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