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Cell Growth and Division
All Living Things Grow and Develop
• Growth means an increase in cell number,
not cell size.
Why are Cells Small?
• Diffusion is effective only over short
distances.
– A small spherical cell has the most volume
compared to its surface area
• There is only so much DNA in the cell,
therefore the cell cannot grow too large.
– Can’t have “DNA overload”
Cell Division
• The process in which a cell divides into two
daughter cells.
• Problems solved by cell division
– Replication of DNA
• Each cell gets its own copy of genetic material
– Reducing cell volume
• Cells stay small
– There is a greater surface area to volume ratio
Process of Mitosis
• Mitosis
– A nuclear division resulting in the production of two
somatic (body) cells having the same genetic
complement as the original cell
• In Eukaryotes mitosis is asexual, meaning oneparent. Both cells will be identical to each other
• Prokaryotic Cell Division
Binary Fission
• Rapid Process
• DNA replicates itself
in a loop
• Attaches to the cell
membrane
• As the cell separates,
the DNA moves into
the two new cells.
Which Cells undergo Mitosis?
• Somatic Cells or Body Cells
– Cells that make up the body
• Examples: skin, hair, blood, bones, nerves, muscle,
etc.
Which Cells undergo Mitosis?
• Somatic Cells
• These cells are DIPLOID (2N)
– Meaning they contain two sets of chromosomes
– One from mom and the other from dad
46 chromosomes in a
human somatic cell.
One from mom and
one from dad
Why must cells reproduce?
• Growth and development
200 µm
20 µm
(b) Growth and development.
(c) Tissue renewal. These dividing
This micrograph shows a
bone marrow cells (arrow) will
sand dollar embryo shortly
give rise to new blood cells (LM).
after
the
fertilized
egg
divided,
Figure 12.2 B, C
forming two cells (LM).
• Repair and replace worn out cells
Rates of Mitosis
• Mitosis does not occur at the same rate in
each cell
• Some cells are always reproducing
– Skin
– Cells of the mouth
– Intestine lining
Rates of Mitosis
• Some cells undergo mitosis rarely or never
– Brain cells
– Muscle cells
– Nerve cells
• Bone marrow will replenish red blood cells
about every 3months.
• But, mitosis is an ongoing process
Stages of Cell Division
Mitosis
Division of the nucleus
Cytokinesis
Division of the cytoplasm
The Cell Cycle
• The series of events that cells go through as
they grow and divide.
• In summary…
– Cell grows
– Prepares for division
– Divides (forms 2 daughter cells)
The phases of the cell cycle
consists of interphase and cell division
G1 phase
Cytokinesis
interphase
telophase
M
phase
anaphase
metaphase
Cell
Division
prophase
mitosis
G2
phase
S
phase
Interphase
Divided into 3 phases
• G1
• S phase
• G2
G1 Phase
• Cell Growth
– Cells increase in size
– Honors Bio -Synthesis of proteins
and organelles
S phase
• Chromosomes are replicated
• Synthesis of DNA
• DNA is dispersed in the form of chromatin
• Chromatin – DNA coiled around proteins
Let’s take a closer look at chromosomes….
Chromosomes
• Carry genetic information
• Genetic information coded in DNA and
proteins called Chromatin
Chromosome Structure
Chromosomes before
they condense
Sister
Chromosome
in Prophase
G2 phase
• Preparation for Mitosis
– honors bio-Synthesis of organelles and
molecules required for cell division
Interphase is now complete
The cell is ready to enter mitosis and begin cell
division
G0 Phase
• A non-dividing stage
• Cells become specialized in their function at
this stage, or “mature”
• Some cells stay here forever like nerve cells
• Some cells can revert back to normal
interphase cycle.
Mitosis (Cell Division)
The 4 phases of mitosis
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Prophase
• Chromatin condenses and
chromosomes become visible.
(Chromatids are attached at the centromere)
• Nuclear envelope breaks down
• Centrosomes move toward
opposite poles.
(remember, centrosomes contain a pair of centrioles)
• Spindle fibers form.
• Nucleolus disappears.
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Metaphase
• Centrosomes are at
opposite ends of the
cell
• Chromosomes line up
across the center of the
cell.
• Spindle fibers attach
to each chromosomes
at their centromere
Anaphase
• Centromeres split.
• Sister Chromatids move
toward opposite ends of
the cell.
Telophase
• Nuclear envelope reforms around each group
of chromosomes.
• Chromosomes disperse.
• Spindle fibers break
apart.
• Nucleolus becomes
visible
*division of the cytoplasm usually occurs with
telophase
Cytokinesis
Division of the cytoplasm
• Usually occurs with Telophase
• In animal cells, the cytoplasm is pinched
inward, forming a cleavage furrow.
• Plant cells form a cell plate, which forms
from the inside out.
Cytokinesis – Animal vs. Plant Cells
Stages of Mitosis – Plant Cells (Onion Root Tip)
Controls on Cell Division
• If there is a cut or an injury the cells at the
edges of the injury are stimulated to divide
rapidly
As the healing process nears completing, the rate slows down, controls on
growth are restored and everything returns to normal
Cell Cycle Regulators
• Made of Proteins
– Cyclins regulate the timing of the cell cycle in
eukaryotic cells.
• They are like stop and go signals
Internal Regulators
• Proteins that respond to events inside the
cell
– They make sure it is ok to enter the next stage
of the Cell Cycle
External Regulators
– Also Proteins
– They direct cells to speed up or slow down the
cell cycle
– They can include growth factors
– They prevent excessive cell growth and keep
the tissues of the body from disrupting one
another.
What controls cell growth?
• As the growth factors and nutrients are used
up, the rate of cell division slows down.
• Continued cell growth and division stops.
Normal cells will….
• Cells will grow in a single layer and stop.
• Need to be anchored to grow.
• Stop growing when they bump into other
cells.
• Commit cell suicide – Apoptosis ap-a-tow'-sis)
Cell Growth
•
Uncontrolled Cell Growth
• What happens to cells that do not respond to
signals that regulate growth?
• Cancer
– a disorder in which some of the body’s own cells lose
the ability to control growth.
– A disease of the cell cycle
Cancer
• Cancer cells don’t respond to signals that
regulate growth
• They divide excessively and form masses of
cells called tumors
• They can break loose and spread throughout
the body.
What causes cancer cells?
• Cells are transformed
• What can cause cells to be transformed?
(carcinogens)
– Chemicals
• Tobacco, pesticides
– X-rays (radiation)
– UV light
– Family History
– Viruses
• HPV (human papillomavirus), increases risk of
cervical cancer.
• Oncogenes are cancer-causing genes found in
certain viruses.
Cancer
• Tumors
– If the abnormal cells remain at the original site it is
called a benign tumor
• Cannot move to other areas
• Usually do not cause serious problems
• Are not cancer
– A malignant tumor will impair the functions of one or
more organs (a person with a malignant tumor is said to
have cancer)
• These cells can spread into nearby tissues. This is called
metastasis
• Are cancerous
Benign and Malignant Tumors
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