L9-12 cell division

• why do cells need to divide?

  • new organisms ->unicellular organisms
  • growth-> adding more cells
  • cell replacement-> programmed cell death, skin exfoliation
• purpose of cell division and how:

  produce 2 genetically identical daughter cells

  1. DNA of parent cell duplicates
  2. chromosomes containing replicated DNA is segregated into 2 daughter cells
  3. daughter cells divided by cytokinesis
• prokaryotic cells divide by:


  binary fusion
• eukaryotic cells divide by:


  mitosis
• how does binary fusion occur?

  • chromosomes replicated
  • each copy of the origin move to opposite ends of the cell
  • plasma membrane grows inwards, new cells formed
• define mitosis:


  physical process of segregating chromosomes into daughter cells
• how many pairs of chromosomes does a human have?

  23 pairs
• cell cycle steps:

  1. G1 -> cell grows, prepares to replicate DNA
  2. S -> cell grows, synthesise DNA
  3. G2-> cell grows, prepare for mitosis
  4. mitosis
  5. cytokinesis
• G2 of interphase characteristics:

  • intact nuclear envelope
  • chromosomes replicated
  • centrosomes replicated
  • microtubules extend forming asters (which become the centrosomes)
• stages of mitosis: 


  PMAT: prophase, prometaphase, metaphase, anaphase, telophase
• prophase characteristics:

  • chromatin fibres condense (chromosomes condense)
  • nucleoli disappear
  • centrosomes move towards opposite ends of the cell
  • mitotic spindle begins to form
• prometaphase characteristics:

  • breakdown of nuclear envelope
  • microtubules begin to attach to chromosomes at their kinetochores
  • other microtubules interact with those from opposite poles
• what is a kinetochore?


  A protein structure on the centromere of a chromosome that attaches to microtubules during cell division
• metaphase characteristics:

  • centrosomes at opposite poles
  • chromosomes align on metaphase plate
  • sister kinetochores attach to microtubules from opposite poles
• anaphase characteristics:

  • separation of centromeres
  • sister chromatids move towards opposite sides of the cell
  • chromatids become new chromosome
  • poles move further apart
• telophase characteristics:

  • elongation of cell by polar microtubules
  • daughter nucleoli begin to form at poles
  • nuclear envelopes form
  • chromatin starts to decondense
  • cleavage furrow
• mitotic spindle function:


  organise chromatids along metaphase plate

  pull sister chromatids apart
• working of mitotic spindles consist of:

  • tubules (astral, kinetochore and non-kinetochore/polar)
  • centrosomes
  • chromatid pairs
• astral motors (dynein) work by:

  • pull astral microtubules towards poles during prophase
  • shortens microtubules
  • hold astral microtubules in place in metaphase and later
• kinetochore motors (use dynein) work by:

  • attach chromosomes to microtubules
  • pull on microtubules during anaphase to move chromosomes towards centrosomes
  • microtubules get shorter
• polar (non-kinetochore) motors (use kinesin) work by:

  • walks in 2 directions at the same time
  • increase microtubule length, polymerise them, causing cell to elongate
  • motors push microtubules in opposite directions in metaphase and anaphase
• motor proteins separate sister chromatids during _

  anaphase
• pulling force from _

  dynein
• pushing force from _


  kinesin
• pulling force:

  • kinetochores pull chromosomes to centrosomes (poles)
  • astral motors pull centrosomes towards inner face of plasma membrane
  • both motors shortened and depolymerise microtubules
• pushing force:

  • polar motors add subunits to drive spindles apart (elongates cells to help telophase and cytokinesis)
• cytokinesis in animal cells characteristics:

  • microfilaments form ring at the furrow
  • ring contracts (actin and myosin filaments interacting)
  • furrow deepens until cell pinches into 2
• cytokinesis in plant cells characteristics:

  • cell plate forms at equatorial plane
  • cell wall forms from the plate contents
• identify stages of cell division:

  a= metaphase

  b= prophase

  c= prometaphase

  d= telophase

  e= interphase

  f= anaphase

  
• why is regulation of cell division important?

  • development ->correct size/shape
  • injury -> divide following injury but stop when damage is repaired
  • adaptive responses -> in low oxygen more RBCs are made, lymphocytes division is triggered by response to antigen
• cell division is regulated in 2 ways:

  1. external signals from other cells e.g. growth factors
  2. internal signals e.g. cyclin dependent kinases
• external signals:

  • promote (mitogens (growth factors)) or inhibit (anti-mitogens) cell division
  • without sustained mitogen stimulation cells will not progress through the G1 checkpoint -> will instead enter G0 (quiet phase/quiescence)
• mitogens lead to cell growth
• mitogens bind to receptors in the plasma membrane and promote cell growth by increasing _ expression


  cyclin
• example of external growth factor:

  platelet-derived growth factor
• internal signals control the cell cycle using promoting factors
• what are the 3 major checkpoints in the cell cycle:

  1. G1 checkpoint
  2. G2 checkpoint
  3. metaphase checkpoint (spindle assembly checkpoint)
• what are cell cycle checkpoints?


  Control points in the cell cycle where regulatory proteins assess if conditions are favourable for the cell to proceed to the next phase
• why are checkpoints important?

  • prevent untimely exit from each cell cycle phase
  • prevent genetic instability
  • prevent cancer
  • enables cells to stop dividing if correct signals are not present
• G1 checkpoint checks:

  • size of cell
  • has it received appropriate external signals