You Say Mitosis and I
Say Meiosis..
or How Cells Reproduce
A. Introduction
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The ability to reproduce distinguishes living
things from nonliving matter.
The continuity of life is based on the
reproduction of cells through cell division.
Cell division occurs as part of the life of a cell
from its origin in the division of a parent cell
until its own division into two.
B. Cell division functions in
reproduction, growth, and repair
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The division of a unicellular organism
reproduces an entire organism, increasing the
population.
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Cell division on a larger scale can produce
offspring for some multicellular organisms.
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Cell division is also central to the growth and
development of a multicellular organism.
Multicellular organisms also use cell division
to repair and renew cells that die from
normal wear and tear or accidents.
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Cell division requires the distribution of
identical genetic material - DNA - to two
daughter cells.
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What is remarkable is the continuity with
which DNA is passed along, without much
change from one generation to the next.
A dividing cell duplicates its DNA,
distributes the two copies to opposite ends of
the cell, and then splits into two daughter
cells.
C. Cell division distributes identical sets of
chromosomes to daughter cells
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A cell’s genetic information, packaged as
DNA, is called its genome.
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In prokaryotes, the genome is often a single long
circular DNA molecule.
In eukaryotes, the genome consists of several
linear DNA molecules.
A human cell must duplicate about 3 meters
of DNA and separate the two copies such that
each daughter cell ends up with a complete
genome.
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DNA molecules are packaged into
chromosomes.
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Every eukaryotic species has a characteristic
number of chromosomes in the nucleus.
 Human somatic cells (body cells) have 46
chromosomes.
 Human gametes
(sperm or eggs)
have 23 chromosomes,
half the number in
a somatic cell.
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Each eukaryotic chromosome consists of a
long, linear DNA molecule.
Each chromosome has hundreds or
thousands of genes, the units that specify an
organism’s inherited traits.
animation
Each gene has a specific
position (LOCUS) on
the chromosome.
http://www.shoreforlife.org/forpatients.html
Organization of
Eukaryotic DNA
http://www.austincc.edu/mlt/mdfund/mdfund_unit3notes.html
 Associated with DNA are
histone proteins that
maintain its structure and
help control gene
activity.
 This DNA-protein
complex, chromatin, is
organized into a long thin
fiber.
 After DNA duplication,
chromatin condenses,
coiling and folding to
make a smaller package.
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Each duplicated chromosome consists of two
sister chromatids which contain identical
copies of the chromosome’s DNA.
The area where the
chromatids connect is
the centromere.
Later, the sister
chromatids are pulled
apart and repackaged
into two new nuclei at
opposite ends of
the parent cell.
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The process of the formation of the
two daughter nuclei, mitosis, is
usually followed by division of the
cytoplasm which is called
cytokinesis.
Mitosis takes one cell and produces
two cells that are the genetic
equivalent of the parent.
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Each human inherited 23 chromosomes from
each parent: one set in an egg and one set in
sperm.
The fertilized egg or zygote underwent trillions
of cycles of mitosis and cytokinesis to produce a
fully developed multicellular human.
These processes continue every day to replace
dead and damaged cells.
Essentially, these processes produce clones cells with the same genetic information.
D: A Day in the Life of a Cell
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The mitotic (M) phase of the cell cycle
alternates with the much longer interphase.
 The M phase includes mitosis and
cytokinesis (about one hour in length for
eukaryotes)
 Interphase accounts for 90% of the cell
cycle or 23 hours out of the day.
 During Interphase the cell is conducting all
life activities and getting ready for the next
round of mitosis to occur (in most cells)
animation
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During interphase the cell is very active, with many
processes occurring in the cytoplasm and nucleus.
Interphase has three subphases:
 the G1 phase (“first gap”) centered on growth
(amino acids are synthesized to produce proteins)
 the S phase (“synthesis”) when the chromosomes
are copied,
 the G2 phase (“second gap”) where the cell
completes preparations for cell division (organelles
are replicated)
The cell divides (M).
The daughter cells may then repeat the cycle.
DNA vs. Cell Cycle Phases
Which phase of the cell cycle is happening at
each part of the graph?
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A checkpoint in the cell cycle is a critical
control point where stop and go signals
regulate the cycle.
Three major checkpoints: G1, G2, and M
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The distinct events of the cell cycle are
directed by a distinct cell cycle control
system.
http://www.cellsalive.com/cell_cycle_js.htm
G1 checkpoint- the cell becomes committed to
entering the cell cycle. If it proceeds past this point, a
new round of cell division occurs;
the cell activates cyclin-CDK factors
which promotes entry into the S phase.
G2 checkpoint- (DNA damage checkpoint)
This ensures that the cell underwent all
of the necessary changes during the
S and G2 phases and is ready to divide.
M checkpoint – (mitotic spindle
checkpoint) – occurs in metaphase where all of the
chromosomes should have aligned along the mitotic
plate. If passed, sister chromatids separate.
http://www.fastbleep.com/biology-notes/31/176/1012
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For many cells, the G1 checkpoint is the most
important.
Great animation
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If the cells receives a go-ahead signal, it usually completes
the cell cycle and divides.
If it does not receive a go-ahead signal, the cell exits the
cycle and switches to a nondividing state, the G0 phase.
 Most mature human cells are in this phase.
 Liver cells can be “called back” to the cell cycle by
external cues (growth factors)
 Heart, muscle, nerve cells and red blood cells remain in
G0 performing their life functions but never dividing.
 Hair, nail, skin cells and cells lining the digestive system
never enter G0
Regulation of the cell cycle
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Protein signals inhibit or activate the cell cycle.
Molecules called cyclins together with CDKs
(cyclin dependent kinases) form MPFs
(maturation promoting factors)
 These proteins are important in regulating the cell
cycle and cause cell proliferation.
 Cyclin levels (G1, S-phase and mitotic cyclins) rise and
fall with the stages of the cell cycle.
 Cdks (G1, S-phase and M-phase) remain fairly stable
but must bind to the appropriate cyclin in order to be
activated.
https://wikispaces.psu.edu/display/230/Mitosis,+Cell+Division+and+the+Cell+Cycle
How do cyclins and Cdks
regulate the cell cycle?
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G1 cyclins bind to Cdk proteins during G1.
Once bound and activated, the Cdk signals the cell's
exit from G1 and entry into S phase.
When the cell reaches an appropriate size and the
cellular environment is correct for DNA replication,
the cyclins begin to degrade.
G1 cyclin degradation deactivates the Cdk and leads
to entry into S phase.
Further in the cycle…
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Mitotic cyclins accumulate gradually during G2.
Once they reach a high enough concentration, they
can bind to Cdks.
When mitotic cyclins bind to Cdks in G2, the resulting
complex is known as Mitosis-promoting factor (MPF).
This complex acts as the signal for the G2 cell to enter
mitosis.
Once the mitotic cyclin degrades, MPF is inactivated
and the cell exits mitosis by dividing and re- entering
G1.
McGraw Hill animation
Test your understanding
A cell cycle "checkpoint" would be best
described as:
A. a site in the cytoplasm where proteins
are inspected for mutations.
B. either G1, S, G2, prophase,
metaphase, anaphase or telophase.
C. specific stages where further progress
of the cell cycle can be halted.
D. any step where the cell cycle is
blocked by a mutated protein.
All of the following statements are true about the
cell cycle EXCEPT:
A. Cdk has to be bound to cyclin to be able to
work.
B. Cdk inhibitors may be useful in controlling
cancer.
C. Cyclin triggers Cdk molecules allowing them
to activate many cellular events
D. Activity of MPFs hold steady throughout the
cell cycle.
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E. Mitosis is a continuum of changes.
 For description, mitosis is usually
broken into five subphases:
prophase,
prometaphase,
metaphase,
anaphase, and
telophase.
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By late interphase (after the “S” phase), the
chromosomes have been duplicated but are
loosely packed.
The centrosomes with centrioles in animal
cells have been duplicated
and begin to organize
microtubules into an
aster (“star”).
In prophase, the chromosomes are tightly
coiled (supercoiling) and become visible, with
sister chromatids joined together.
 The nucleoli and nuclear
membrane begin to
disappear.
 The mitotic spindle begins
to form and appears to
push the centrosomes away
from each other toward
opposite ends (poles)
of the cell.
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During prometaphase, the nuclear envelope
diappears and microtubules from the
spindle interact with the chromosomes.
Microtubules from one
pole attach to one of two
kinetochores, special
regions of the centromere,
kinetochores
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In metaphase the
sister chromatids line
up at the metaphase
plate, an imaginary
plane equidistant
between the poles,
defining metaphase.
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At anaphase, the centromeres divide,
separating the sister chromatids.
Each is now pulled toward the pole to which
it is attached by spindle fibers.
By the end, the two
poles have equivalent
collections of
chromosomes.
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At telophase, the cell continues to elongate
as free spindle fibers from each centrosome
push off each other.
Two nuclei begin for form, as the nuclear
envelope begins to
reappear.
Cytokinesis, division
of the cytoplasm,
ends.
Animation
Animation #2
F.Cytokinesis divides the cytoplasm:
a closer look
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Cytokinesis, division of
the cytoplasm, typically
follows mitosis.
In animals, the first sign
of cytokinesis (cleavage)
is the appearance of a
cleavage furrow in the
cell surface near the old
metaphase plate.
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On the cytoplasmic
side of the cleavage
furrow a contractile
ring of actin
microfilaments and
the motor protein
myosin form.
Contraction of the
ring pinches the cell
in two.
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Cytokinesis in plants, which have cell walls,
involves a completely different mechanism.
During telophase, vesicles
from the Golgi fuse at
the metaphase plate,
forming a cell plate.
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The plate enlarges until its
membranes fuse with the
plasma membrane at the
perimeter, with the contents
of the vesicles forming new
wall material in between.
G. Mitosis in eukaryotes may have evolved from
binary fission in bacteria
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Prokaryotes reproduce by binary fission, not
mitosis.
Most bacterial genes are located on a single
bacterial chromosome which consists of a
circular DNA molecule and associated
proteins.
While bacteria do not have as many genes or
DNA molecules as long as those in eukaryotes,
their circular chromosome is still highly
folded and coiled in the cell.
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In binary fission, chromosome replication
begins at one point in the circular
chromosome, the origin of replication site.
These copied regions begin to move to
opposite ends of the cell.
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The mechanism behind the movement of the
bacterial chromosome is still an open
question.
As the bacterial chromosome is replicating
and the copied regions are moving to
opposite ends of the cell, the bacterium
continues to grow until it reaches twice its
original size.
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Cell division
involves inward
growth of the
plasma membrane,
dividing the parent
cell into two
daughter cells,
each with a
complete genome.
Possible intermediate
evolutionary steps between
binary fission & mitosis are
seen in two types of
unicellular algae.
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In dinoflagellates,
replicated chromosomes
are attached to the
nuclear envelope.
In diatoms, the spindle
develops within the
nucleus.
How does one cell
(zygote) develop into a
multicellular organism
using mitosis?
-CLEAVAGE – rapid
cell division without
cell growth
-BLASTULATIONformation of hollow
ball of cells
-GASTRULATION –
formation of tissue
layers
Fate of the Germ Layers Formed at Gastrulation
Ectoderm
Nervous system
Sense organs
Outer layer of
skin (epidermis,
nails, hair, etc.)
Pituitary gland
Endoderm
Lining of
digestive tube
Lining of the
respiratory
system
Mesoderm
Notochord
Skeleton (bone and
cartilage)
Muscles
Circulatory system
Excretory system
Reproductive system
Inner layer of skin (dermis)
Outer layers of digestive
tube
animation
https://b51ab7d9e5e1e7063dcb70cee5c33cf7f4b7bad8.googledrive.com/host/0Bx6hk6AUBHxDc2d4TDJZTFIy
MGs/files/Bio%20102/Bio%20102%20lectures/Animal%20Diversity/Lower%20Invertebrates/sponges.htm
What induces cell
differentiation?
Chemicals called
Morphogens!
(Cell signaling
pathway)
It is not birth, marriage, or
death, but gastrulation,
which is truly the most
important time in your life
Wolpert 1986
I. What is a stem cell and how can they be
used in medicine?
Click here to find out!
• Cells from which all
other cells with
specialized functions
are generated.
• Under the right
conditions in the body
or a laboratory, stem
cells divide to form
more cells called
daughter cells.
Learn.genetics animation
DNA interactive website
HHMI animation
Where do stem cells come from?
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Embryonic stem cells. These stem cells
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come from embryos that are three to five days
old. At this stage, an embryo is called a
blastocyst and has about 150 cells.
These are pluripotent (ploo-RIP-uh-tunt) stem
cells, meaning they can divide into more stem
cells or can become any type of cell in the
body.
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Why are the use of these stem cells
considered controversial?
Alternative stem cell origins
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Adult stem cells-These stem cells are
found in small numbers in most adult
tissues, such as bone marrow or fat.
Compared with embryonic stem cells,
adult stem cells have a more limited
ability to give rise to various cells of the
body.
The best source?
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Adult cells altered to have properties of embryonic stem
cells (induced pluripotent stem cells)
Scientists have successfully transformed regular adult cells
into stem cells using genetic reprogramming.
By altering the genes in the adult cells, researchers can
reprogram the cells to act similarly to embryonic stem cells.
This new technique may allow researchers to use these
reprogrammed cells instead of embryonic stem cells and
prevent immune system rejection of the new stem cells.
However, scientists don't yet know if altering adult cells will
cause adverse effects in humans.
animation
Stem cells can come from a child
before or at birth.
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Perinatal stem cells
Researchers have discovered stem cells
in amniotic fluid in addition to umbilical
cord blood stem cells.
These stem cells also have the ability to
change into specialized cells.
Why are these valuable to the person
who they originated from?
The Importance of Stem Cell Research
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The capacity of stem cells to divide and differentiate
along different pathways is necessary in embryonic
development and makes stem cells suitable for
therapeutic uses.
The use of stem cells in the treatment of disease is
mostly at the experimental phase. However,
scientists anticipate the use of stem cell therapies as
a standard method of treating a range of diseases in
the near future (ex. heart disease and diabetes).
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Use of stem cells to treat Stargardt’s disease, a disease
where the light-sensitive retina in the eye degenerates.
Ethical argument regarding stem cell research
Stargardt’s Disease Treatment
with Embryonic Stem Cells
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The stem cells have been tested in animal models of eye
disease. - Robert Lanza, MD, November 2010.
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"In rats, we've seen 100 percent improvement in visual performance
over untreated animals without any adverse effects," Lanza said.
"Our studies showed that the cells were capable of extensive rescue
of photoreceptors in animals that otherwise would have gone blind.
Near-normal function was also achieved in a mouse model of
Stargardt's disease."
July 2012 Phase I/II clinical trial: 3 patients received
intraocular injections of 50,000 human embryonic stem cellderived retinal pigment epithelial (RPE) cells. One patient in
the study received an intraocular injection of 100,000 cells.
http://www.allaboutvision.com/conditions/stargardts.htm
Therapeutic uses of stem cells
also include…
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Stem cell transplants such as in bone
marrow to replace cells damaged by chemo
or to fight blood related diseases like
leukemia
Stem cells transplants are being used to
treat degenerative disease such as heart
failure.
See animation on uses of stem cell
pbs animation
J. How is Cancer Related to Mitosis?
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Cancer is uncontrolled mitosis!
Cancer cells divide excessively and invade
other tissues because they are free of the
body’s control mechanisms.
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Cancer cells do not stop dividing when growth
factors are depleted either because they
manufacture their own, have an abnormality in
the signaling pathway, or have a problem in the
cell cycle control system.
If and when cancer cells stop dividing, they do
so at random points, not at the normal
checkpoints in the cell cycle.
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Cancer cell may divide indefinitely if they
have a continual supply of nutrients.
 In contrast, nearly all mammalian cells
divide 20 to 50 times under culture
conditions before they stop, age, and die.
 Cancer cells may be “immortal”.
 Cells from a tumor removed from a
woman (Henrietta Lacks) in 1951 are
still reproducing in culture.
HeLa Cells
In 1951, a scientist at Johns Hopkins Hospital in Baltimore,
Maryland, created the first immortal human cell line with a
tissue sample taken from a young black woman with cervical
cancer. Those cells, called HeLa cells, quickly became
invaluable to medical research.
HeLa Cells
Dividing
Significance of HeLa Cells
- essential to developing the polio
vaccine.
- went up in the first space missions to
see what would happen to cells in
zero gravity.
- Used in developing cloning, gene
mapping and in vitro fertilization
techniques.
Normal human female karyogram:
HeLa karyogram:
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The abnormal behavior of cancer cells begins
when a single cell in a tissue undergoes a
series of genetic transformations that convert
it from a normal cell to a cancer cell.
A MUTAGEN is a physical or chemical agent that
changes genetic material (DNA) (ex: radiation,
formaldehyde)
 Some genetic changes (mutations) turn ON
ONCOGENES (cancer-causing genes) or turn OFF
tumor-suppressor genes that stop tumor development.
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Normally, the immune system recognizes
and destroys mutated cells.
However, cells that evade destruction
proliferate to form a tumor, a mass of
abnormal cells.
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If the abnormal cells remain at the
originating site, the lump is called a
benign tumor (primary tumor).
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Most do not cause serious problems and
can be removed by surgery.
In a malignant tumor, the cells leave
the original site (metastasis) and impair
the functions of one or more organs.
animation
An overview with several animations showing
how cancer develops. Click here…
Unregulated Cell Division
Nova How Cancer Grows and Spreads
interactive animation
animation
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Treatments for metastasizing cancers
include high-energy radiation and
chemotherapy with toxic drugs.
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These treatments target actively dividing cells.
Researchers are beginning to understand
how a normal cell is transformed into a
cancer cell.
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The causes are diverse.
However, cellular transformation always
involves the alteration of genes that influence
the cell cycle control system.
Cancer Treatments
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Surgery If a tumor has not metastasized, removal of the mass may
completely cure the patient. But, if it has it is nearly impossible to remove
all the cancer cells.
Radiation Kills cancer cells by focusing a high energy beam of gamma or
x rays on the cancer cells themselves. This damages the molecules of the
cancer cells, causing them to commit suicide. This treatment also kills
normal, healthy tissue.
Chemotherapy Chemicals are used to damage DNA or proteins of cancer
cells, forcing them to die. This targets rapidly dividing cells, which could
mean it could target normal, healthy cells.
Immunotherapy A medicine like interferon is administered that causes
the immune system to fight the cancer cells.
Hormone therapy Designed to change the hormone production of the
body that may either stop the cancer from growing or kill it entirely.
Gene therapy Replace damaged genes with ones that are normal. Targets
the main cause of cancer: the damage to DNA.
How can Natural Killer Cells in Our
Immune System help to fight cancer?
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See this youtube video…
Cancer and gene regulation
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http://learn.genetics.utah.edu/cont
ent/epigenetics/control/
Where does cancer (uncontrolled
mitosis) come from?
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Faulty signaling pathways
Mutations in human genome (p53 site)
Click here to learn about the p53
site of the genome and cancer
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http://highered.mcgrawhill.com/sites/007337797x/student_view0/chapter9/animation_quiz__how_tumor_suppressor_genes_block_cell_division.html
K. A close look at mitosis as a form of
reproduction.
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In asexual reproduction, a single individual
passes along copies of all its genes to its
offspring.
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Single-celled eukaryotes reproduce asexually by
mitotic cell division to produce two identical
daughter cells.
 Fission: asexual
reproduction in which
a parent separates into
two or more
approximately equal
sized individuals.
Examples of asexual reproduction
Invertebrates:
 Budding: asexual
reproduction in which
new individuals split off
from existing ones.
Video of hydra budding
Fragmentation: the breaking of the body into
several pieces, some or all of which develop
Lizard loses tail video
into complete adults.
 Requires regeneration of lost body parts.
 Example – a lizard trying to escape a
predator biting its tail can lose this part of
its body and regrow a new tail.
 Stories have been told of oyster fisherman
who try to rid their oyster beds of
predatory starfish by slicing them up and
throwing them back to sea. Why would
this technique backfire?

Parthenogenesis (virgin birth) is the process by
which an unfertilized egg develops into (often)
haploid adult or the individual produces a diploid
fertilized egg.
 Parthenogenesis plays a role in the social
organization of species of bees, wasps, and
ants.
 Male honeybees are haploid and female
honeybees are diploid.
 Several genera of fishes, amphibians, and
lizards produce by a form of parthenogenesis
that produces diploid zygotes.
 Asexual
reproduction in plants
 Meristematic tissues with dividing
undifferentiated cells can sustain or
renew growth indefinitely- usually
found at the tips of a stem or the tip
of a root.
http://bio1151.nicerweb.com/Locked/media/ch29/apical.html
Vegetative propagation of plants is
common in agriculture
 Various
methods have been
developed for the asexual
propagation of crop plants, orchards,
and ornamental plants.
These can be reproduced asexually
from plant fragments called
cuttings (pieces of shoots or stems).
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Ex:African violets, can be propagated
from single leaves
One
advantage of asexual
reproduction is that a plant well
suited to a particular environment
can clone many copies of itself
rapidly.
The offspring of vegetative
reproduction are not as fragile as
seedlings produced by sexual
reproduction.
What are the benefits of asexual
reproduction
Advantages of asexual reproduction:
Can reproduce without needing to find a
mate (less energy required)
Can have numerous offspring in a short
period of time (faster reproduction)
In stable environments, allows for the
perpetuation of successful genotypes.
Disadvantage of asexual reproduction:
Lack of genetic variety leaves the population
susceptible to disease or other environmental change
L. Meiosis is another type of cell
reproduction used in sexually
reproducing organisms
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Meiosis is a type of cell division whereby
gametes –sex cells are made in animals or
spores are made in plants or fungi.
Meiosis occurs in the gonads- sex organs of
sexually reproducing organisms
In animals, the gonads are the ovaries and
testes.
In flowering plants (angiosperms), the
female gonad is the ovary and the male gonad
is the anther.
M. Why is Meiosis Necessary for
Organisms that Reproduce Sexually?
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Meiosis produces cells with half of the chromosome
number.
When gametes unite during fertilization, the full
chromosome # is restored.
The alternation of meiosis and fertilization maintains
the chromosome # through generations.
For example, if fruit flies have a chromosome
number of 8, what would happen if their gametes
had 8 chromosomes and fertilization occurred?
N. Meiosis results in variation in the
species.
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Sexual reproduction results in greater
variation among offspring than does asexual
reproduction which is key to evolution.
Two parents give rise to offspring that have
unique combinations of genes inherited
from the parents.
Offspring of sexual
reproduction vary
genetically from
their siblings and
from both parents.
O. Background information and vocabulary
needed for our understanding of meiosis
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A karyotype is a property of the cell (the number and
type of chromosomes present in the nucleus) and a
karyogram is a photograph or diagram of an
individual’s homologous pairs of chromosomes in
decreasing length.
In humans,
each somatic cell
(all cells other
than sperm or
ovum) has 46
chromosomes.
A human
karyogram
shows 23
homologous
chromosome
pairs, each pair
with the same
length,
centromere
position, and
staining
pattern.
These homologous chromosome pairs
carry genes that control the same
inherited characters.
The genes
are the same,
but the
specific form
of the gene
(allele) could
be different.
Ex: Brown eyes and blue eyes
are different alleles for eye color.
Karyograms can detect unusual
chromosomes numbers of a fetus.
Ex: Down Syndrome, Turner’s Syndrome,
Kleinfelter’s Syndrome
Karyogram for a
Down Syndrome
female.
http://bookbing.org/downs-syndrome-powerpointpresentations/karyotype-down-syndrome/
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An exception to the rule of homologous
chromosomes is found in the sex
chromosomes, the X and the Y.
The pattern of inheritance of these
chromosomes determine an individual’s sex.
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Human females have a homologous pair of X
chromosomes (XX).
Human males have an X and a Y chromosome
(XY).
Because only small parts of these have the
same genes, most of their genes have no
counterpart on the other chromosome.
The other 22 pairs are called autosomes.
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Homologous pairs of chromosomes are a
consequence of sexual reproduction.
We inherit one chromosome of each homologous
pair from each parent.
 The 46 chromosomes in a somatic cell can be
viewed as two sets of 23, a maternal set and a
paternal set.
Sperm cells or ova (gametes) have only one set of
chromosomes - 22 autosomes and an X or a Y.
A cell with a single chromosome set is haploid symbol is “n” where n is the number of
homologous pairs
 For humans, the haploid number of
chromosomes is 23 (n = 23).
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
Fertilization is the process where, a haploid
sperm reaches and fuses with a haploid
ovum.
The fertilized egg (zygote) now has two
haploid sets of chromosomes bearing genes
from the maternal and paternal family lines.
The zygote and all cells with two sets of
chromosomes are diploid cells symbolized as
“2n” (twice the # of homologous pairs)
 For humans, the diploid number of
chromosomes is 46 (2n = 46).


As an organism develops from a zygote to a
sexually mature adult, the zygote’s genes are
passed on to all somatic cells by mitosis.
Gametes, which develop in the gonads, are
not produced by mitosis.

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If gametes were produced by mitosis, the
fusion of gametes would produce offspring
with four sets of chromosomes after one
generation, eight after a second and so on.
Instead, gametes undergo the process of
meiosis in which the chromosome number is
halved.

Human sperm or ova have a haploid set of 23
different chromosomes, one from each
homologous pair.


Fertilization restores the diploid condition
by combining two haploid sets of
chromosomes.
Fertilization and
meiosis
alternate in sexual
life cycles.
P. Meiosis reduces chromosome number
from diploid to haploid: a closer look

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Many steps of meiosis resemble steps in
mitosis.
Both are preceded by the replication of
chromosomes.
However, in meiosis, there are two
consecutive cell divisions, meiosis I and
meiosis II, which results in four daughter
cells.
Each final daughter cell has only half as many
chromosomes as the parent cell.

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
Meiosis reduces
chromosome number
by copying the
chromosomes once,
but dividing twice.
The first division,
meiosis I, separates
homologous
chromosomes.
The second, meiosis II,
separates sister
chromatids.

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Division in meiosis I occurs in four phases:
prophase, metaphase, anaphase, and
telophase.
During the preceding interphase the
chromosomes are replicated to
form sister chromatids.


These are genetically identical
and joined at the centromere.
Also, the single centrosome
is replicated.

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In prophase I, crossing over can occur!
The chromosomes condense and
homologous chromosomes pair up to form
tetrads.
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In a process called synapsis, special proteins
attach homologous chromosomes
tightly together.
At several sites the chromatids of
homologous chromosomes are
crossed (chiasmata) and segments
of the chromosomes are traded.
A spindle forms from each
centrosome and spindle fibers
attached to kinetochores on
the chromosomes begin to
move the tetrads around.

At metaphase I, the tetrads are all arranged
at the metaphase plate.


Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad,
while those from the other pole are attached to
the other.
In anaphase I,
the homologous
chromosomes
separate and
are pulled toward
opposite poles.

In telophase I, movement of homologous
chromosomes continues until there is a
haploid set at each pole.

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Each chromosome consists of linked sister
chromatids.
Cytokinesis by the same
mechanisms as mitosis
usually occurs simultaneously.
Interkinesis occurs-a brief
intermission - but there is
no further replication
of chromosomes.

Meiosis II is very similar to mitosis.
 During prophase II a spindle apparatus
forms, attaches to kinetochores of each
sister chromatids, and moves them
around.
 Spindle fibers from one pole
attach to the kinetochore of
one sister chromatid and
those of the other pole to
the other sister chromatid.


At metaphase II, the sister chromatids are
arranged at the metaphase plate.
At anaphase II, the
centromeres of sister
chromatids separate
and the now separate
sisters travel toward
opposite poles.

In telophase II, separated sister
chromosomes arrive at opposite poles.

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Nuclei form around
the chromosomes
Cytokinesis separates
the cytoplasm.
At the end of meiosis,
there are four haploid
daughter cells.
Click on
animation
Click on
animation

Q. Mitosis and meiosis have several key
differences.
 The chromosome number is reduced
by half in meiosis, but not in mitosis.
 Mitosis produces daughter cells that
are genetically identical to the
parent and to each other.
 Meiosis produces cells that differ
from the parent and each other.

Three events, unique to meiosis, occur
during the first division cycle.
1. During prophase I, homologous
chromosomes pair up in a process called
synapsis when crossing over occurs.

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A protein zipper, the synaptonemal complex,
holds homologous chromosomes together
tightly.
Later in prophase I, the joined homologous
chromosomes are visible as a tetrad.
At X-shaped regions called chiasmata, sections
of nonsister chromatids are exchanged.
CROSSING OVER LEADS TO GENETIC
VARIATION!!!
2. At metaphase I homologous pairs of
chromosomes, not individual chromosomes
are aligned along the metaphase plate.

In humans, you would see 23 tetrads.
3. At anaphase I, it is homologous
chromosomes, not sister chromatids, that
separate and are carried to opposite poles of
the cell.

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Sister chromatids remain attached at the
centromere until anaphase II.
The processes during the second meiotic
division are virtually identical to those of
mitosis.

Mitosis produces two identical daughter
cells, but meiosis produces 4 very different
cells.
Found on a
Biology tshirt
Mcgraw Hill animation
Animation showing mitosis vs.
meiosis
R. Sexual life cycles produce genetic
variation among offspring


The behavior of chromosomes during meiosis
and fertilization is responsible for most of the
variation that arises each generation during
sexual reproduction.
Three mechanisms contribute to genetic
variation:



independent assortment (random orientation of
homologous pairs)
crossing over
random fertilization

Independent assortment of chromosomes
contributes to genetic variability due to the
random orientation of tetrads at the
metaphase plate.

There is a fifty-fifty chance that a
particular daughter cell of meiosis I
will get the maternal chromosome of
a certain homologous pair and a
fifty-fifty chance that it will
receive the paternal chromosome.

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Each homologous pair of chromosomes is
positioned independently of the other pairs
at metaphase I.
Therefore, the first meiotic division results
in independent assortment of maternal and
paternal chromosomes into daughter cells.
The number of combinations possible when
chromosomes assort independently into
gametes is 2n, where n is the haploid number
of the organism.


If n = 3, there are eight possible combinations.
For humans with n = 23, there are 223 or about
8 million possible combinations of
chromosomes.
Click on animation


Independent
assortment alone would
find each individual
chromosome in a
gamete that would be
exclusively maternal or
paternal in origin.
However, crossing over
produces recombinant
chromosomes which
combine genes
inherited from each
parent.


Crossing over begins very early in prophase
I as homologous chromosomes pair up gene
by gene.
In crossing over, homologous portions of
two nonsister chromatids trade places.

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
For humans, this occurs two to three times per
chromosome pair.
One sister chromatid may undergo different
patterns of crossing over than its match.
Independent assortment of these
nonidentical sister chromatids during
meiosis II increases still more the number of
genetic types of gametes that can result from
meiosis.

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The random nature of fertilization adds to
the genetic variation arising from meiosis.
Any sperm can fuse with any egg.

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A zygote produced by mating of a woman and
man has a unique genetic identity.
An ovum is one of approximately 8 million
possible chromosome combinations (actually
223).
The successful sperm represents one of 8
million different possibilities (actually 223).
The resulting zygote is composed of 1 in 70
trillion (223 x 223) possible combinations of
chromosomes.
Crossing over adds even more variation to
this.

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The three sources of genetic variability in a
sexually reproducing organism are:
 Independent assortment of homologous
chromosomes during meiosis I and of
nonidentical sister chromatids during meiosis
II.
 Crossing over between homologous
chromosomes during prophase I produces new
combinations of alleles.
 Random fertilization of an ovum by a sperm.
All three mechanisms reshuffle the various genes
carried by individual members of a population.
Mutations, still to be discussed, are what
ultimately create a population’s diversity of genes.
S. Evolutionary adaptation depends on a
population’s genetic variation

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
Darwin recognized the importance of genetic
variation in evolution via natural selection.
A population evolves through the differential
reproductive success of its variant members.
Those individuals best suited to the local
environment leave the most offspring,
transmitting their genes in the process.
This natural selection results in adaptation,
the accumulation of favorable genetic
variations.

As the environment changes or a population
moves to a new environment, new genetic
combinations that work best in the new
conditions will produce more offspring and
these genes will increase.



The formerly favored genes will decrease.
Sex and mutations are two sources of the
continual generation of new genetic
variability.
Gregor Mendel, a contemporary of Darwin,
published a theory of inheritance that helps
explain genetic variation.

However, this work was largely unknown for
over 40 years until 1900.
T. Formation of sperm and
eggs in humans
Meiosis in males is called
“spermatogenesis” : the birth of sperm
 Meiosis in females is called
“oogenesis” : the birth of an egg or ovum
The timing of meiosis, and the size and
number of gametes formed differ in
males and females.

Spermatogenesis and oogenesis both involve
mitosis, cell growth, meiosis and
differentiation, but differ in significant ways

Spermatogenesis is the production of mature sperm
cells from spermatogonia.
 A continuous and prolific process in the adult
male.
 Each ejaculation contains 100 – 650 million
sperm.
 Occurs in seminiferous tubules.
 As spermatogenesis progresses the developing
sperm cells move from the seminiferous tubule
to the epididymis where they can be temporarily
stored.

Sperm structure:


Haploid nucleus.
Tipped with an acrosome.


Contains enzymes that help
the sperm penetrate to the
egg.
A large number
of mitochondria
provide ATP to
power the
flagellum.

Oogenesis is the production of ova from
oogonia.
 Differs from spermatogenesis in three
major ways:
 At birth an ovary contains all of the
primary oocytes it will ever have.
 Unequal cytokinesis during meiosis
results in the formation of a single
large secondary oocyte and three small
polar bodies.
 The polar bodies degenerate.
 Oogenesis has long “resting” periods.
animation
What happens when meiosis
makes a mistake?

Nondisjunction – failure of chromosomes
to separate in Anaphase I or Anaphase II

when spindle fibers not attached properly
http://www.nicerwe
b.com/bio1151/Lock
ed/media/ch15/nond
isjunction.html
Animation on
nondisjunction in meiosis I
Animation on nondisjunction in
meiosis II
 Consequence: Gamete receives too many or too few
of a particular kind of chromosome.
 The fusion of an abnormal gamete with a normal
one can lead to the production of offspring with an
abnormal number of chromosomes. (aneuploidy)
 Nondisjunction can cause
 Down Syndrome (trisomy 21) – leads to mental
retardation, heart defects, short stature, sterility usually
 Turner syndrome (XO) – sterile females
 Klinefelter syndrome (XXY) – infertile males with
several feminine body characteristics
Older parents are more likely to
have babies with Down Syndrome
Nondisjunction
is more likely in
older parents.
https://www.geneyouin.ca/gene-silencing-for-down-syndrome-prevention-using-stem-cells/
Click here to see a youtube video
that summarizes our unit!