BIOCELL GEN Lecture 10 cell cycle

• Cell Cycle


  • Structure (Interphase; M phase)
  • Function [control of cell division: for organism growth, cell repair/replacement; cell multiplication (plant)]

  
• Interphase


  • Structure (G1- / S- / G2- phases)
  • Function [cell growth and doubling the genetic material in preparation to cell division]

  
• M Phase


  • Structure (Mitosis; Cytokynesis)
  • Function [nuclear and cytoplasm division]

  
• Mitosis


  • Structure (Prophase; Prometaphase; Metaphase; Anaphase; Telophase)
  • Function [genetic material division between two daughter cells]

  
• Cytokinesis


  • Structure (cleavage furrow)
  • Function [cytoplasm/cell membrane division / equal organelles distribution between two daughter cells]
• Why do cells divide?

  • Growth/development: by increasing the n◦ of somatic cells
  • Repair: by replacing cells lost through Apoptosis (programmed cell death)
  • Reproduction: through 2 different forms (Asexual: 1 single individual passes genes to its identical offspring (clone); Sexual: 2 individuals give rise to offspring with unique combinations of genes (recombination))

  
• Not all cells divide
• DNA and chromosomes


  • Prokaryotes: a single DNA molecule
  • Eukaryotes: a number of DNA molecules
  • DNA molecules in a cell are packaged into chromosomes (carrying several hundreds genes)
  • Eukaryotic chromosomes: DNA + protein (chromatin), condense during cell division
  • Every eukaryotic species has a characteristic n. of chromosomes in each cell nucleus
  • Somatic cells (nonreproductive cells) have two sets of chromosomes (diploid)
  • Gametes (reproductive cells: sperm and eggs) have 1 set of chromosomes (aploid)

  
• Eukaryotic Chromosomes Organization


  • Normally, chromosomes are spread out in a dispersed form called chromatin
  • During Interphase, in preparation for cell division, DNA is replicated
  • During mitosis, chromosomes condense
  • Metaphase chromosomes: Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), attached along their lengths by cohesins
  • The two sister chromatids are most closely attached at centromeres
  • Each duplicated chromosome contains 2 identical DNA molecules, one in each chromatid

  
• Eukaryotic Cell Cycle


  • 4 phases (G1 phase, S phase, G2 phase, M phase)
  • 3 events (DNA replication, Mitosis, Cytokinesis)
  • These events are triggered in a 'time-controlled' manner by an independent cell-cycle control system (similar in all eukaryotes)
  • Major events are separated by Gap phases (G1 and G2)

  
• Interphase


  • Appearance: Intact nuclear envelope, Dispersed (decondensed) chromatin, Interphase MT array
  • Key functions: Most cell growth and organelle biogenesis occurs, DNA replicates (in S phase), Centrosomes duplicate

  
• M Phase


  • Comprises MITOSIS and CYTOKINESIS
  • Mitosis = nuclear division
  • Cytokinesis = cytoplasmic division
  • SISTER CHROMATIDS segregate into two cells, such that each daughter cell inherits one complete set of chromosomes
  • Each daughter cell must also receive one CENTROSOME and the appropriate complements of cytoplasm and organelles

  
• Mitosis


  1 - Prophase: Breakdown of nuclear envelope, Chromosomes become more condensed and sister chromatids start to "resolve"

  2 - Prometaphase: Chromosomes align at the "metaphase plate", Kinetochore microtubules attach sister chromatids to opposite poles of the spindle

  3 - Metaphase: Chromosomes aligned at metaphase plate

  4 - Anaphase: Cohesins at centromeres is destroyed by separase, Sister chromatids separate and move towards opposite poles

  5 - Telophase: Daughter chromosomes arrive at the spindle poles and decondense, Mitotic spindle disassembles, 2 new nuclear envelopes re-assemble, Cytokinesis begins

  
• Cytokinesis in animal cells


  Contractile rings of actin and myosis create a cleavage furrow that pinches the cell in two, Fusion of intracellular vesicles with the plasma membrane insert new membrane
• Cytokinesis in plant cells

  No cleavage furrow, During telophase, vesicles derived from the Golgi Apparatus move along microtubules to the middle of the cell forming the cell plate, Cell wall material carried in the vesicles collect in the cell plate as it grows, The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell, A new cell wall forms between the 2 daughter cells from the contents of the cell plate

  
• Metaphase spindle is very dynamic
• Centrosome duplication allows the assembly of a bipolar spindle
• Kinetochores Attach Sister Chromatids to the Spindle


  Kinetochores are protein complexes that form a plaque on the surface of the centromere, In animal cells, each kinetochore binds 10-40 spindle microtubules, Kinetochore proteins binds to the sides of microtubule near its plus end, with multiple low-affinity and labile bonds that break and reform, This allows polymerization and depolymerization while the microtubule remains attached to kinetochore
• How chromosomes move during mitosis

  Microtubules are characterized by dynamic instability, They alternate between a period of growth (RESCUE) and a period of rapid disassembly (CATASTROPHE) depending on the presence or absence of a GTP capping at their end

  
• What is the result of mitotic cell division?

  
• Cell replication in eukaryotes proceeds through highly regulated stages to produce progeny cells that have identical copies of the nuclear DNA and half of the cytoplasmic content

  
• Centrosome duplication allows the assembly of a bipolar spindle• Centrosome is the microtubule-organizing center• Centrosome duplicates in a semiconservative way• Only once per cell cycle• Centrosomes and microtubules are in the cytoplasm, nuclear envelope breakdown is needed for MTsto access chromosomes