Download Cell Division Reproduction

Cell Division and Reproduction
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A basic tenant of biology is that cells
arise from preexisting cells. This was
first proposed by Rudolf Virchow in
1858 and is known as ‘Virchow’s
Principle’.
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In eukaryotes, such as plants and
animals, cell division allows:
– Growth of an individual from a
fertilized egg.
– The production of sperm and egg
cells.
– Asexual reproduction by
budding, grafting, or air layering.
Reproduction in prokaryotes
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Bacteria and archaea reproduce by
binary fission.
These groups have a single circular
chromosome.
As seen in the figure on the right, the
chromosome duplicates itself and the
cell elongates.
Once there is a chromosome in each
side of the elongated cell, the cell
(plasma) membrane grows inward
and divides the cell into two cells.
Each of the new cells is just like the
original cells.
Binary fission can quickly give rise to
a large population of identical
prokaryotic cells
Eukaryote chromosomes
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The number of chromosomes in
eukaryotic cells is species
dependent.
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Human cells (except sperm and egg)
have 46 chromosomes.
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Chromosomes are a combination of
DNA (which carries genetic
information) and proteins (which form
the framework on which the DNA is
held)
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The DNA of the nucleus is usually
diffuse and spread throughout the
nucleus.
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This diffuse DNA is called
chromatin.
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Chromosome duplication
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Before cell division can occur each
chromosome in the nucleus must
be replicated.
When this process is complete the
two new chromosomes are called
chromatids and are connected at
one point called the centromere.
During the production of two new
cells, each cell will get one of the
chromatids.
The Cell Cycle
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Every cell goes through a series of events from its production until it, in tern,
divides into two new cells. These events are called the cell cycle.
Most of the life of the cell is spent in interphase. At this time no activity can
be seen in the nucleus. However, this period is divided into G1, S, and G2
phases.
The G1 phase is when the cell is carrying out its normal activities. This may
last for hours, days, or even years.
If the cell is divide, at some point regular activity ceases and the
chromosomes in the nucleus duplicate to form the chromatids seen in the
previous slide. This is the S pase of the cell cycle.
The cell then passes into the G2 stage. This may last for several hours
during which time the cell continues to grow and to complete preparation for
nuclear division.
At the end of the G2 stage the cell begins mitosis. At the end of mitosis
cytokinesis occurs, and divides the cell into two new daughter cells.
The series of slides which follow examine the cell cycle and the process of
mitosis.
The Cell Cycle
Mitosis - Prophase
• The first sign of mitosis is the
condensation of DNA into the
structure of chromosome
• Note the strands that can be
seen in the nucleus
• Later in prophase the nuclear
membrane will disappear, and
• Centrioles migrate to opposite
sides of the nucleus and
spindle fibers appear.
Mitosis - Prometaphase
• At this point in time the
chromosomes can easily be
seen.
• The microtubles of the mitotic
spindle can be seen radiating
out from the centrioles.
• The nuclear membrane is
gone.
• Spindle fibers from both
centrioles have attached to
the centomere region of each
chromosome.
Mitosis - Metaphase
• Metaphase is the point in time
when all the chromosomes
are lined up in the same plane
of the cell .
Mitosis - Anaphase
• Anaphase begins as the two
chromatids separate and are
drawn to opposite side of the
cell by the receding
microtubles which are
attached to them.
• Eventually the two new sets of
chromosomes are positioned
at opposite sides of the cell.
Mitosis - Telophase
• When the nuclear membranes
reform, but while the
chromosomes are still visible
the cell is in telophase.
• At this time cytokinesis occurs
and the cell is split into two
new cells.
• Quickly the DNA unravels, the
chromosomes are no longer
seen, and the cell is back in
interphase.
animal cell division
• In the illustration to the right, a
cleave furrow is forming and
the cell is dividing into two
cells as the plasma membrane
pinches inward.
Plant Cell Division
• Plant cell division involves the
production of a cell wall to
divide a cell intro two new
cells.
• Vesicles of new cell wall
material accumulate in the
mid-plane of the cell.
• These fuse forming a partition
between the two sides of the
cell, then the plasma
membrane connects around
all sides of the new cells.
Control of cell growth
• Cell do not replicate
themselves randomly, but
divide in response to chemical
signals that they receive.
• These signals are called
growth factors, and operate at
specific points in the cell
cycle.
• If no signal is received at the
G1 checkpoint, the cell
switches to a non-dividing
state called G0
Control of cell growth
• In the illustration below you can see a model of how a growth
factor, that attaches to a membrane protein, can start a cascade of
reaction that lead to the start of the cell cycle.
Uncontrolled Cell Growth
• When a cell does not respond to normal growth factors, it can
replicate unchecked by any control.
• These cells form tumors which may be benign (confined to their
site of origin) or malignant (capable of spreading and invading
other tissues or parts of the body).
Cancer
• The spread of cancer cells to other parts of the body via the
circulatory system is called metastasis.
• Techniques to destroy tumor cells include:
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Surgical removal of the tumor
Chemotherapy
Radiation
Immune system response
Different types of tumors may have very different levels of
response to the above methods of treatment.
Homologous Chromosomes
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It is important to remember that a
normal somatic (body) cell in
humans contains 46
chromosomes.
These chromosomes consist of 23
matched pairs.
Matched pairs of chromosomes
are called homologous
chromosomes.
In humans pairs 1-22 are called
autosomes, and the last pair
which may be XX or XY are the
sex chromosomes.
Each chromosome of a pair of
autosomes carry the same gene,
but not necessarily the same exact
information.
Chromosomes in other species
• Each species has a set number of chromosomes. For example:
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Humans have 46
Cats have 38
Squirrels have 40
Porpoises have 44
Chickens have 78
Alligators have 32
Drosophila (fruit flies) have 8
• How many chromosomes does a dog have? I’ll let you find the
answer to that question!