Mitotic Spindle:  A very complex
process allows the chromosomes to
separate during the M phase of the
cell cycle.  There are really 5 parts
to mitosis that together take about
1 hr. During Prophase the condense
to form the chromosomes.  Near the
end of prophase is called
prometaphase.  In this phase the
centrosome begin to project fibers
that make up the spindle apparatus. 
These fibers are made of
microtubules.  The centrosome
(purple) actually begin to duplicate
about the time of S phase.  The begin
to be recognizable during
prometaphase when they develop the
fibers and move towards opposite
poles.  At the same time the nuclear
envelop disappears.  Inside each
centrosome are CENTRIOLES (purple
boxes) are found in animal cells but
not in flowering plants, while
centrosome are found in all
eukaryotic cells.  The exact role of
the centrioles is unknown, but the
centrosome around them act as
crystallizing centers for the
formation of microtubules.   Each
microtubule has 2 ends one called the
plus end (+) the other the minus (-
) end.  Under the conditions found
in the cell microtubules always grow
and fall apart (disassemble) from the
+ end.   The plus end is the end
farthest from the centrosome. There
are 3 types of spindle fibers in the
spindle apparatus based on their
location.  All are made of
microtubules and all originate from
the centrosome.  The first is called
polar fibers. These fibers (seen in
this image in red) extend from the
centrosome towards the opposite
pole(centrosome).  Some make it to
the other side some only reach part
way.  In electron micrographs some
of these fibers coming from opposite
ends are connected together by
microtubule associated proteins
(MAPS).  These interdigitating
microtubules help to hold the spindle
together.  They are depicted in this
image connected by small red boxes. 
The second type of spindle fiber is
called a CHROMOSOMAL FIBER OR
KINETOCHORE MICROTUBULE.   These
fibers run from centrosome to the
kinetochore of the chromosome.   The
chromosome at this time is already
replicated and consists of 2
chromatids connected at their
CENTROMERE.  Attached to the
centromere is a protein blob called
the kinetochore.   The kinetochore
is the attachment site for the
chromosomal fibers.  About 15 to 30
fibers attach for each side
chromatid.   During prometaphase
their is a continual process of
assembly of chromosomal microtubules
and their disassembly.   As the
fibers attach to the chromosome they
pull it towards that centrosome.  As
fibers from the opposite centrosome
attach they pull it the other way. 
This process continues until their
are equal length and numbers of
chromosomal fibers on each side of
the chromosome.  This equal length
of fibers cause the chromosomes to
align in the midline of the cell. 
This is the part of mitosis we call
METAPHASE.  Metaphase itself is very
short.   At this point a store of
calcium ions are released from inside
the cell and the centromere splits.
This begins ANAPHASE.  During
anaphase the chromatids are pulled
to their centrosome.  No one knows
how this occurs.  There are two basic
theories.  The oldest is called
ASSEMBLY AND DISASSEMBLY.  In this
theory the microtubules themselves
provide the force for movement of the
chromatids.  In this process,
microtubule subunits are disassembled
from the kinetochore end of the
chromosomal fibers.  The chromosome
is then attached to the existing
shorter chromosomal spindle fibers
, like a swimmer reaching for a
lifeline.  This process of
disassembly leads to the chromosome
moving closer towards their pole. The
extra microtubule subunits end up in
the polar fibers which help to
elongate the cell.  Another
hypothesis uses a SLIDING FILAMENT 
idea.   In this process there is some
associated protein that acts as a
motor. This motor works between the
chromosomal fibers to slide them past
each other.  In the process pieces
are broken off the kinetochore end
and the chromosomes get close to the
poles.  The motor protein has not
been isolated, but various scientists
have shown that ATP is required in
vitro to get the chromosomes to move. 
This would indicate that a motor
protein is involved.   The final type
of spindle fiber is the ASTRAL
FIBERS.  It has no role in chromosome
separation.  It is not found in
flowering plants.  They have what are
called an ANASTRAL SPINDLE APPARATUS. 
  With the completing of anaphase
begins the process of cytoplasm
separation.  This production of 2
daughter cells is called CYTOKINESIS. 
 When anaphase is completed the
remaining polar fibers collapse in
the center of the cell in a pile
called the MIDBODY.  This signals the
actin MICROFILAMENTS just under the
cell membrane to begin to contact. 
The contractile ring pinches off the
cytoplasm into two cells.  The cells
remain connected for some time until
the midbody can be broken.     