Microtubule Involvement in Centrosomes in the Cell Cycle

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  1. Before mitosis
    • When an animal cell exits mitosis, the cytoplasm contains a single centrosome containing two centrioles; even before cytokinesis is complete, the two centrioles of each daughter cell lose their close association to one another
    • this is accomplished by the enzyme separase, which is activated late in mitosis and cleaves the proteinaceous link holding the centrioles together
  2. G1
    • Centrosome splits into two adjacent centrosomes, each containing a pair of mother-daughter centrioles
  3. S phase
    • Each centrioles of the centrosome replicates
    • -          appearance of procentriole next to each preexisting centrioles and oriented at right angles to it
    • microtubule elongation converts each procentriole into a full-length daughter centrioles
  4. G2
    • Disassembly of microtubules of the cytoskeleton occurs in preparation for reassembly as components of the mitotic spindle
  5. prophase
    formation of the mitotic spindle and breakdown of the nuclear envelope
  6. prometaphase: general overview
    • Dissolution of the nuclear envelope
    • Spindle assembly is completed and chromosomes are moved into position at the center of the cell
  7. prometaphase: beginning
    • compacted chromosomes are scattered throughout the space that was the nuclear region
    • the free ends of the microtubules are seen to grow and shrink in a dynamic fashion, searching for a chromosome
    • -          Once found, chromosomes can move along wall of the microtubule until stable association with the plus end occurs
    • -          a chromosome that is attached to microtubules from only one spindle pole is unstable
    • -          attachment of microtubules at the other spindle pole occurs
  8. prometaphase: microtubule
    • Microtubules grow out from the chromosome by incorporation of tubulin subunits at the kinetochore end
    • Congression occurs, in which the chromosomes are moved to the center of the mitotic spindle
    • -          shortening and lengthening are governed by differences in pulling forces on the two sister kinetochores
    • shortening and elongation occur primarily by loss or gain of subunits at the plus ends
  9. Metaphase
    • Aligned at the spindle equator (the metaphase plate)—with both chromatids connected by their kinetochores to microtubules from both poles
    • -          Subunits are rapidly lost and added at the plus ends (even though attached to the kinetochore, the kinetochore does not act like a cap); a net addition occurs
    • -          the minus ends experience a net loss
    • Subunits move along the chromosomal microtubules from the kinetochore toward the pole
    • Different from anaphase because, in metaphase, subunits are added to the plus ends, and in anaphase, subunits are lost from the plus ends
  10. Anaphase: beginning
    • Begins when sister chromatids of each chromosome split apart and start their movement toward opposite poles
    • Very slow to ensure accuracy
    • Poleward movement is accompanied by shortening of chromosomal microtubules lost from both plus and minus ends
  11. Anaphase A and B
    • Anaphase A: the movement of the chromosomes toward the poles
    • Anaphase B: the two spindle poles move farther apart accompanied by net addition of tubulin subunits to the plus ends of the polar microtubules
    • Subunits are added to polar microtubules and removed from chromosomal microtubules at the same time 
  12. Telophase
    • Chromosomes collect in a mass and decondense
    • Mitotic spindle disassembles
    • Nuclear envelope reforms
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Microtubule Involvement in Centrosomes in the Cell Cycle
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