Mitosis and
Cytokinesis
Section 6-3
In Mitosis, Chromatids are
pulled by Microtubules
 During mitosis the nucleus divides to
form two nuclei, each containing a
complete set of chromosomes and the
chromatids on each chromosome are
physically moved to opposite sides of
the dividing cell with the help of the
spindle.
 During cytokinesis the cytoplasm is
divided between the two resulting
cells.
 Spindles – cell structures made up of
both centrioles and individual
microtubule fibers that are involved in
moving chromosomes during cell
division.
 Forming the Spindles in Animal Cells
1. One pair of centrioles, at right angles
to one another
2. During G2 phase – the centriole pair
is replicated so that the cell has two
pairs of centrioles as it enters the
mitotic phase.
3. When the cell enters mitotic phase,
centriole pairs begin to separate,
moving towards the opposite poles of
the cell.
4. As centrioles separate, spindles
begin to form.
5. Centrioles and spindle fibers – both
made of hollow tubes of protein
called microtubules.
6. Each spindle fiber made of an
individual microtubule. Each
centriole, however, is made of nine
triplets of microtubules arranged
in a circle.
Forming Spindle Fibers in Plant Cells
 Plant cells do not have centrioles,
but they form a spindle almost
identical to that of an animal cell.
The Mitotic Spindle
Separating Chromatids by
Attaching Spindle Fibers
 Some microtubules in spindle
interact with each other, others
attach to a protein structure
found on each side of the
centromere.
 The two sets of microtubules
extend out toward opposite
poles of the cell.
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
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Once microtubules attach to
centromeres and poles, chromatids can
be separated.
Chromatids moved to each pole of cell
in a manner similar to bringing in a
fish with a fishing pole. When
microtubule is “reeled in”, the
chromatids are dragged to opposite
poles.
Reeling occurs because ends of spindle
fibers are broken down bit by bit at
each of the poles. Fibers become
shorter – chromatids move closer.
 As soon as chromatids separate
from each other – called
chromosomes.
 When finally arrive at the poles,
one complete set is present.
Mitosis and Cytokinesis
Divide Cells

Mitosis - Continuous process, but
divided into four stages
P – Prophase
M – Metaphase
A – Anaphase
T – Telophase
Stage One – Prophase
 Chromosomes coil up and
become visible
 Nuclear envelope dissolves
 Spindle forms
Stage Two – Metaphase
 chromosomes move to center
of cell and line up along the
equator
 spindle fibers link chromatids
to opposite poles
Stage Three – Anaphase
 centromeres divide
 two chromatids (now called
chromosomes) move toward
opposite poles of cell as spindle
fibers shorten
Stage Four – Telophase
 nuclear envelope forms around
chromosomes at opposite pole
of cell
 chromosomes uncoil
 spindle dissolves
After four stages are complete, so
is mitosis.
http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html
http://www.johnkyrk.com/mitosis.html
http://www.accessexcellence.org/RC/VL/GG/mitosis.html
http://www.cellsalive.com/mitosis.htm
Cytokinesis
 Begins as mitosis ends
 cytoplasm of cell is divided in half
 cell membrane grows to enclose
each cell, which forms two cells
 end result of mitosis and
cytokinesis – two identical cells
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
Animal cells and other cells
without cell walls – cell is pinched
in half by a belt of protein threads
Plant cells and other cells with cell
walls have a different method of
dividing cytoplasm.

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vesicles formed by the Golgi apparatus
fuse at midline of dividing cell and
form a cell plate
Cell Plate – membrane bound cell wall
that forms across the middle of a plant
cell in cytokinesis

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New cell wall forms on both sides
of the cell plate.
When complete – cell plate
separates plant cell into two new
plant cells.
In plant and animal cells,
offspring cells are about
the same size. Each has an
identical copy of the
original cell’s chromosomes
and about ½ of the
cytoplasm and organelles.
Summary
 Before mitosis begins, the cell will
form the mitotic spindle during the
G2 phase of the cell cycle.
 Mitosis occurs in four stages – PMAT
 Cytokinesis occurs after mitosis –
division of the cell’s cytoplasm
 http://school.discoveryeducation.com
/quizzes6/muskopf/mitosis1.html
HOMEWORK
 Section 6-3 Review Questions
 Math Lab p. 129