Why do cells go through mitosis?

1. So we can grow.

2. So we can replace damaged cells.

3. So we can replace cells that have died.


Why do cells only grow to a certain size before they have to divide?

1. “DNA Overload”: large cells would require more DNA (to make the necessary proteins) than can be supported by a single nucleus.

2. Exchanging materials: as a cell grows, its volume increases much faster than its surface area so the amount of materials needed to support a large cell can not diffuse through the cell membrane. 


The cell cycle is the series of events that cells go through as they grow and divide.

Consists of 3 phases:

1. Interphase

a. G1 phase : period of growth.
b.S phase: DNA replicates itself.
c. G2 phase: the cell prepares for mitosis.

2. M phase: period of division which includes mitosis.

3. Cytokinesis: the division of the cytoplasm




DNA goes through changes during the cell cycle:


·      Called chromatin

·      DNA is uncoiled

·      Invisible under the microscope

·      Looks like this during interphase


·      DNA coiled up like a slinky

·      Visible under the microscope

·      Called two sister chromatids

·      Sister chromatids are held together by a centromere

·      Looks like this during prophase and metaphase


·      Two sister chromatids separate

·      Each is now called a chromosome

·      Looks like this during anaphase and telophase



Gap 1(G1):

·    the cell grows in size

·    protein production is high

·    the longest part of the cell cycle


Synthesis (S):

·    the chromosomes copy themselves in a process called DNA synthesis or Replication.


Gap 2 (G2):

·    organelles are made

·    cell parts needed for division are made (centrioles)

·    usually the shortest part of the cell cycle.



Definition: the division of the nucleus






·    chromatin coils up or condenses into visible double chromosomes (two sister chromatids).

·    the nuclear envelop and nucleolus disappear.

·    the centrioles begin to migrate toward the poles (animal cells only).

·    the spindle (a foot-ball shaped structure) begins to form and the fibers attach to the centromeres of the sister chromatids.






·    the doubled chromosomes line up at the equator (middle) of the cell moved by the spindle fibers attached to their centromere.



·    the centromeres split separating the sister chromatids.

·    the spindle fibers pull the identical chromosomes towards the opposite poles.


Telophase: mainly the opposite of prophase


·    the chromosomes uncoil and become invisible.

·    the nuclear envelopes and nucleolus reappear.

·    the spindle fibers break down and disappear.

·    The cytoplasm begins to divide (cytokinesis).



Definition: the division of the cytoplasm


Animal cells: the plasma membrane pinches in along the equator causing a furrow.  The furrow deepens until the cell is pinched in half.




Plant cells: a cell plate develops along the equator of the cell and grows outward until it reaches the cell wall. New cell wall material is secreted on each side of the cell plate until separation is complete.