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Mitosis - Advanced
Douglas Wilkin, Ph.D.
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Printed: January 15, 2015
AUTHOR
Douglas Wilkin, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Mitosis - Advanced
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Mitosis - Advanced
• Identify and describe the main processes in mitosis.
How do all your cells stay genetically identical?
Mitosis, of course. As you can see, mitosis is a multistage process that ensures separation of sister chromatids, and
ensures that daughter cells are just like the parent cell.
Mitosis
Mitosis is the division of the cell’s nucleus, the final step before two daughter cells are produced. Mitosis begins
immediately at the conclusion of interphase, specifically at the end of the G2 phase. The cell enters mitosis as
it approaches its size limitations. Four distinct phases of mitosis have been recognized: prophase, metaphase,
anaphase, and telophase, with each phase merging into the next one ( Figure 1.1). See All cells arise from
pre-existing cells at http://www.dnaftb.org/7/animation.html for an animation of Walther Flemming discussing
his description of mitosis. http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/MitosisFlash.html has an
excellent animation of mitosis.
The Phases
The phases of mitosis are discussed in the video: http://www.youtube.com/watch?v=LLKX_4DHE3I (20:42).
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FIGURE 1.1
Mitosis is the phase of the eukaryotic cell
cycle that occurs between DNA replication and the formation of two daughter
cells. What happens during mitosis? During mitosis, the nucleus divides, paving
the way for two cells to be produced
after cell division, each with a complete
makeup of genetic material.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/271
Prophase
Prophase is the first and longest phase of mitosis, see Figure 1.2. During prophase, the chromatin (DNA) coils
up into visible chromosomes, each made up of two sister chromatids held together by the centromere. Also during
this phase, the nucleolus disappears, and the spindle begins to form from the centrioles. Most eukaryotic cells
contain structures known as centrosomes, consisting of a pair of centrioles. During prophase, the centrioles begin
to move to opposite ends, or poles, of the cell. As the centrioles migrate, the fiber-like spindle begins to elongate
between the centrioles. The spindle is a thin, cage-like structure made out of microtubules. In plant cells, the spindle
forms without centrioles. The spindle plays an essential role moving chromosomes and in the separation of sister
chromatids.
Preprophase
As plant cells have some structural differences compared to an animal cell, an additional stage prior to prophase is
necessary. In plant cells only, prophase is preceded by a preprophase stage. Plant cells have a large central vacuole
encompassing the center of the cell. Prior to the division of the nucleus, the nucleus must migrate to the center of the
plant cell. To accomplish this, the cell forms a phragmosome, a sheet of cytoplasm that bisects the middle of the
cell. The phragmosome suspends the cell nucleus in the center of the cell in preparation for prophase. Additionally,
during this phase the plane of cell division is established. The accurate control of division planes, which establishes
the placement of the future cell wall, is crucial for the correct structure of plant tissues and organs.
The mitotic spindle also starts to form during preprophase in plant cells. Unlike animal cells, plant cells do not have
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Chapter 1. Mitosis - Advanced
FIGURE 1.2
The
spindle
starts
prophase of mitosis.
to
form
during
Kinetochores on
the spindle attach to the centromeres of
sister chromatids.
centrosomes to organize their mitotic spindles. Instead, in these cells, the nuclear envelope acts as a microtubule
organizing center (MTOC) for spindle formation. The preprophase spindle forms by self-assembly of microtubules
in the cytoplasm surrounding the nuclear envelope.
Prometaphase
During early prometaphase, the nuclear membrane disintegrates and microtubule spindles invade the center of the
cell. Also during this phase, the spindle attaches to the centromere of each chromatid. Specifically, the spindle
attaches to the kinetochore, a protein structure on the centromere where the spindle fibers attach.
Metaphase
During metaphase, the centromeres of the chromosomes line up along the metaphase plate or equatorial plane, in
essence the approximate middle of the cell. This orientation of the chromosomes at the equator of the cell helps to
ensure proper chromosome separation. This alignment allows the spindle fibers to correctly pull the chromatids to
either pole of the cell, resulting in separation of sister chromatids from a chromosome, see Figure 1.3.
FIGURE 1.3
Chromosomes, consisting of sister chromatids, line up at the equator
(metaphase plate) of the cell during metaphase.
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Anaphase
Anaphase is the phase in which the sister chromatids separate. The sister chromatids are pulled apart by the
shortening of the microtubules of the spindles, similar to the reeling in of a fish by the shortening of the fishing
line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite
pole. This process occurs when the proteins that bind sister chromatids together are cleaved, resulting in unattached
identical chromosomes, essentially separate daughter chromosomes. These separate chromosomes are pulled apart
by shortening spindle fibers, and pulled toward the centrosomes to which they are attached. At the end of anaphase
the spindle fibers degrade. At this time, each pole of the cell has a complete set of chromosomes, identical to the
amount of DNA at the beginning of G1 of the cell cycle.
Telophase
Telophase is essentially the opposite of prophase and prometaphase. The chromosomes begin to unwind back
into chromatin in preparation to direct the cell’s metabolic activities. A new nucleus forms around each set of
chromosomes. This is followed by cytokinesis, the division of the cytoplasm, resulting in two genetically identical
cells, ready to enter G1 of the next cell cycle. The phases of mitosis are summarized in Figure 1.4.
FIGURE 1.4
Mitosis in the Eukaryotic Cell Cycle. Mitosis is the multi-phase process in which the
nucleus of a eukaryotic cell divides. In this
diagram, prometaphase is not included as
a separate phase, but incorporated into
prophase.
Cytokinesis
Cytokinesis is the final step in cell division. It often occurs concurrently with telophase, though it is a separate
process. Cytokinesis ( Figure 1.6) differs between plant and animal cells. In animal cells, the plasma membrane
pinches inward along the cell’s equator until two cells are formed. Specifically, a cleavage furrow containing a
contractile ring develops in approximately the middle of the cell (similar to the position of the metaphase plate),
essentially pinching off the two nuclei and forming separate cells. In plant cells, a cell plate forms along the cells
equator. A new membrane grows along each side of the cell plate, with a new cell wall forming on the outside of
each new membrane.
At the end of cytokinesis, each daughter cell has a complete copy of the genome of its parent cell. The end of
cytokinesis marks the end of the M-phase, the end of one cell cycle, and the beginning of G1 and interphase of the
next cell cycle.
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Chapter 1. Mitosis - Advanced
FIGURE 1.5
This is a representation of dividing plant
cells. Cell division in plant cells differs
slightly from animal cells as a cell wall
must form. Note that most of the cells are
in interphase. Can you find examples of
the different stages of mitosis?
FIGURE 1.6
Cytokinesis is the final stage of eukaryotic cell division. It occurs differently
in animal (left) and plant (right) cells.
FIGURE 1.7
In this electron micrograph of a cell, the
formation of two new cells is almost complete, as new membrane grows and divides the parent cell.
Inside the Cell: The Stages of Mitosis can be viewed at http://www.youtube.com/watch?v=2WwIKdyBN_s (1:22).
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MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/272
Vocabulary
• anaphase: Phase of mitosis during which sister chromatids separate and move to opposite poles of the cell.
• cell plate: Forms during cytokinesis in plant cells; a new membrane grows along each side of the cell plate,
with a new cell wall forming on the outside of each new membrane.
• centriole: A cylindrical shaped cell structure composed of nine triplets of microtubules; structure from which
spindle fibers originate.
• cytokinesis: Division of the cytoplasm, forming two daughter cells.
• kinetochore: The protein structure on chromatids where the spindle fibers attach during cell division.
• metaphase: Phase of mitosis during which chromosomes line up at the equator of the cell.
• metaphase plate: The center (equator) of a cell during mitosis; chromosomes line up at the metaphase plate
to ensure the proper separation of the sister chromatids.
• mitosis: The division of the nucleus into two genetically identical nuclei.
• phragmosome: A sheet of cytoplasm that forms in highly vacuolated plant cells to prepare for mitosis; forms
in the approximate center of the cell.
• preprophase: Phase of mitosis prior to prophase; only in plant cells; ensures the nucleus is positioned in the
center of the cell.
• prometaphase: Phase of mitosis during which the nuclear envelope breaks down, centrioles separate, and a
spindle moves through the center of the cell.
• prophase: First phase of mitosis during which chromatin condense into chromosomes, the nuclear envelope
breaks down, centrioles separate, and a spindle begins to form.
• telophase: Last stage of mitosis during which chromosomes uncoil to form chromatin, the spindle breaks
down, and new nuclear membranes form.
Summary
• Mitosis is the division of the nucleus.
• Mitosis is the division of the nucleus; five distinct phases of mitosis have been recognized: prophase, prometaphase,
metaphase, anaphase, and telophase.
• Cytokinesis is the division of the cytoplasm. Cytokinesis occurs after mitosis.
• At the end of cytokinesis there are two genetically identical daughter cells.
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Chapter 1. Mitosis - Advanced
Explore More
• Animal Cell Mitosis at http://www.cellsalive.com/mitosis.htm .
Review
1.
2.
3.
4.
Order the phases of mitosis.
Describe the main processes involved in mitosis.
Differentiate cytokinesis in animal and plant cells.
Four phases of mitosis: can you describe what happens in each phase?
References
1.
2.
3.
4.
5.
6.
7.
Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0
Courtesy of Nogales group and Lawrence Berkeley National Laboratory. . Public Domain
Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0
Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0
Edmund Beecher Wilson. . Public Domain
Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0
Wadsworth Center. . Public Domain
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