THE LIFE CYCLE OF A CELL

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

  Interphase

     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

  Mitosis

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

  Definition:     The process through 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.

  The Phases of Mitosis:

 1.      PROPHASE

•    nucleolus disappears

•    nuclear membrane begins to disappear (p. 57, fig. 3.2)

•    chromosomes coil, shorten, and thicken, and are thus visible (p. 57, fig. 3.3)

•    chromosomes are attached at a single site called a centromere

•    centrioles migrate to opposite poles of the cell (p. 57, fig. 3.2)

•    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 prophase 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