I.  Eukaryotic and Prokaryotic Cells.

        Living cells are divided into two types - procaryotic and eucaryotic. This
        division is based on internal complexity.

       Eukaryotes: The cells of protozoa,higher plants and animals are highly structured. These cells
       tend  to be larger than the cells of bacteria, and have developed specialized packaging and
       transport mechanisms with recognizable organelles that may be necessary to support their larger size.

       Prokaryotes: These cells are simple in structure, with no recognizable organelles. They have an
       outer cell wall that gives them shape. Just under the rigid cell wall is the more fluid cell membrane.
       The cytoplasm enclosed within the cell membrane does not exhibit much structure when viewed by
       electron microscopy.

II.  Eukaryotic Cells -- Animal Cells

   Cell Organelles                                         Structure                 Function
   Nucleus
   Nucleolus
   Endoplasmic Reticulum (ER)
   Centriole and Centrosome
   Golgi Complex
   Cytoskeleton

   Cytosol
   Mitochondrion
   Lysosome
   Peroxisome
   Vacuole
   Cell Membrane

III.  Cell Cycle

    During development from stem to fully differentiated, cells in the body alternately divide (mitosis) and
    "appear" to be resting (interphase). This sequence of activities exhibited by cells is called the
    Cell Cycle.

    Interphase:

    Interphase generally lasts at least 12 to 24 hours in mammalian tissue. During this period, the
    cell is constantly synthesizing RNA, producing protein and growing in size. By studying molecular
    events in cells, scientists have determined that interphase can be divided into 4 steps:
    Gap 0 (G0), Gap 1 (G1), S (synthesis) phase, Gap 2 (G2).

        Gap 0 (G0): There are times when a cell will leave the cycle and quit dividing. This may be a
        temporary resting period or more permanent. An example of the latter is a cell that has reached
        an end stage of development and will no longer divide (e.g. neuron).

        Gap 1 (G1): Cells increase in size, produce RNA and synthesize protein. An important cell cycle
        control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready
        for DNA synthesis.

        S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must
        be duplicated. DNA replication occurs during this S (synthesis) phase.

        Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to
        grow and produce new proteins. At the end of this gap is another control checkpoint (G2
        Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.
 

    Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of
    the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis
    is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there
    is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to
    complete cell division.