A Review of Mitosis: Eukaryotic Nuclear Division
Cameron Incognito
March 22, 2013
Introduction
The ability of an organism to reproduce is one of the characteristics dividing living things from
non-living things. Mitosis is a specific form of reproduction that occurs in eukaryotic, nongamete cells involving the division of nucleic genetic material into two identical daughter nuclei.
Preceding the mitotic (M) phase is the interphase (consisting of the G1 phase, S phase, and the G2
phase) in which genetic material is duplicated and synthesized in preparation for the formation of
daughter cells. Once interphase has been successfully completed, the cell progresses to the
mitotic (M) phase, which is generally divided into five main stages: prophase, prometaphase,
metaphase, anaphase, and telophase/cytokinesis. After mitosis and cytokinesis have been
completed, a parent cell will have divided into two genetically identical daughter cells. The
mitotic (M) phase is described in detail below in Figure 1:
Figure 1. The five stages of the mitotic (M) phase
Image from: < http://www.shmoop.com/cell-cycle/mitosis.html>
Prophase
Before the start of prophase, the nuclear envelope encases the nucleus, the chromosomes are
loosely condensed, and two centrosomes have been developed in the G2 phase of interface.
Prophase then begins starting with the chromatin fibers, a complex of DNA and proteins,
becoming condensed into chromosomes. These chromosomes appear as two sister chromatids
connected in the center at the centromere. The nucleoli begin to be degraded by cellular proteins
causing the nucleoli to disappear. Lastly, the two centrosomes start to produce microtubules that
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begin to extend, pushing the centrosomes towards opposite ends of the cell. The complex of
centrosomes and microtubules is termed the mitotic spindle.
Figure 2. A cell in prophase during the mitotic (M) phase of reproduction
Image from: < http://www.biology.iupui.edu/biocourses/N100/2k4ch8mitosisnotes.html>
Prometaphase
During prometaphase, the nucleoli are completely degraded, releasing the chromosomes into the
cytoplasm. The chromosomes continue to condense and form kinetochores at the centromere of
chromatids. The microtubules expand through the cell entering into the nuclear space where
specific microtubules, termed kinetochore microtubules, connect to the newly present
kinetochores. The kinetochore microtubules are then moved around in a jerking motion
attempting to move the chromatids towards the metaphase plate, a plane equidistant from each of
the two centrosomes at the poles of the cell. Non-kinetochore microtubules begin to interact with
complementary non-kinetochore microtubules being generated from the opposing centrosome.
Figure 3. A cell in prometaphase during the mitotic (M) phase of reproduction
Image from: < http://www.biology.iupui.edu/biocourses/N100/2k4ch8mitosisnotes.html>
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Metaphase
The centrosomes extend completely to opposite poles of the cell so the spindle is stretched as far
as possible, providing ample space for the preparation of two cells. The chromosomes, connected
to the spindle at the kinetochore, then reach equilibrium with their centromeres coming to rest at
the metaphase plate. The kinetochores of sister chromatids are all fully attached to kinetochore
microtubules branching from the opposite pole.
Figure 4. A cell in metaphase during the mitotic (M) phase of reproduction
Image from: < http://www.biology.iupui.edu/biocourses/N100/2k4ch8mitosisnotes.html>
Anaphase
At the beginning of anaphase, cohesion proteins are cleaved following a complex signaling
cascade, allowing paired sister chromatids to split and form independent daughter chromosomes.
The daughter chromosomes are pulled, centromere first, towards opposite poles of the cell due to
the shortening of their kinetochore microtubules. The non-kinetochore microtubules continue to
lengthen effectively elongating the cell. At the end of anaphase, each side of the cell contains
equivalent and complete sets of chromosomes capable of forming a functioning cell.
Figure 5. A cell in anaphase during the mitotic (M) phase of reproduction
Image from: < http://www.biology.iupui.edu/biocourses/N100/2k4ch8mitosisnotes.html>
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Telophase and Cytokinesis
Telophase begins with the formation of two daughter nuclei in the cell. The nuclear envelope
begins to form using the materials from the original parent nuclear envelope and the cell’s
endomembrane system. The nucleoli begin to reappear inside of the envelope and the daughter
chromosomes begin to de-condense inside of the nuclear space, transforming into accessible
genetic information. The spindle microtubules, both kinetochore and non-kinetochore,
depolymerize and are recycled in the cell. Mitosis, or the division of a single parent nucleus into
two identical daughter nuclei, is complete after telophase. The cell, however, has not fully
divided until cytokinesis, or the division of the cytoplasm, occurs. Cytokinesis begins with the
development of a groove, termed the cleavage farrow, at the site of the metaphase plane. Actin
microfilaments on one side of the groove interact with myosin proteins on the opposing side to
cause a contraction until the cell is pinched in two. Two separate cells containing identical
genetic information have now been produced.
Figure 6. A cell after the completion of telophase and cytokinesis stages of reproduction
Image from: < http://www.biology.iupui.edu/biocourses/N100/2k4ch8mitosisnotes.html>
Conclusion
Mitosis is the end stage of reproduction for all eukaryotic, non-gamete cells during which the
cell’s genetic material is divided into two sets of identical daughter genetic information. With the
successful completion of the interphase (G1 phase, S phase, and G2 phase), the cell proceeds to
the mitotic (M) phase. Inside of the cell the nuclear envelope begins to degrade leaving the
chromatin fibers exposed. The chromatin fibers tightly coil and condense producing
chromosomes with a structure consisting of two identical sister chromatids joined at the
centromere. The centrosomes start to produce the mitotic spindle and move away from each
other towards opposite poles of the cell. Next, the nuclear envelope completely fragments
allowing the extending kinetochore microtubules to attach at the chromatid centromeres. The
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non-kinetochore microtubules continue to expand encompassing the entire cell until the
centrosomes are at opposite poles of the cell. Once the chromosomes are aligned with the
kinetochore on the metaphase plate, cohesion proteins are cleaved via a signaling cascade
causing a transition to anaphase. During anaphase the two sister chromatids are pulled apart in
opposite directions by kinetochore microtubules resulting in both sides of the cell containing
equivalent and complete sets of genetic material. The non-kinetochore microtubules continue to
expand elongating the cell. In telophase, two daughter nuclei are formed via the formation of
new nuclear envelopes and the re-emergence of nucleoli. The microtubules are depolymerized,
the chromosomes de-condense, and the cell proceeds to cytokinesis during which the cell divides
its cytoplasm. The original parent cell has now been divided to create two identical daughter
cells.
References
Reece, Jane B., Lisa A. Urry, Michael Lee Cain, Steven A. Wasserman, Peter V. Minorsky, and
Robert Bradley Jackson. Campbell Biology. Boston [etc.: Benjamin Cummings, 2011.
Print.
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