Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CYTOKINESIS
1001
preprophase band of microtubules
and actin filaments
remains of interpolar
spindle microtubules
cortical array of
interphase microtubules
early cell
plate
new
cell wall
vacuole
mother
cell wall
Golgiderived
vesicles
phragmoplast
microtubules
G 2 telophase cytokinesis G 1
vesicle fusion until it reaches the plasma membrane and the original cell wall and
divides the cell in two. Later, cellulose microfibrils are laid down within the matrix
of the cell plate to complete the construction of the new cell wall (Figure 17–49).
MBoC6 m17.57/17.49
Membrane-Enclosed Organelles Must Be Distributed to Daughter
Cells During Cytokinesis
The process of mitosis ensures that each daughter cell receives a full complement
of chromosomes. When a eukaryotic cell divides, however, each daughter
cell must also inherit all of the other essential cell components, including the
membrane-enclosed organelles. As discussed in Chapter 12, organelles such as
mitochondria and chloroplasts cannot be assembled de novo from their individual
components; they can arise only by the growth and division of the preexisting
organelles. Similarly, cells cannot make a new endoplasmic reticulum (ER) unless
some part of it is already present.
How, then, do the various membrane-enclosed organelles segregate when a
cell divides? Organelles such as mitochondria and chloroplasts are usually present
in large enough numbers to be safely inherited if, on average, their numbers
roughly double once each cycle. The ER in interphase cells is continuous with
the nuclear membrane and is organized by the microtubule cytoskeleton. Upon
entry into M phase, the reorganization of the microtubules and breakdown of
the nuclear envelope releases the ER. In most cells, the ER remains largely intact
and is cut in two during cytokinesis. The Golgi apparatus is reorganized and fragmented
during mitosis. Golgi fragments associate with the spindle poles and are
thereby distributed to opposite ends of the spindle, ensuring that each daughter
cell inherits the materials needed to reconstruct the Golgi in telophase.
Figure 17–49 The special features of
cytokinesis in a higher-plant cell. The
division plane is established before
M phase by a band of microtubules and
actin filaments (the preprophase band)
at the cell cortex. At the beginning of
telophase, after the chromosomes have
segregated, a new cell wall starts to
assemble inside the cell at the equator of
the old spindle. The interpolar microtubules
of the mitotic spindle remaining at
telophase form the phragmoplast. The
plus ends of these microtubules no
longer overlap but end at the cell equator.
Golgi-derived vesicles, filled with cell-wall
material, are transported along these
microtubules and fuse to form the new cell
wall, which grows outward to reach the
plasma membrane and original cell wall.
The plasma membrane and the membrane
surrounding the new cell wall fuse,
separating the two daughter cells.
Some Cells Reposition Their Spindle to Divide Asymmetrically
Most animal cells divide symmetrically: the contractile ring forms around the
equator of the parent cell, producing two daughter cells of equal size and with
the same components. This symmetry results from the placement of the mitotic
spindle, which in most cases tends to center itself in the cytoplasm. Astral microtubules
and motor proteins that either push or pull on these microtubules contribute
to the centering process.
There are many instances in development, however, when cells divide asymmetrically
to produce two cells that differ in size, in the cytoplasmic contents they
inherit, or in both. Usually, the two different daughter cells are destined to develop