19: Cell Division and the Cell Cycle
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- 19.1: Introduction
- Mitosis is the condensation of chromosomes from chromatin and their separation into dividing cells. Cytokinesis is the process that divides a cell into two new cells after duplicated chromosomes are safely on opposite sides of the cell. Mitosis and Cytokinesis together are a relatively short time in the cell cycle. While cell cycle times vary, imagine a cell that divides every 20 hours.
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- 19.2: Bacterial Cell Division and the Eukaryotic Cell Cycle
- The life of actively growing bacteria is not separated into a time for duplicating genes (i.e., DNA synthesis) and one for binary fission (dividing and partitioning the duplicated DNA into new cells). Instead, the single circular chromosmome of a typical bacterium is replicating even before fission is complete, so that the new daughter cells already contained partially duplicated chromosomes. Cell growth, replication and fission are illustrated below.
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- 19.3: Regulation of the Cell Cycle
- Progress through the cell cycle is regulated. The cycle can be controlled or put on ‘pause’ at any one of several phase transitions. Such checkpoints monitor whether the cell is on track to complete a successful cell division event. Superimposed on these controls are signals that promote cell differentiation.
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- 19.4: When Cells Die
- As noted, few cell types live forever; most live for a finite time. Most are destined to turn over (another euphemism for dying), mediated by programmed cell death, or apoptosis. This occurs in normal development when cells are only temporarily required for a maturation process (e.g., embryonic development, metamorphosis).
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- 19.5: Disruption of the Cell Cycle Checkpoints Can Cause Cancer
- If a checkpoint fails or if a cell suffers physical damage to chromosomes during cell division, or if it suffers a debilitating somatic mutation in a prior S phase, it may selfdestruct in response to a consequent biochemical anomaly. This is another example of apoptosis. On the other hand, when cells die from external injury, they undergo necrosis, an accidental rather than a programmed death.
Thumbnail: Life cycle of the cell. (CC BY-SA 4.0; BruceBlaus).