DNA Part IV:
Cellular
ReproductionMitosis and
Cytokinesis
Eukaryotic Cell Reproduction
• Cell division includes
mitosis and cytokinesis.
• Mitosis it the division of
the nucleus.
• Cytokinesis is the
division of the cytoplasm.
• Daughter cells are
genetically identical.
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Unicellular vs. Multicellular Mitosis
1. Unicellular organisms undergo
cell division to reproduce
themselves.
2. Multicellular organisms undergo
cell division for growth or repair,
or to make a new organism from a
fertilized egg,.
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Purpose of Chromosomes
There are six feet of DNA in a
human somatic cell.
•DNA is wrapped around histone
proteins and coiled forming
chromosomes.
•Packaging DNA in
chromosomes prevents DNA
breakage and helps ensure that
each cell gets one copy of each
chromosome.
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Coiling of DNA to Form Chromosomes
During interphase, the DNA forms chromatin. There are
areas of DNA that are tightly wound around histones, and
there are areas of DNA that are just loosely wound around
histones. This depends on the DNA is being transcribed
or replicated.
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DNA Coiling to Form Chromosomes
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Chromatids Versus Chromosome
• Double stranded chromosomes are
held together by centromere.
• One half of a double stranded
chromosome is called a chromatid.
• The lengths of the chromatids may
also be held together by proteins
called cohesins.
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Chromatids Versus Chromosome
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Centromere vs. Kinetochore
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Prophase
•Chromatin fibers begin to condense
into chromosomes and are visible
under the microscope.
•Cohesins hold chromatid arms
together (vertebrates only at the
centromere).
•Nucleoli disappears.
•Mitotic spindle forms asters
radiating out from the centrosome.
•After replicating, the centrosomes
are moving to opposite poles.
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Prometaphase
Prometaphase
(really Prophase cont.)
•Nuclear envelopes fragments and
nucleolus is no longer visible.
•Centrosomes are at opposite ends of the
nuclear area.
•The microtubules extend through the
nuclear area
•Two opposing kinetochores form on the
centromere on each chromatid.
•Kinetochore microtubules attach to the
kinetochores. Moving the chromosomes
back and forth until they reach the middle
of the cell.
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Nonkinetochore Microtubules
Nonkinetochore microtubules overlap from opposite poles.
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Metaphase
Metaphase
•Double stranded chromosomes line
up on the metaphase plate.
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Anaphase
Anaphase•Cohesin proteins are cleaved and
the sister chromatids separate.
•The chromosomes are pulled to
opposite poles.
•The kinetochore microtubules are
disassembled at the chromosome
end.
•Spindle poles move apart by
interacting with nonkinetochore
microtubules.
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Which End of the Microtubule is Shortened?
Experiment
•During anaphase, mark the
microtubules to form a stripe.
•Observe which side of the
microtubules shorten.
•They shortened on the side of the
chromatids, so therefore the
kinetochores are disassembling the
kinetochores microtubules and not the
centrosome.
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Telophase
• Two daughter nuclei form in the
cell.
• Nuclear envelope forms from the
fragments of the disassembled
nuclei and the endomembrane
system.
• Chromosomes unwind forming
chromatin.
• Beginning of cytokinesis
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Cytokinesis in Animals
•Mitosis without cytokinesis results
in multinucleated cells. This happens
in certain algae, plants, fungi, and
even a few animals.
•Animals cells do cytokinesis by the
pinching in of the cell membrane.
•Rings of actin form under the cell
membrane associated with myosin
contracts like a “pull-string” purse
forming a cleavage furrow.
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Cytokinesis in Algae
In algal cells, cytokinesis
occurs by an inward
growth of new cell wall
and membrane.
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Cytokinesis in Higher Plants
In higher plants, cytokinesis begins in
the middle and proceeds toward the
periphery as membranous vesicles fuse
to form the cell plate..
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Comparing Cytokinesis in Plants and Animals
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Mitosis in a Plant Root Tip
Mitosis in a plant occurs in the root tips, shoot tips and other
specialized areas. Division in the onion cells happens in the meristem
(previous years this word has been on tests!).
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Prokaryotes Reproduce by Binary Fission
This is a much more
primitive, thus simpler
mechanism of cell
division!
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Evolution of Mitosis
Yeast cells and diatoms do not
breakdown the nuclear membrane.
Instead it pinches inward like the
binary fission of prokaryotes. The
microtubules are contained within
in the nucleus.
Dinoflagellates also do not
breakdown the nuclear membrane
but the spindle fibers penetrate the
nuclear membrane and attach to the
plasma membrane.
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