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Eukaryotic Cell Reproduction
Mitosis Defined
DNA Part IV:
Mitosis or M Phase of
the Cell Cycle
Mitosis is the process by
which a cell separates
the chromosomes in its
cell nucleus into two
identical sets, in two
separate nuclei.
• Cytokinesis is the division
of the cytoplasm.
• Also occurs during M of the
cell cycle.
• Daughter cells are
genetically identical.
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Unicellular vs. Multicellular Mitosis
Purpose of Chromosomes
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Coiling of DNA to Form Chromosomes
There are six feet of DNA in a
human somatic cell.
• DNA is wrapped around
histone proteins and coiled
forming chromosomes.
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,.
• Packaging DNA in
chromosomes prevents DNA
breakage and helps ensure that
each cell gets one copy of each
chromosome.
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DNA Coiling to Form Chromosomes
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Chromatids Versus Chromosome
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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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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Centromere vs. Kinetochore
Prophase
Mitosis & Cell Division Animation
•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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Prophase
Prometaphase
Prometaphase
Prometaphase
Prometaphase (cont.)
• Nuclear envelopes fragments and
nucleolus is no longer visible.
• Two opposing kinetochores form on the
centromere on each chromatid.
• Centrosomes are at opposite ends of the
nuclear area.
• Kinetochore microtubules attach to the
kinetochores. Moving the chromosomes
back and forth until they reach the
middle of the cell.
• The microtubules extend through the
nuclear area.
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Prometaphase
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Prometaphase
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Nonkinetochore Microtubules
Nonkinetochore microtubules overlap from opposite poles.
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Metaphase
Metaphase
Anaphase
Metaphase
• Longest phase of mitosis.
• Double stranded chromosomes line
up on the metaphase plate.
Anaphase
• Cohesin proteins are cleaved
and the sister chromatids
separate.
• The chromosomes are pulled to
opposite poles.
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Anaphase
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Anaphase
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Anaphase
Anaphase (cont.)
• 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
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Telophase
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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
• 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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Cytokinesis in Animals
Cytokinesis in Animals
Cytokinesis in Animals
• In animal cells, rings of
actin form under the cell
membrane associated
with myosin (much like
skeletal muscles)
• Mitosis without cytokinesis
results in multinucleated
cells. This happens in
certain algae, plants, fungi,
and even a few animals.
• Next, the fibers contract
like a “pull-string” purse.
Forming a cleavage
furrow.
• Animals cells undergo
cytokinesis by the pinching
in of the cell membrane.
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Cytokinesis in Algae
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Cytokinesis in Higher Plants
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Comparing Cytokinesis in Plants and Animals
In higher plants, cytokinesis begins in
the middle and proceeds toward the
periphery as membranous vesicles fuse
to form the cell plate..
In algal cells, cytokinesis occurs
by an inward growth of new cell
wall and membrane.
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Comparing Cytokinesis in Plants and Animals
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Mitosis in a Plant Root Tip
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Overview of Mitosis
Mitosis in a plant occurs in the root tips,
shoot tips and other specialized areas.
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Prokaryotes Reproduce by Binary Fission
Evolution of Mitosis
Mitosis: A Review
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.
This is a much
more primitive,
thus simpler
mechanism of cell
division!
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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