Essential Cell Biology 5th edition

638 CHAPTER 18 The Cell-Division Cycle
interphase
mitosis
(anaphase)
cytokinesis
interphase
Figure 18−34 Animal cells change shape during M phase. In these micrographs of a mouse fibroblast dividing in culture, the
same cell was photographed at successive times. Note how the cell becomes smaller and rounded as it enters mitosis; the two
daughter cells then flatten out again after cytokinesis is complete. (Courtesy of Guenter Albrecht-Buehler.)
QUESTION 18–8
Draw a detailed view of the
formation of the new cell wall
that separates the two daughter
cells when a plant cell divides (see
Figure 18−35). In particular, show
where the membrane proteins of
the Golgi-derived vesicles end
up, indicating what happens to
the part of a protein in the Golgi
vesicle membrane that is exposed
to the interior of the Golgi vesicle.
(Refer to Chapter 11 if you need a
reminder of membrane structure.)
flatten out again (Figure 18−34). When cells divide in an animal tissue,
this cycle of attachment and detachment presumably allows the cells to
rearrange their contacts with neighboring cells and with the extracellular
matrix, so that the new cells produced by cell division can be accommodated
within the tissue.
ECB5 E18.33/18.34
Cytokinesis in Plant Cells Involves the Formation of a
New Cell Wall
The mechanism of cytokinesis in higher plants is entirely different from
that in animal cells, presumably because plant cells are surrounded by
a tough cell wall (discussed in Chapter 20). The two daughter cells are
separated not by the action of a contractile ring at the cell surface but
instead by the construction of a new wall that forms inside the dividing
cell. The positioning of this new wall precisely determines the position of
the two daughter cells relative to neighboring cells. Thus, the planes of
cell division, together with cell enlargement, determine the final form of
the plant.
The new cell wall starts to assemble in the cytoplasm between the two
sets of segregated chromosomes at the start of telophase. The assembly
process is guided by a structure called the phragmoplast, which
is formed by the remains of the interpolar microtubules at the equator
of the old mitotic spindle. Small membrane-enclosed vesicles, largely
derived from the Golgi apparatus and filled with polysaccharides and glycoproteins
required for the cell wall matrix, are transported along the
microtubules to the phragmoplast. Here, they fuse to form a disclike,
membrane-enclosed structure, which expands outward by further vesicle
fusion until it reaches the plasma membrane and original cell wall,
thereby dividing the cell in two (Figure 18−35). Later, cellulose microfibrils
are laid down within the matrix to complete the construction of the
new cell wall.
Membrane-enclosed Organelles Must Be Distributed to
Daughter Cells When a Cell Divides
Organelles such as mitochondria and chloroplasts cannot assemble
spontaneously from their individual components; they arise only from the
growth and division of the preexisting organelles. Likewise, endoplasmic
reticulum (ER) and Golgi apparatus also derive from preexisting organelle
fragments. How, then, are these various membrane-enclosed organelles
segregated when the cell divides so that each daughter gets its share?