Images courtesy of The Biology Project (Dept. of Biochemistry and Molecular
Biophysics, University of Arizona)
|Definition of Mitosis
|Mitosis is the process of cell division where
chromosomes are visibly condensed and the nucleus divides to form
daughter chromosomes. Mitosis is central to the cellular
reproduction in which microtubules play a significant role in nearly all
stages of the cell cycle. Although mitosis is generally defined as
the five M phases (prophase, prometaphase, metaphase, anaphase, and
telophase) for the sake of microtubulin discussion, we also include
descriptions of interphase and cytokinesis that complete the entire cell
cycle resulting in two daughter cells.
Phases of the Cell Cycle:
The cell is engaged in metabolic activity and performing its prepare for
mitosis (the next four phases that lead up to and include nuclear
division). Chromosomes are not clearly discerned in the nucleus,
although a dark spot called the nucleolus may be visible. The cell may
contain a pair of centrioles which are organizational sites for
Chromatin in the nucleus begins to condense and becomes visible in the
light microscope as chromosomes. The nucleolus disappears. The two
centromeres of the cell, each with its pair of centrioles, move to
opposite "poles" of the cell. The mitotic spindle begins to form
which is an array of spindles each containing approximately 20
microtubules that are synthesized from the the tubulin polymers in the
The nuclear envelope disintergrates due to the dissolution of the lamins
that stabalize the internal membrane. Chromosomes can now attach
to spindle microtubules via kinetochores, and undergo movement.
Spindle fibers align the chromosomes along the metaphase plate in the
middle of the cell nucleus. The proposed mechanism is that
microtubules attached to opposite sides shrink or grow until they are at
equilibrium. Microtubule motors move them towards the minus end of
shrinking microtubules, or vice versa.
The paired chromosomes separate at the kinetochores and move to opposite
sides of the cell. This is thought to result from movement along the
microtubules powered by minus-end motors while the microtubules
themselves shorten at both ends. The overlapping spindle fibers
move past each, pushing the poles farther apart, powered by plus-end
Chromatids arrive at opposite poles of cell, and new membranes form
around the daughter nuclei. The chromosomes disperse and are no longer
visible under the light microscope. The spindle fibers disperse, and
cytokinesis or the partitioning of the cell may also begin during this
stage which is visible under light microscopy.
In animal cells, cytokinesis results when a fiber ring composed of a
protein called actin around the center of the cell contracts pinching
the cell into two daughter cells, each with one nucleus.
The goal of our study was to characterize
mitosis using light microscopy for in vivo observations of the cell cycle
starting from fertilization of sea urchins to
complete cell division over several cell cycles. Given the significant
role that microtubules play in mitosis, our study of mitosis in vitro
involved microtubulin polymerization which was quantified
and characterized using fluorescent microscopy.
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