cell division

• Cell division is necessary for the growth of multicellular organisms, the renewal of cells in multicellular organisms, and the production of differentiated cells in multicellular organisms
• Prokaryotic cells


  Have no nucleus, have a single circular chromosome, divide by binary fission
• Eukaryotic cells


  Must divide their nucleus as well as the rest of the organelles, genetic material replicates before nucleus divides
• Cell division


  • Can be Mitosis or Meiosis

  
• Mitosis


  Results in genetically identical eukaryotic cells (diploid), occurs in somatic cells
• Meiosis


  Results in genetically different eukaryotic cells with half the genetic material (haploid), occurs in gametes

  
• Haploid cells


  Have one complete set of chromosomes

  
• Diploid cells


  Have double set of chromosomes

  
• DNA in a cell constitutes the cell's genome, DNA molecules in a eukaryotic cell are packaged into chromosomes

  
• Before cell division


  DNA is replicated and the chromosomes condense, each duplicated chromosome has two sister chromatids

  
• Centromere


  The narrow "waist" of the duplicated chromosome, where the two chromatids are most closely attached

  
• Cell cycle


  • Consists of Mitotic (M) phase and Interphase
• Interphase


  G1 phase, S phase, G2 phase
• Mitotic spindle


  Structure made of microtubules that controls chromosome movement during mitosis
• Centrosome


  Microtubule organizing center, contains a pair of centrioles in animal cells
• Plant cells do not have centrosomes but they do make perfect mitotic spindles
• Metaphase plate


  Imaginary structure at the midway point between the spindle's two poles where the chromosomes line up

  
• Kinetochores


  Protein complexes associated with centromeres, microtubules attach to kinetochores
• Mitosis


  Prophase, Prometaphase, Metaphase, Anaphase, Telophase

  
• Cytokinesis


  Division of cytoplasm, formation of new cell membrane, separation of cells
• Cell cycle control system


  Directs the sequential events of the cell cycle, has checkpoints where the cell cycle stops until a go-ahead signal is received
• G1 checkpoint


  Most important checkpoint, if cell receives go-ahead signal it will usually complete the cell cycle, if not it will exit the cycle into G0 phase
• Cyclin-dependent protein kinases (Cdks)

  Enzymes that catalyze the phosphorylation of various proteins necessary for cell cycle events, their activity is controlled by cyclins
• Cyclins


  Regulatory proteins essential for the enzymatic activation of Cdks, their concentration fluctuates cyclically throughout the cell cycle
• MPF (maturation-promoting factor)


  A cyclin-Cdk complex that triggers a cell's passage past the G2 checkpoint into the M phase
• Activation of M-Cdk


  Requires phosphorylation and dephosphorylation of specific sites, dephosphorylation of inhibitory sites by a phosphatase causes the sudden, explosive increase in M-Cdk activity
• Activated M-Cdk


  Indirectly activates more M-Cdk, creating a positive feedback loop
• Phosphorylated


  Having a phosphate group added to a molecule
• Removal of inhibitory phosphate groups from a specific protein phosphatase

  Activates M-Cdk
• Activation of M-Cdk by selective phosphorylation and dephosphorylation


  Causes a sudden, explosive increase in M-Cdk activity that rapidly promotes the cell in the M phase
• 1. cyclin-Cdk complex


  Enzymatically inactive when first formed
• Activation of M-Cdk


  1. Cdk is phosphorylated at sites necessary for its activation and other sites that inhibit its activity

  2. Finally activated by a phosphatase that dephosphorylates the inhibitory phosphate groups
• Activated M-Cdk indirectly activates more M-Cdk
• Activated M-Cdk


  Phosphorylates and activates more molecules of activating phosphatase, which can now activate more M-Cdk complexes
• Activation of M-Cdk is explosive
• Sudden drop in cyclin concentration during mitosis

  1. Due to its rapid degradation by the ubiquitin-dependent proteolytic system

  2. Each molecule of M-cyclin covalently binds many ubiquitin molecules and leads it to degradation in the proteasome

  3. Ubiquitination of cyclin ultimately causes its destruction
• Destruction of cyclin


  Inactivates Cdk
• Anaphase promoting complex (APC)


  Protein complex that adds ubiquitin to cyclin and other proteins involved in mitotic regulation
• APC is not active at all stages of the cell cycle


  At advanced phases of mitosis, M-Cdk is activated, which leads to APC complex activation, which leads to ubiquitination and degradation of M-cyclin and thus inactivation of M-Cdk

  
• 1. Cdk contributes to its own deactivation