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

1002 Chapter 17: The Cell Cycle
along different pathways. To create daughter cells with different fates in this way,
the mother cell must first segregate certain components (called cell fate determinants)
to one side of the cell and then position the plane of division so that the
appropriate daughter cell inherits these components (Figure 17–50). To position
the plane of division asymmetrically, the spindle has to be moved in a controlled
manner within the dividing cell. It seems likely that changes in local regions of
the cell cortex direct such spindle movements and that motor proteins localized
there pull one of the spindle poles, via its astral microtubules, to the appropriate
region. Genetic analyses in C. elegans and Drosophila have identified some of the
proteins required for such asymmetric divisions, and some of these proteins seem
to have a similar role in vertebrates.
Mitosis Can Occur Without Cytokinesis
Although nuclear division is usually followed by cytoplasmic division, there are
exceptions. Some cells undergo multiple rounds of nuclear division without intervening
cytoplasmic division. In the early Drosophila embryo, for example, the
first 13 rounds of nuclear division occur without cytoplasmic division, resulting in
the formation of a single large cell containing several thousand nuclei, arranged
in a monolayer near the surface. A cell in which multiple nuclei share the same
cytoplasm is called a syncytium. This arrangement greatly speeds up early development,
as the cells do not have to take the time to go through all the steps of
cytokinesis for each division. After these rapid nuclear divisions, membranes are
created around each nucleus in one round of coordinated cytokinesis called cellularization.
The plasma membrane extends inward and, with the help of an actin–
myosin ring, pinches off to enclose each nucleus (Figure 17–51).
Nuclear division without cytokinesis also occurs in some types of mammalian
cells. Megakaryocytes, which produce blood platelets, and some hepatocytes and
heart muscle cells, for example, become multinucleated in this way.
After cytokinesis, most cells enter G 1 , in which Cdks are mostly inactive. We
end this section by discussing how this state is achieved at the end of M phase.
The G 1 Phase Is a Stable State of Cdk Inactivity
A key regulatory event in late M phase is the inactivation of Cdks, which is driven
primarily by APC/C-dependent cyclin destruction. As described earlier, the inactivation
of Cdks in late M phase has many functions: it triggers the events of late
mitosis, promotes cytokinesis, and enables the synthesis of prereplicative complexes
at DNA replication origins. It also provides a mechanism for resetting the
anterior
posterior
40 µm
Figure 17–50 An asymmetric cell
division segregating cytoplasmic
components to only one daughter cell.
These light micrographs illustrate the
controlled asymmetric segregation
of specific cytoplasmic components to
one daughter cell during the first division
of a fertilized egg of the nematode
C. elegans. The fertilized egg is shown in
the left micrographs and the two daughter
cells in the right micrographs. The cells
above have been stained with a blue,
DNA-binding, fluorescent dye to show the
nucleus (and polar bodies); they are viewed
by both differential-interference-contrast
and fluorescence microscopy. The cells
below are the same cells stained with an
antibody against P-granules and viewed
by fluorescence microscopy. These small
granules are made of RNA and proteins
and determine which cells become germ
cells. They are distributed randomly
throughout the cytoplasm of the unfertilized
egg (not shown) but become segregated to
the posterior pole of the fertilized egg. The
cleavage plane is oriented to ensure that
only the posterior daughter cell receives
the P-granules when the egg divides.
The same segregation process is
repeated in several subsequent cell
divisions, so that the P-granules end up
only in cells that give rise to eggs and
sperm. (Courtesy of Susan Strome.)