Biol 19

• Prokaryotic cell cycle

  • B period: cell growth
  • C period: DNA replication
  • D period: fission

  
• Bacterial growth

  Exponential in theory
• Phases of bacterial growth

  • Lag: cells gather nutrients and prepare for division
  • Log: exponential reproduction (geometric progression)
  • Stationary: nutrients are limited
  • Decay: poisonous metabolites accumulate, nutrients run out
• Resources are never limitless
• Exponential growth


  In optimal conditions
• Binary fission in prokaryotes

  1. DNA replication

  2. Cell expansion & DNA migration

  3. Contraction of the Z-ring

  4. Septum formation

  5. Division (separation)

  
• ParM


  Prokaryotic actin homologue, moves plasmids to opposite ends of the cell prior to division

  
• FtsZ


  Prokaryotic tubulin homologue, forms a band around the midpoint of the cell - the "Z ring", creates a strangulation in the cell during division
• Endospore formation


  1. Fission starts but stops at cell expansion

  2. A double-membrane capsule is formed around one chromosome

  3. The rest of the cell dissolves

  
• Endospore


  Resistance structure that includes a double membrane, one chromosome, a few copies of DNA pol and ribosomes, excludes water (mostly)
• The endospore can ride it out in stasis until conditions improve
• Endospore germination


  Outer membrane breaks and inner spore emerges
• Eukaryotic cell cycle

  M is for "mitosis", G's are "gap" phases, S is the "synthesis" phase

  
• G1 phase


  Growth and RNA synthesis, no DNA synthesis
• S phase


  Synthesis of DNA, proteins and DNA replication

  
• G2 phase


  Further growth and synthesis of mitosis-related RNA and proteins

  
• M phase


  Can be subdivided into distinct mitotic phases: Prophase, Prometaphase, Metaphase, Anaphase, Telophase

  
• G0 phase


  Cell synthesises proteins and ribosomes, and reproduces organelles in preparation for cell division

  
• Cohesin proteins


  Trimeric proteins that hold sister chromatids together
• Centrosome


  A pair of centrioles replicated during the S phase, along with DNA (but in the cytoplasm)

  
• Condensin proteins


  Contribute to chromatid condensation
• Karyotype


  Diploid organisms have pair of homologous chromosomes
• Prometaphase


  Spindle formation and nuclear dissolution
• Kinetochore proteins


  Link the mitotic chromosomes to the spindle
• Anaphase


  Motor proteins ratchet along the microtubules, cell also elongates
• Cytokinesis in animal cells

  Microfilaments divide the cell
• Mitosis in plant cells

  • Cells expand during interphase, never during anaphase
  • No centrosomes but still have a mitotic spindle
  • No furrow in cytokinesis, a phragmoplast forms a scaffold and vesicles deposit new cell wall

  
• G0 phase

  Cells can exit the cycle and enter a permanently (neurons) or temporarily (liver cells) non-dividing state
• Contact inhibition


  A major trigger for cells to enter the G0 state
• Cell reproductive activity


  • Cells that reproduce continuously (e.g. epithelial, bone marrow)
  • Cells that reproduce if required (e.g. white blood cells, liver cells)
  • Cells that cannot/will not reproduce (e.g. neurons, red blood cells)
• Checkpoint regulators


  Regulatory proteins that give the "go ahead" at each cell cycle checkpoint
• G1 checkpoint


  Controls the entry into the S phase, the "commit to division" checkpoint
• G2 checkpoint


  Controls the entry into mitosis, checks if DNA replication and repair is complete

  
• M checkpoint


  Controls the entry into anaphase, checks if every chromosome is attached to kinetochore microtubules