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Cell Division

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  1. Cell Division

  2. Cell Increase and Decrease • Cell division increases the number of somatic cells • Consists of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm) • Apoptosis, programmed cell death, decreases the number of cells

  3. The Cell Cycle • Cell cycle: orderly set of stages that take place between the time a cell divides and the time the resulting cells also divide.

  4. The Stages of Interphase • Most of the cell cycle is spent in interphase • In between episodes of mitosis • Cell carries on its usual functions • Prepares to divide (grows larger, number of organelles doubles, amount of DNA doubles) • Lasts 20 hours in mammals

  5. The Stages of Interphase • 3 stages • G1 occurs before DNA synthesis • S includes DNA synthesis • G2 occurs after DNA synthesis • G = growth • S = synthesis • M = mitosis

  6. The Stages of Interphase • G1 stage • Cell doubles its organelles • Accumulates materials needed for DNA synthesis

  7. The Stages of Interphase • S stage • DNA replication occurs • At the beginning of stage, each chromosome is composed of one DNA molecule (chromatid). At the end, each chromosome consists of two identical DNA molecules (sister chromatids) • DNA replication has resulted in duplicated chromosomes

  8. The Stages of Interphase • G2 stage • Cell synthesizes proteins needed for cell division • G0 stage • “Resting stage” • Cell is neither dividing nor preparing to divide • Ex. nerve cells

  9. The Mitotic Stage • Sister chromatids of each chromosome separate, becoming daughter chromosomes that are distributed to two daughter nuclei • After cytokinesis, two daughter cells are left • About 4 hours in mammal cells

  10. Control of the Cell Cycle • Controlled by internal and external signals. • Growth factors: hormones that act as external signals that regulate mitosis

  11. Control of the Cell Cycle • The stop signs represent checkpoints when the cell cycle can possibly stop

  12. Control of the Cell Cycle • Cyclin: protein that control the progression of cells through the cell cycle • Must be present from G2 to M, and G1 to S

  13. Control of the Cell Cycle • G1 checkpoint: cell cycle will stop if DNA is damaged • In mammals, protein p53 stops cell cycle at G1. • If p53 cannot repair DNA, will bring about apoptosis

  14. Control of the Cell Cycle • G2 checkpoint: Cell cycle stops if DNA has not finished replicating • If DNA is damaged, stopping the cell cycle allows time for damage to be repaired

  15. Control of the Cell Cycle • M checkpoint: Mitosis stops if chromosomes not properly aligned

  16. Apoptosis • Programmed cell death • Cell fragments engulfed by white blood cells or neighbouring cells

  17. Apoptosis • Cells harbour the enzymes that bring about apoptosis • Held in check by inhibitors • Unleashed by internal or external signals • “Initiators” receive signal to activate “executioners” which activate enzymes that dismantle cell • ex. executioners turn on enzymes that tear apart the cytoskeleton and enzymes that chop up DNA.

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