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

1004 Chapter 17: The Cell Cycle
Table 17–2). Like Cdh1, Sic1 is inhibited by M-Cdk, which phosphorylates Sic1
during mitosis and thereby promotes its ubiquitylation by SCF. Thus, Sic1 and
M-Cdk, like Cdh1 and M-Cdk, inhibit each other. As a result, the decline in M-Cdk
activity that occurs in late mitosis causes the Sic1 protein to accumulate, and this
CKI helps keep M-Cdk activity low after mitosis. A CKI protein called p27 (see
Figure 17–14) may serve similar functions in animal cells.
In most cells, decreased transcription of M-cyclin genes also inactivates
M-Cdks in late mitosis. In budding yeast, for example, M-Cdk promotes the
expression of these genes, resulting in a positive feedback loop. This loop is turned
off as cells exit from mitosis: the inactivation of M-Cdk by Cdh1 and Sic1 leads
to decreased M-cyclin gene transcription and thus decreased M-cyclin synthesis.
Gene regulatory proteins that promote the expression of G 1 /S- and S-cyclins are
also inhibited during G 1 .
Thus, Cdh1–APC/C activation, CKI accumulation, and decreased cyclin gene
expression act together to ensure that the early G 1 phase is a time when essentially
all Cdk activity is suppressed. As in many other aspects of cell-cycle control,
the use of multiple regulatory mechanisms allows the system to operate with reasonable
efficiency even if one mechanism fails. So how does the cell escape from
this stable G 1 state to initiate a new cell cycle? The answer is that G 1 /S-Cdk activity,
which rises in late G 1 , releases all the braking mechanisms that suppress Cdk
activity, as we describe later, in the last section of this chapter.
Summary
After mitosis completes the formation of a pair of daughter nuclei, cytokinesis finishes
the cell cycle by dividing the cell itself. Cytokinesis depends on a ring of actin
and myosin filaments that contracts in late mitosis at a site midway between the
segregated chromosomes. In animal cells, the positioning of the contractile ring is
determined by signals emanating from the microtubules of the anaphase spindle.
Dephosphorylation of Cdk targets, which results from Cdk inactivation in anaphase,
triggers cytokinesis at the correct time after anaphase. After cytokinesis, the
cell enters a stable G 1 state of low Cdk activity, where it awaits signals to enter a new
cell cycle.
MEIOSIS
Most eukaryotic organisms reproduce sexually: the genomes of two parents mix
to generate offspring that are genetically distinct from either parent. The cells of
these organisms are generally diploid: that is, they contain two slightly different
copies, or homologs, of each chromosome, one from each parent. Sexual reproduction
depends on a specialized nuclear division process called meiosis, which
produces haploid cells carrying only a single copy of each chromosome. In many
organisms, the haploid cells differentiate into specialized reproductive cells called
gametes—eggs and sperm in most species. In these species, the reproductive cycle
ends when a sperm and egg fuse to form a diploid zygote, which has the potential
to form a new individual. In this section, we consider the basic mechanisms
and regulation of meiosis, with an emphasis on how they compare with those of
mitosis.
Meiosis Includes Two Rounds of Chromosome Segregation
Meiosis reduces the chromosome number by half using many of the same molecular
machines and control systems that operate in mitosis. As in the mitotic cell
cycle, the cell begins the meiotic program by duplicating its chromosomes in meiotic
S phase, resulting in pairs of sister chromatids that are tightly linked along
their entire lengths by cohesin complexes. Unlike mitosis, however, two successive
rounds of chromosome segregation then occur (Figure 17–53). The first of
these divisions (meiosis I) solves the problem, unique to meiosis, of segregating
the homologs. The duplicated paternal and maternal homologs pair up alongside