Download Lec.14 Dr:Buthaina Al-Sabawi Date:21/12/2016 Mitosis

Events during Mitosis
Lec.14
Dr:Buthaina Al-Sabawi
Date:21/12/2016
Mitosis
Mitosis is a process where a single cell divides into two identical
daughter cells (cell division).
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During mitosis one cell divides ONCE to form two identical cells.
The major purpose of mitosis is for growth and to replace worn out cells.
Mitosis is nuclear division plus cytokinesis, and produces two identical
daughter cells during prophase, prometaphase, metaphase, anaphase, and
telophase.
Interphase: Cells may appear inactive during this stage, but they are
quite the opposite. This is the longest period of the complete cell cycle
during which DNA replicates, the centrioles divide, and proteins are
actively produced. For a complete description of the events during
Interphase, read about the Cell Cycle.
Interphase
Prophase:
 Chromosomes become more coiled and can be viewed under a
light microscope.
 Each duplicated chromosome is seen as a pair of sister chromatids
joined by the duplicated but unseparated centromere.
(1)
 The nucleolus disappears during prophase.
 In the cytoplasm, the mitotic spindle, consisting of microtubules
and other proteins, forms between the two pairs of centrioles as
they migrate to opposite poles of the cell.
 The nuclear envelope disappears at the end of prophase. This
signals the beginning of the substage called prometaphase.
Prophase
Prometaphase: In this stage the nuclear envelope breaks down so there is
no longer a recognizable nucleus. Some mitotic spindle fibers elongate
from the centrosomes and attach to kinetochores, protein bundles at the
centromere region on the chromosomes where sister chromatids are
joined. Other spindle fibers elongate but instead of attaching to
chromosomes, overlap each other at the cell center.
Metaphase: The chromosomes become arranged on the metaphase plate
and are attached to the now fully formed spindle.
(2)
(3)
Kinetochore microtubules: each sister chromatid is attached to
a kinetochore microtubule; in cell division during anaphase the
microtubules hold onto the kinetochore and pull the two sister
chromatids apart to opposite poles.
Polar microtubules: microtubules that connect to each other
from opposite poles; by pushing against each other they elongate
the cell; during anaphase they disconnect and are pulled apartd.
Aster microtubules: microtubules that extend to the top of the
cell; above the centrosome; during prophase they start extending
into polar microtubules.
Anaphase: Spindle fibers shorten, the kinetochores separate . Anaphase
begins when the duplicated centromeres of each pair of sister chromatids
separate, and the now-daughter chromosomes begin moving toward
opposite poles of the cell due to the action of the spindle.
At the end of anaphase, a complete set of chromosomes has assembled at
each pole of the cell.
(4)
Telophase:
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.
Cytokinesis: 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. In plant
cells, the rigid wall requires that a cell plate be synthesized between the
two daughter cells.
Anaphase
(5)
Telophase
(6)
(7)