Imagine the life cycle of a cell if it were to live for 24 hours….



  •               the first 10 hours are spent in the first growth period (G1) where the cell differentiates into a specific kind or type that has a specific function
  •               the next 9 hours are spent in the synthesis period (S) where the DNA is produced and every chromosome is duplicated -- giving rise to carbon copies called sister chromatids
  •               the next 4 hours are spent in the second growth period (G2) where the cell undergoes final preparation for the mitosis stage



  •               the last hour is taken up the mitosis which is the most visibly obvious stage


The process throgh which:  (i) single cells are reproduced and (ii) multicellular organisms grow.  The cells which arise from the original parent cells are called daughter cells.

  •               once the cell reaches a maximum size, it usually has two alternatives:  to reproduce or to die
  •               mitosis ensures that vital cells are replaced by new ones once they wear out and die
  •               mitosis must be precise - the nuclear material must be duplicated precisely, since the daughter cells must have the genetic capacity to carry out all of the many cell processes of the parent
  •               mitosis is a continuous process that has very distinct events or phases
  •               these phases are analogous to frames or "snapshots" of a motion picture


1.             PROPHASE

  •               nucleolus disappears
  •               nuclear membrane begins to disappear
  •               chromosomes coil, shorten, and thicken, and are thus visible
  •               chromosomes are attached at a single site called a centromere
  •               centrioles migrate to opposite poles of the cell 
  •               spindle fibres form (like clothes lines fixed at both ends)

2.             METAPHASE

  •               sister chromatid pairs of each chromosome align themselves along the equator of the cell

3.             ANAPHASE

  •               the centromere breaks in half and both chromosome pairs separate into daughter chromosomes
  •               each chromatid migrates (along the spindle fibres) to opposite ends of the cell  

4.             TELOPHASE - the reverse of all the events of phrophase take place

  •               centrioles replicate
  •               chromosomes uncoil
  •               nuclear membrane reforms
  •               nucleolus reappears
  •               cell begins to divide


C.            Cytoplasmic Division


Cytokinesis:          The division of the cytoplasm to ensure the equal distribution of organelles to each of  the daughter cells.  Cytokinesis is different in plant cells and animal cells.


  •                                                                 - plant cells produce a cell plate between the two nuclei of the daughter cells
  •                                                                   that effectively separates the cytoplasm of each cell
  •                                                                 - animal cells produce a cleavage furrow around the middle of the parent cell
  •                                                                   that effectively "pinches" it into two equal halves