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Mitosis: Prophase
 A cells chromosomes remain at least partially
uncondensed during interphase to allow for
transcription and DNA replication.
 However, in preparation for nuclear division, the
chromosomes condense into tightly coiled packages
that are more easily transported.
 Once the chromosomes have condensed enough
that they are visible under the light microscope, we
say that the cell is in PROPHASE.
Mitosis: Prophase
 In animal cells, a region near the nucleus that organizes
microtubules while they are forming is called a centrosome.
 The centrosome also contains two barrel-shaped structures
called centrioles.
 The centrosome is duplicated just before prophase begins.
One centrosome (along with its two centrioles) moves to
opposite ends (or poles) of the cell during prophase.
 Microtubules then begin to grow from each centrosome.
 These microtubules will form the spindle- a dynamic network
of microtubules that attaches to and moves chromosomes
during mitosis.
Mitosis: Prophase
 At the end of prophase, the nuclear envelope breaks
down and microtubules enter the region where the
nucleus had previously been.
 Some microtubules stop growing when they reach
the middle of the cell, while others continue to grow
until they reach a chromosome and attach to it.
 At some point, different microtubules from opposite
poles of the cell will be attached to each sister
chromatid to ensure that each sister chromatid will
be pulled to opposite ends of the cell later during
mitosis.
Mitosis: Prophase
Mitosis: Metaphase
 The microtubules from opposite poles that are
attached to each sister chromatid begin a “tug-ofwar” in which the lengthen and shorten (by adding
tubulin subunits), pushing and pulling the
chromosomes with them.
 Eventually, all of the microtubules reach the same
length.
 At this point, the chromosomes are aligned in the
middle (or equator) of the cell along what is called
the metaphase plate.
Mitosis: Metaphase
Mitosis: Anaphase
 During anaphase, the microtubules that are attached
to the sister chromatids of each chromosome begin
to shorten.
 As this shortening occurs, the two sister chromatid
of each chromosome are pulled apart so that each
moves to opposite poles of the cell.
 Each DNA molecule (sister chromatid) has now
become a separate chromosome.
Mitosis: Anaphase
Mitosis: Telophase
 Telophase begins when the two clusters of
chromosomes reach opposite poles of the cell.
 Each cluster of chromosomes contains the same
number as the parent cell had. Thus the
chromosome number of each new cell that will arise
from each pole will be diploid (2n).
 A new nucleus forms around each cluster of
chromosomes and the chromosomes begin to
partially uncondense again.
 The end result is two diploid nuclei.
Mitosis: Telophase
Cytokinesis
 After mitosis is over, the cell usually divides its cytoplasm so that
two separate diploid daughter cells are formed.
 Animal cells typically pinch in two.
 This happens when a contractile ring composed of actin and
myosin protein filaments that is part of the cell cortex (a layer of
cytoskeletal proteins just under the plasma membrane) begins to
contract.
 As the contractile ring contracts, it drags the plasma membrane
inward, forming an indentation that can be seen with a
microscope. This indentation is called a cleavage furrow.
 As the cleavage furrow deepens, the cytoplasm, and the cell, are
pinched in two.
Cytokinesis: Animal Cell
Cytokinesis
 Plant cells go through cytokinesis differently than animal cells due to the
presence of the cell wall composed of cellulose, which is a rigid structure
that can’t be pinched in two.
 By the end of anaphase in a plant cell, a set of short microtubules has
formed on either side of the cell’s equator.
 These microtubules guide vesicles from Golgi bodies and the cell surface to
the equator of the cell.
 At the cell’s equator, these vesicles carrying their cell wall building
components fuse together, forming a disk-shaped structure called the cell
plate.
 The cell plate grows outward until it reaches the plasma membrane, where
it attaches and, in doing so, partitions the cytoplasm into two
approximately parts.
 In time, the cell plate develops into cell wall and joins with the original cell
wall of the parent cell, producing two daughter cells with their own plasma
membranes and cell walls.
Cytokinesis: Plant Cell