Cell Division (SL IB Biology)

• According to the cell theory, new cells are produced from pre-existing ones
• The cells that make up a multicellular organism are the result of a single cell undergoing many cycles of cell division
  • This single cell will initially form embryonic stem cells which will specialise to form the organs and tissues of the organism
• A cell that divides is known as a parent (or 'mother') cell 
  • Each parent cell will produce two daughter cells after cell division has occurred
• Two types of cell division exist
  • One results in daughter cells that are genetically identical to each other and to the parent cell
  • The other produces cells that are genetically different from each other and from the parent cell
    • This type of cell division is an important source of genetic variation within populations

• During cell division, the nucleus of the cell will divide first
  
  • This is known as nuclear division
• Once the nucleus has divided into two nuclei, the cytoplasm divides in two with one nucleus moving into each cell to create two daughter cells
  • The division of the cytoplasm is called cytokinesis

Cytokinesis in animal and plant cells

• The process differs slightly in animal and plant cells
• In animal cells:
  • A ‘cleavage furrow’ forms and separates the daughter cells
  • The cleavage furrow forms when actin and myosin proteins form a contractile ring just under the plasma membrane
  • This ring is formed at the equator (centre) of the cell
  • As the proteins contract, they pull the plasma membrane towards the centre eventually separating the cell into two daughter cells
• In plants cells:
  • A ‘cell plate’ (the precursor to a new cell wall) forms at the equator. Once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells
  • The cell plate is formed from vesicles carrying carbohydrates, lipids and proteins fusing together to create the two plasma membranes
  • After this other vesicles, carrying pectin and cellulose, deposit these substances by exocytosis in the gap between the two new membranes leading to the formation of new cell walls

Cytokinesis in an animal cell and a plant cell

• During cytokinesis, the division of the cytoplasm is usually equal
  • This produces daughter cells that are of a similar size
  • It is also important to ensure that each daughter cell receives at least one mitochondrion for cellular respiration
    • Plant cells would also need to receive at least one chloroplast in order to photosynthesise
  • These organelles can only be made by dividing a pre-existing structure
• There are however cases where the division of the cytoplasm is not equal, such as:
  • Oogenesis in humans
  • Budding in yeast

Oogenesis as an example of the unequal division of cytoplasm

• The production of ova begins in the ovaries of the female foetus before birth
  • Germinal epithelial cells will divide to form an immature ovum called a primary oocyte
• During puberty, the primary oocyte will divide to form a secondary oocyte and a smaller structure called a polar body
  • These structures are the result of the unequal division of cytoplasm
• The secondary oocyte will divide again to form an ovum and another polar body
  • The polar bodies will degenerate and form part of the final ovum

Oogenesis is an example of unequal cytokinesis