Cell cycle (eg, mitosis, meiosis, structure of spindle apparatus, cell cycle regulation)
mitosis vs meiosis

Interphase

  • G1: Grow, make organelles.
  • S: Replicate DNA.
  • G2: Grow, make organelles.

Mitosis

  • Prophase: Condense chromosomes, nuclear membrane break down, spindle assembly, centrioles move to opposite poles.
  • Metaphase: Chromosomes line up, attach to spindle at kinetochore.
  • Anaphase: Sister chromatids pulled apart.
  • Telophase: Decondense chromosomes, reform nuclear membrane, spindle break down, contractile rings (myosin + actin) assemble to begin cytokinesis.

Meiosis

  • Meiosis I
    • Prophase I: same as prophase but with synapsis + cross over.
    • Metaphase I: line up of homologous pairs.
    • Anaphase I: homologous pairs pulled apart.
    • Telophase I: same as telophase.
  • Meiosis II: same as mitosis
    • Prophase II
    • Metaphase II
    • Anaphase II
    • Telophase II

Structure of Spindle Apparatus

mitotic spindle: kinetochore, interpolar and aster microtubules
  • Kinethochore microtubules = those that attach to the chromosomes at the kinetochore.
  • Interpolar microtubules = those than span the poles, but don't attach to chromosomes.
  • Aster microtubules = those that don't span the poles.

Cell Cycle Regulation

cell cycle
  • Cyclins
    • Cyclin → Cyclin-dependent kinases (CDKs) → phosphorylates key proteins → cell cycle progresses → Cyclin then degraded by ubiquitination
      • S-cyclin → S-CDK → phosphorylates pre-replication complex → Enter S phase DNA synthesis → S-cyclin destroyed by the end of S phase.
      • M-cyclin → M-CDK → phosphorylates key proteins responsible for chromosome condensation, nuclear envelope break down, spindle formation → Enter mitosis prophase → Anaphase promoting complex (APC) ubiquitinates M-cyclin, which is destroyed by the end of mitosis.
    • Cyclin's concentration goes up and down, while CDK concentration is constant, but only active when cyclin is around.
  • Checkpoints:
    • G1 Checkpoint (aka restriction point): environment favorable? Cell big enough? Damaged DNA?
      • p53 prevents transition to S if there's DNA damage. p53 → p21 → inhibits S-CDK. Without p53, damaged DNA gets replicated, results in mutation. p53 mutation found in 1/2 of all cancers.
      • A cell may enter G0 at this stage instead of going to the S phase.
    • S Checkpoint: incompletely replicated DNA?
    • G2 Checkpoint: damaged/incompletely replicated DNA? Environment favorable? Cell big enough?
    • M Checkpoint: Chromosomes all properly attached to spindles?
      • Chromosomes without attachment to spindle sends signal that blocks activation of APC.