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Cell Cycle

cell cycle - 5 phases 

  • G1 – primary growth phase of cell
    • includes major part of a cell’s life for most organisms
  • S - phase where genome is replicated
  • G2 - 2nd growth phase; preparations made for separation of genomes
    • organelles replicate, chromosomes condense, microtubules assemble
  • interphase - collective name for G1, S, G2 phases
  • M (mitosis) - phase where microtubules pull sister chromatids apart
    • divided into prophase, metaphase, anaphase, telophase
  • C (cytokinesis) - cytoplasm divides, forms 2 daughter cells
    • actin acts as drawstring to pinch animal cells in 2
    • plate forms between dividing cells w/ cell walls
  • embryonic cells have shortest cell cycles
  • G0 phase - resting state before DNA replication
    • most cells in body are in this state at any given time
    • neurons/muscle cells never leave this phase after maturing

interphase - prepares for mitosis 

  • major portion of growth during G1 phase
  • chromosome creates 2 sister chromatids attached at centromere during S phase
  • kinetochore - protein disk bound to specific DNA sequence at centromere
  • proteins made, organelles produced during G1/G2 phases
  • DNA only replicates during S phase
  • condensation - DNA coils together w/ help of motor proteins
  • centrioles - microtubule-organizing centers that form during G2
  • tubulin - protein that makes up microtubules

mitosis  

  • prophase - forming mitotic apparatus
    • begins when condensed chromosomes become visible to light microscope
    • ribosomal RNA synthesis stops when area of chromosome that codes for rRNA condenses
    • centrioles move towards poles as spindle fibers form between them; spindle apparatus made of microtubules form
    • nuclear envelope breaks down, gets absorbed by endoplasmic reticulum during spindle formation
    • aster - radial arrangement of microtubules on centrioles towards membrane; braces centrioles against membrane; no asters in plant cells
    • microtubules must link sister chromatids to opposite sides or they won’t separate later
  • metaphase - centromere alignment
    • chromosomes align in center of the cell in circular array
    • metaphase plate - imaginary plane perpendicular to axis of chromosome circle
  • anaphase - shortest phase
    • centromeres split in 2, freeing sister chromatids
    • separase - enzyme that cleaves the cohesin protein holding the chromatids together
    • anaphase-promoting complex (APC) - makes centromeres divide at the same time
    • poles move apart, centromeres move towards poles
    • microtubules shortens as tubulin subunits are removed (microtubules don’t contract)
  • telophase - nuclei reforms
    • spindle disassembles
    • microtubules broken down into tubulin that can be used for cytoskeleton of daughter cells
    • nucleus forms around sister chromatids

cytokinesis - phase where cell actually divides 

  • relocation of organelles takes place in S/G2 phase
  • cleaves cell into equal halves
  • animal cytokinesis - uses constricting actin filament belt
    • actin filaments slide by each other, forms cleavage furrow, eventually slices into cell’s center
  • plant cytokinesis - creates cell plates between daughter cells
    • middle lamella - space between daughter cells, filled w/ pectins
  • fungi/protist cytokinesis - nucleus doesn’t dissolve during mitosis
    • nucleus divides after mitosis completes
  • nothing determines how organelles get distributed
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