Cell Reproduction / division

• Why do cells need to divide?

  new organisms

  growth

  cell replacement
• Purpose of cell division: to produce two identical daughter cells from the original parent cell.
• What are the basic requirements for a cell to divide effectively?

  • The DNA of the parent cell must be duplicated
  • The chromosomes containing the replicated DNA must then be physically segregated into two daughter cells
  • The daughter cells must physically divide from each other.
• Binary fission

  1. chromosome replication begins. Soon thereafter, one copy of the origin move rapidly toward the other end of the cell
  2. replication continues, one copy of the origin is now at each of the cell.
  3. replication finishes. The plasm membrane grows inward, and new wall is deposited.
  4. two daughter cells result
• Binary fission

  Relatively simple process by which bacteria duplicate their DNA and segregate their chromosome into two daughter cells.

  much less complicated than the mechanisms employed by eukaryotic cells
• The cell cyle

  G1

  S

  G2

  M
• Mitosis: the physical process of segregating chromosomes into daughter cells
• What occurs in G2?

  1. intact nuclear envelope
  2. chromosomes replicated - remain indistinct as loosely pack chromatin (DNA + protein) fibres
  3. Centrosome replicated (MTOCs)
  4. Microtubules extend radially forming asters
• Prophase

  1. chromatin fibres condense - forming discrete chromosomes
  2. Nucleoli disappear
  3. centrosomes move away from each other
  4. mitotic spindles begin to form
• prometaphase

  1. breakdown of nuclear envelope
  2. Some microtubules attach to chromosomes at their kinetochores
  3. Other microtubules interact with those from opposite poles
• Metaphase

  1. centrosomes at opposite poles
  2. Chromosomes align on the metaphase plate
  3. Sister kinetochores attached to microtubules coming from opposites poles
• Anaphase

  1. begins with separation of centromeres
  2. Sister chromatids move towards opposite poles of the cell
  3. each chromatid becomes a new chromosome
  4. poles move further apart
• Telophase

  1. elongation of cell by polar microtubules
  2. daughter nucleoi begin to form at poles of cell
  3. nucelar envelopes form
  4. chromatin begins to decondense
• Mitotic spindle consists:

  1. tubules (astral, kinetochore, non-kinetochore/polar)
  2. centrosomes
  3. chromatid pair - arranged on the metaphase plate
• mitotic spindle function:

  • to organise chromatids along the metaphase plate and then to pull sister chromatids apart
• astral motors (dynein)
• astral motors (dynein)

  pull astral microtubules towards poles during prophase

  microtububles de-polymerise and shorten

  hold astral microtubules in place during metaphase and later
• Kinetochore motors

  attach chromosomes to microtubules

  pull on microtubules during anaphase, chromosomes move towards centrosomes

  Microtubules de-polymerise and get shorter
• non-kinetochore/polar motor

  motos are attached to a microtubule from either side where the polar microtubules overlap

  motor push the microtubules away in opposite directions during metaphase and anaphase

  microtubules polymerise and get longer

  cell elongates
• Motor proteins separate sister chromatids during anaphase
• How is chromosomal separation/segregation achieved?

  Combination of pushing and pulling
• Pulling (dynein)

  • kinetochore motor pull chromosomes towards the centrosome/pole
  • Astral motor pull centrosomes toward inner face of the plasma membrane
  • both shorten depolymerise microtubules
• Pushing (kinesin)

  • non-kinetochore/polar motors add subunits (polymerises), microtubules drive the poles of the spindle apart. This elingates the cell to aid telophase/cytokinesis
• When does the physical separation of chromosome pair by the mitotic spindle?


  in anaphase
• Physical separation of chromosomes in anaphase

  1. proteins holding sister chromatids together are inactived
  2. chromatids separate
  3. kinetochore microtubules have a motor proteins (dynein) which walk a chromosome to the nearest pole
  4. microtubules shorten by depolymerisation at their kinetochore ends
  5. non-kinetochore microtubules elongate whole cell during anaphase (motor proteins -kinesine, attach to microtubules and lengthen them by addition of subunits)
• cytokinesis in animal cell

  1. microfilaments form a ring at the furrow
  2. Ring contracts - owing to interaction between actin and myosin filaments
  3. furrow deepens until cell is pinched in two
• Cytokinesis in plant cells

  1. cell plate forms at equatorial plane of the cell
  2. cell wall forms- form plate content