Essential Cell Biology 5th edition

624 CHAPTER 18 The Cell-Division Cycle
Figure 18−16 The initiation of DNA
replication takes place in two steps.
During G 1 , Cdc6 binds to the ORC, and
together these proteins load a pair of
DNA helicases on the DNA to form the
prereplicative complex. At the start of S
phase, S-Cdk triggers the firing of this
loaded replication origin by guiding the
assembly of the DNA polymerase (green)
and other proteins (not shown) that initiate
DNA synthesis at the replication fork
(discussed in Chapter 6). S-Cdk also blocks
re-replication by phosphorylating Cdc6 (not
shown) and the ORC. This phosphorylation
keeps these proteins inactive and prevents
the reassembly of the prereplicative
complex until the Cdks are turned off in
the next G 1 .
G 1
DNA
DNA helicase
Cdc6
prereplicative complex (preRC)
S-Cdk
ORC (origin recognition complex
sitting on origin)
HELICASE BINDS,
Cdc6 DISSOCIATES
HELICASE ACTIVATED,
REPLICATION MACHINE
RECRUITED
ORIGIN
LOADED
S
replication
fork
P
DNA polymerase
P
P
ORIGIN
FIRED
P
COMPLETION OF
DNA REPLICATION
phosphates from M-Cdk. As a result, M-Cdk remains inactive and M
phase is delayed until DNA replication is complete and any DNA damage
is repaired.
Once a cell has successfully replicated its DNA in S phase, and progressed
through G 2 , it is ready to enter M phase. During this relatively
brief period, the cell will accomplish a remarkable reconfiguration, dividing
its nucleus (mitosis) and then its cytoplasm (cytokinesis; see Figure
18−2). In the next three sections, we describe the events that occur during
M phase. We first present a brief overview of M phase as a whole and
then discuss, in sequence, the mechanics of mitosis and of cytokinesis,
with a focus on animal cells.
M PHASE
Although M phase (which includes mitosis plus cytokinesis) takes place
ECB5 e18.16/18.16
over a relatively short amount of time—about one hour in a mammalian
cell—it is by far the most dramatic phase of the cell cycle. During this brief
period, the cell reorganizes virtually all of its components and distributes
them equally into the two daughter cells. The earlier phases of the cell
cycle, in effect, set the stage for the drama of M phase.
The central problem for a cell in M phase is to accurately segregate the
chromosomes that were duplicated in the preceding S phase, so that
each new daughter cell receives an identical copy of the genome. With
minor variations, all eukaryotes solve this problem in a similar way: they
assemble two specialized cytoskeletal machines—one that pulls the
duplicated chromosomes apart (during mitosis) and another that divides
the cytoplasm into two halves (during cytokinesis). We begin our discussion
of M phase with an overview of how the cell sets the processes of
M phase in motion.