Chapter 9, Cellular Growth
and Mitosis
WHY ARE CELLS SO SMALL?
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As cells get larger, their surface area to volume
ratio keeps getting smaller.
In other words, as cells get bigger, the cell
membrane gets proportionately smaller in
relation to the cytoplasm.
WHY IS THIS A PROBLEM?
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Remember, the cell membrane is responsible for
taking in and getting rid of everything that enters
and exits the cell.
If the cell membrane is not large enough to
transport everything the cell needs in its
cytoplasm, then the cell will die.
As a consequence, when a cell gets too big, it
must reproduce and divide into another cell or
else it will stop growing and eventually die.
THE CELL CYCLE
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The life cycle of a cell known as the Cell Cycle.
The cell cycle is divided into three main stages.
The first stage is called Interphase.
Interphase is the longest stage of the cell cycle.
During interphase a cell grows, performs normal cell
functions (like making proteins), makes a copy of its
DNA, and then prepares for mitosis.
INTERPHASE
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Think of interphase as the life of a person from the
time they are born, until the time they get married.
They grow up, eat, drink, sleep (perform normal
functions), they get educated (DNA is replicated),
and start a career, buy a house, get married, start a
family etc. (prepare for mitosis).
The three main parts to interphase are
G1 – Cell grows and performs normal functions
S – DNA is replicated
G2 – Cell prepares for mitosis
MITOSIS
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The next phase of the cell cycle is Mitosis.
Mitosis is the stage in which the cell's nucleus and
nuclear material divide.
Mitosis is divided into four substages.
The next and final stage of the cell cycle is
Cytokinesis.
Cytokinesis is when the cell (cytoplasm and all parts
of the cell) divides into two cells.
CELL CYCLE
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G1 – Growth and
normal functions
S – DNA is
replicated
G2 – Prepares for
mitosis
Mitosis – Nucleus
divides
Cytokinesis – Cell
divides
THE FOUR SUBSTAGES OF MITOSIS
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Mitosis is a complex series of stages.
The first of the four substages of mitosis is called
Prophase.
In prophase, the cell's genetic material (called
chromatin) tightens and condenses into x-shaped
structures called chromosomes.
Each half of the chromosome is referred to as a
Sister Chromatid.
At the center of the chromosome, where the sister
chromatids meet is called a Centromere.
PROPHASE AND CHROMOSOMES
LATE PROPHASE
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During late prophase, the nucleolus and the nuclear
membrane disappear.
Little structures from the cytoskeleton called
centrioles start to appear and go to opposite sides
(called the poles) of the cell.
Then a net-like structure called the spindle
apparatus starts to form from long strands that
attach to the centrioles called spindle fibers.
Small strands also surround the centrioles in a starlike appearance called aster fibers.
LATE PROPHASE
METAPHASE
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The second of the four substages of mitosis is called
Metaphase.
During metaphase the chromosomes are pulled to
the center of the cell and lined up in a straight line
along the equator of the cell.
The centromeres of each chromosome attach to a
spindle fiber along the equator of the cell.
METAPHASE
ANAPHASE
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The third of the four substages of mitosis is called
Anaphase.
During anaphase, the sister chromatids are
separated at the equator of the cell.
The spindle fibers pull apart the sister chromatids at
the centromeres.
The spindle fibers begin to shorten and move toward
the centrioles (located at the poles).
The result is two identical copies of the
chromosomes at each pole.
ANAPHASE
TELOPHASE
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The last of the four substages of mitosis is called
Telophase.
Telophase is the stage of mitosis when the
chromosomes reach the poles of the cell and begin
to relax and decondense.
The nuclear membranes begin to reform around the
genetic material called chromatin.
Then the spindle apparatus gets disassembled and
the parts become recycled by the cell.
TELOPHASE
CYTOKINESIS
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The final stage of the cell cycle is cytokinesis.
This is when the cytoplasm of the cell divides.
The result is two cells with identical nuclei.
In animals, microfilaments (parts of the cytoskeleton)
pinch or constrict the cytoplasm creating a
cleavage furrow
In plants, the cell wall divides by creating a new wall
in between the two new nuclei. This new wall is
called a cell plate
CYTOKINESIS
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Cleavage furrow in
animals
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Cell plate in plants
THE CELL CYCLE
ABNORMAL CELL CYCLE: CANCER
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Cancer is when the cells in your body begin to grow
and divide uncontrollably.
Once cancer begins, cancer cells begin to crowd
out and kill normal cells.
Cancer cells spend less time in interphase than
normal cells do.
Substances and agents that are known to cause
cancer are called carcinogens.
Tobacco and too much exposure to the sun are
known carcinogens.
STEM CELLS
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Stem cells are cells that are unspecialized, but can
become specialized under the right conditions.
This means that they have not developed into a
certain type of tissue cell yet.
When embryonic stem cells are dividing in the
cleavage phase (100-150 cells) they are still
unspecialized stem cells.
Research in stem cells is highly controversial
because one common source of stem cells is from
human embryos that have been aborted.
STEM CELLS