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Cellular Reproduction
http://embryology.med.unsw.edu.au
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Cells must reproduce for several
reasons:
 Growth and Development – as an
organism grows, it needs more cells (baby
vs adult) since the size of a single cell is
limited
 Repair and Replacement – injuries or
replacement of dead cells (cut on skin –
more skin cells needed to repair the cut)
 Asexual Reproduction – single celled
organisms must make more of themselves
▪ Binary fission – cell reproduces an identical
copy of itself
▪ Includes 2 stages 1. DNA replicates, 2. cell
divides
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Cells have a size limitation.
Cells grow until they reach their
size limits, then they either stop
growing or divide.
Cells that divide can then each
grow to the size limitation for
that type of cell, allowing the
organism to increase in size.
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Most cells are smaller than a period (.) typed
on a paper.
The key factor that keeps cells small is the
ratio of the cells surface to area volume.
 Surface area of the cell – area covered by the
plasma membrane (structure through which all
nutrients and waste products must pass)
 Volume – space taken up by inner contents of cell,
includes organelles in the cytoplasm and nucleus
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As a cell grows the ratio of surface area to
volume decreases
Cells that are too BIG have difficulty
supplying nutrients and expelling enough
waste products – too far to travel
Small cells have a higher ratio of surface to
volume – more efficient!

Small cells are more efficient.
 Transport of substances – substances
move by diffusion or by motor
proteins along the cytoskeleton –
shorter distance = faster
 Cellular communication – signaling
proteins work more efficiently
because they travel a shorter
distance
▪ For example: The signals that trigger
protein synthesis might not reach the
ribosome fast enough for protein
synthesis to occur to sustain the cell
Cytoskeleton used for
transporting within a cell

Cell division
 prevents the cell from becoming too large
 The way the cell reproduces allowing growth and
healing

Cells reproduce by a cycle of growing and
dividing called the cell cycle

Each time a cell goes through one complete
cycle, it becomes two cells.

Interphase – the stage during which the cell
 grows
 carries out cellular functions
(is alive and doing cell stuff)
 and replicates DNA
 3 substages

Mitosis – divided into 4 substages
 Cell’s nucleus and nuclear material divide

Cytokinesis – occurs near the end of mitosis
 Cytoplasm is divided resulting in 2 daughter cells
How long the cell cycle
takes depends on the
cell that is dividing.
 In Eukaryotic cells, the
length of the cell cycle
varies from 8 minutes
to one year.
 For most normal
animal cells – 12 to 24
hours
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Most of the cell’s life is spent in Interphase
Three substages:
 G1, S, G2
 G1 (Gap 1) – cell grows and
performs normal functions
▪ Period immediately after the cell divides
▪ Preparations are made to replicate DNA
▪ Some cells such as muscle and nerve cells, exit the cell
cycle and do not divide again

S (synthesis) – cell copies DNA
 Replication of DNA occurs at this time
(Chromosomal Replication)

Key Terms
 Chromosomes – structures that contain
the genetic material
 Chromatin – relaxed form of DNA is the
cell’s nucleus – looks like spaghetti
▪ Chromatin makes the nucleus appear dark
and dense at this stage.
Rat liver cells – the specks are
relaxed chromatin
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G2 (Gap 2) – preparation for division
 The cell synthesizes a protein that makes
microtubules for cell division
 The cell takes inventory and makes sure it’s ready
to continue with mitosis
▪ (Performs a double check to ensure the new cells have
what they need to function properly)
Mitosis is the
division of the
cell’s nuclear
material.
Four substages:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
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First stage of mitosis is prophase
 The cell’s chromatin tightens into
chromosomes, which are shaped like an X
 Each half of the chromosome “X” is called
a sister chromatid
 Sister chromatids are attached at the
center by a centromere and contain
identical copies of DNA
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What is the location of the sister chromatids?
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As prophase continues,…
 The nucleolus and nuclear membrane disappear
 Spindle fibers (microtubules) form in the cytoplasm
 In animal cells and most protists, another pair of
microtubule structures called centrioles migrate to
the ends, or poles, or the cell
 Aster fibers begin coming out of the centrioles
 Spindle fibers attach to the sister chromatids on both
sides of the centromere and then to opposite sides of
the cell – ensures each new cell receives one complete
copy of DNA
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The second stage of mitosis – metaphase
 “meta” means middle, one of the shortest stages of mitosis
 Sister chromatids are pulled by motor proteins along the
spindle apparatus (spindle fibers, centrioles and aster
fibers) to the center of the cells
 Chromatids line up along the equator of the cell
 Ensures that the new cells have accurate copies of the
chromosomes
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The third stage of mitosis – anaphase
 Chromatids are pulled apart
 Microtubules of the spindle apparatus begin to shorten
causing the two sister chromatids to separate into two
identical chromosomes
 Separation happens simultaneously
 At the end the chromosomes have moved with the
help of the spindle apparatus to each of the poles of
the cell
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The fourth and last stage of mitosis – telophase
 The chromosomes arrive at the poles and begin to
relax, or decondense
 Two new nuclear membranes begin to form and the
nucleoli reappear
 Spindle apparatus disassembles and some of the
microtubules are recycled by the cell to build various
parts of the cytoskeleton
 The nuclear material is divided ending mitosis
The final process of the cell
cycle is cytokinesis.
 Cyto – Greek word kytos,
meaning hollow vessel –
refers to cell
 Kinesis – Greek word
kinetikos, meaning putting in
motion

During this stage, the
cytoplasm is divided.

Cytokinesis results in two cells, each with
identical nuclei
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Animal cells – cytokinesis occurs due to
microfilaments constricting, or pinching, the
cytoplasm (the pinched area is called a
furrow)
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Plant cells have a rigid cell wall covering the
cytoplasm so instead of pinching, a structure
called a cell plate forms between the two daughter
nuclei
Cell walls then form on either side of the cell plate
- the outcome is two genetically identical cells
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Prokaryotic cells divide by binary
fission and finish cell division a
different way

When prokaryotic DNA is
duplicated, both copies attach to
the plasma membrane
As the plasma membrane grows,
the attached DNA molecules are
pulled apart
The cell complete fission,
producing two new prokaryotic
cells
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 The timing and rate of cell division
 are important to the health of an
organism
 varies depending on the type of cell
 a mechanism involving proteins and
enzymes controls the cell cycle
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Proteins called cylins bind to enzymes (CDK)
in the stages of interphase and mitosis to
start the various activities
Different cyclin/enzyme combinations signal
activities such as…
 Start of cell cycle, DNA replication, protein
synthesis, and nuclear division
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Built-in check points monitor
the cell cycle and can stop the
process if an error is detected
 G1 check point includes a check
for DNA damage and can stop
the cycle before entering the S
stage of interphase (before the
damaged DNA is replicated)
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When control of the cell cycle fails, cancer results
Cancer is the uncontrolled growth and division of
cells
 Grow and divide quickly as long as they receive nutrients
 Cancer cells crowd normal cells and can kill an organism
 Cancer can occur in a healthy, active, young organism
not just a weak one
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Mutations – changes in the segments of DNA
that code for the production of proteins
 Cells will often fix mutations in DNA but if the
repair system fails, cancer can result
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Environmental factors – increase the risk of
cancer
 Carcinogens – cancer causing agents like tobacco,
tobacco smoke, alcohol, some viruses, and
radiation from the Sun or X-rays
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Avoid carcinogens
 All types of tobacco are dangerous and
should be avoided – even smokeless
tobacco and secondhand smoke have been
proven to cause cancer

FDA (Food and Drug Administration) –
government agency that works to
ensure that food and drink are safe
 Labels and warnings are required for
products that might be carcinogens
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Radiation – amount of exposure
should be limited
 (Example: ultraviolet radiation from the
Sun and X-rays)
 Sunscreen is recommended for
everyone who is exposed to the Sun
 Lead aprons are used when X-rays of
teeth are taken
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More than one change in DNA is required to
change an abnormal cell into a cancer cell
Over time, changes occur
 May explain why the risk of cancer increases with
age
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An individual who inherits one or more
change from a parent is at a higher risk for
cancer
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Apoptosis – a programmed cell death
 Not all cells are intended to survive
 Cells undergoing apoptosis typically shrink and
shrivel
 All animal cells have a “death program” that can
be activated
Normal
Cell
Dying
Cell
Examples of apoptosis
 Plants – lose leaves in autumn – cells attached to
the tree undergo apoptosis
 The development of the human hand and foot
 Cells occupy the spaces between the fingers and toes
giving a webbed appearance
 These cells undergo apoptosis so that the webbing will
not be present in the mature organism
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Cells that are damaged
beyond repair, including
cells with DNA damage
that could lead to
cancer will also undergo
apoptosis
Apoptosis can help
protect organisms from
developing dangerous
cancerous growth
Cancer cell undergoing apoptosis
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Most cells in an organism
are designed for
specialized function
 Skin, heart, muscle, blood
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In 1998, scientists
discovered a way to
isolate a unique type of
cell in humans called the
stem cell
 Stem cells – unspecialized
cells that can develop into
specialized cells under the
right conditions
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Stem cells can remain in an organism for
many years, continually dividing
There are two types: Embryonic and Adult
 Embryonic – mass of cells that are the result of a
fertilized egg (100-150 cells) – controversial due to
ethical concerns to use in research
 Adult – found in various tissues in the body (adults and
infants) and might be used to maintain and repair the
same kind of tissue in which they have been found
▪ Adult does not refer to age, even newborns have adult stem
cells
 Harvard scientists have
been able to use adult
stem cells to repair bone
and brain tissue in mice
 Less controversial
because the donors can
give consent