthesis, the cell cycle will stop at this point. The cell cycle may also stop at this checkpoint if the cell needs a rest period. Certain cells pass into the G0 phase at this checkpoint. Many cells that have passed into the G0 phase will never divide again.

2. DNA synthesis (G2) checkpoint. At this point in the G2 phase, DNA repair enzymes check the results of DNA replication. If this checkpoint is passed, proteins will signal the cell to begin the molecular processes that will allow the cell to divide mitotically.

3. Mitosis checkpoint. If a cell passes this checkpoint, proteins signal the cell to exit mitosis. The cell then enters into the Gj phase, the major growth phase of the cell cycle, once again.

When Control Is Lost: Cancer

The proteins that regulate cell growth and division are coded for by genes. If a mutation occurs in one of these genes, the proteins may not function properly. Cell growth and division may be disrupted as a result. Such a disruption could lead to cancer, the uncontrolled growth of cells. Cancer cells do not respond normally to the body's control mechanisms. Some mutations cause cancer by overproducing growth-promoting molecules, which can lead to increased cell division. Other mutations may interfere with the ability of control proteins to slow or stop the cell cycle.

figure 8-10

The cell cycle in eukaryotes is controlled at three inspection points, or checkpoints. Many proteins are involved in the control of the cell cycle.

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