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

1000 Chapter 17: The Cell Cycle
(A)
The Phragmoplast Guides Cytokinesis in Higher Plants
In most animal cells, the inward movement of the cleavage furrow depends on an
increase in the surface area of the plasma membrane. New membrane is added
at the inner edge of the cleavage furrow and is generally provided by small membrane
vesicles that are transported on microtubules from the Golgi apparatus to
the furrow.
Membrane deposition is MBoC6 particularly m17.55/17.47
important for cytokinesis in higher-plant
cells. These cells are enclosed by a semirigid cell wall. Rather than a contractile
ring dividing the cytoplasm from the outside in, the cytoplasm of the plant cell is
partitioned from the inside out by the construction of a new cell wall, called the
cell plate, between the two daughter nuclei (Figure 17–48). The assembly of the
cell plate begins in late anaphase and is guided by a structure called the phragmoplast,
which contains microtubules derived from the mitotic spindle. Motor
proteins transport small vesicles along these microtubules from the Golgi apparatus
to the cell center. These vesicles, filled with polysaccharide and glycoproteins
required for the synthesis of the new cell wall, fuse to form a disc-like, membraneenclosed
structure called the early cell plate. The plate expands outward by further
(B)
Figure 17–47 Localization of cytokinesis
regulators at the central spindle of the
human cell. (A) At center is a cultured
human cell at the beginning of cytokinesis,
showing the locations of the GTPase RhoA
(red) and a protein called Cyk4 (green),
which is one of several regulatory proteins
that form complexes at the overlapping
plus ends of interpolar microtubules.
These proteins are thought to generate
signals that help control RhoA activity at
the cell cortex. (B) When the same threedimensional
image is viewed in the plane
of the contractile ring, as shown here,
RhoA (red) is seen as a ring beneath the
cell surface, while the central spindle
protein Cyk4 (green) is associated with
microtubule bundles scattered throughout
the equatorial plane of the cell. (Courtesy of
Alisa Piekny and Michael Glotzer.)
50 µm
Figure 17–48 Cytokinesis in a plant cell in telophase. In this light micrograph, the early cell plate
(between the two arrowheads) has formed in a plane perpendicular to the plane of the page. The
microtubules of the spindle are stained with gold-labeled antibodies against tubulin, and the DNA
in the two sets of daughter chromosomes is stained with a fluorescent dye. Note that there are no
astral microtubules, because there are no centrosomes in higher-plant cells. (Courtesy of Andrew
Bajer.)
MBoC6 m17.56/17.48