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
II. Eukaryotic Cells -- Animal Cells
Endoplasmic Reticulum (ER)
Centriole and Centrosome
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
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
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
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.