Chapter 17
The Cell Cycle
An Overview of the Cell Cycle
1. The eucaryotic cell cycle is divided into four phases
2. Cell-cycle control is similar in all eucaryotes
3. Cell-cycle control system dissected genetically in yeasts
4. Cell-cycle control system analyzed biochemically in animal embryos
5. Cell-cycle progression studied in various ways
The major events of the cell cycle
Events of eucaryotic cell division as seen under a microscope
The four phases of the cell cycle
Cell-cycle control system dissected genetically in yeasts
Behavior of a temperature-sensitive Cdc mutant
room temperature
36°C
Morphology of budding yeast cells arrested by a Cdc mutation
Normal yeast cells – buds vary in size
according to the cell-cycle stage
In a Cdc15 mutant, grown at the
restrictive temperature, cells complete
anaphase but cannot complete the exit
from mitosis and cytokinesis. They
arrest uniformly with the large buds,
which are characteristic of late M phase
Cell-cycle control system analyzed biochemically in animal embryos
A mature Xenopus egg, ready for fertilization
Oocyte growth and egg cleavage in Xenopus
Studying cell cycle in a cell-free system
Cell-cycle progression studied in various ways
Labeling S-phase cells
Analysis of DNA content with a flow cytometer
The cell-cycle control system
1.
Cell-cycle control system triggers the major events of the cell cycle
2.
The cell-cycle control system depends on cyclically activated cyclindependent protein kinases (Cdks)
3.
Inhibitory phosphorylation and Cdk inhibitory proteins (CKIs) can
suppress Cdk activity
4.
The cell-cycle control system depends on cyclical proteolysis
5.
Cell-cycle control also depends on transcriptional regulation
6.
The cell-cycle control system functions as a network of biochemical
switches
Control of the cell cycle
Two key components of the cell-cycle control system
Cyclin-Cdk complexes of the cell-cycle control system
Inhibitory phosphorylation and Cdk inhibitory proteins (CKIs)
can suppress Cdk activity
The structural basis of Cdk activation
The regulation of Cdk activity by inhibitory phosphorylation
The inhibition of a cyclin-Cdk complex by a CKI
The cell-cycle control system depends on cyclical proteolysis
Cell-cycle control also depends on transcriptional regulation
In budding yeast, about 10% of the genes encode mRNAs
whose levels oscillate during the cell cycle
The cell-cycle control system functions as a network of
biochemical switches
An overview of the cell-cycle control system
The two central events of the cell cycle are:
- replication of DNA during the S phase
- chromosome segregation and cell division during the M phase
Both these events are controlled by the cyclin-Cdk complexes
S phase
1.
S-Cdk initiates DNA replication once per cycle
2.
Chromosome duplication requires duplication of chromatin structure
3.
Cohesins help hold sister chromatids together
Control of chromosome duplication
Control of the initiation of DNA replication
The ORC remains associated with the ori site
throughout the cell cycle.
In early G1, Cdc6 and Cdt1 (helicase loading
proteins) associate with the ORC and the
resulting complex allows the assembly of the
Mcm ring and the formation of the
prereplicative complex.
In the S phase, S-Cdk stimulates the assembly
of several additional proteins to form the
preinitiation complex. Other proteins are
recruited to the origin and replication begins.
S-Cdk blocks rereplication by triggering the
destruction of Cdc6 and the inactivation of
the ORC.
The cell is able to assemble the pre-RC only
after M-Cdk is inactivated and APC/C is
activated at the end of the M-phase
S-Cdk activity is high during G2 and early mitosis. This prevents
rereplication from occurring after the S phase
M-Cdk also prevents rereplication from occurring during mitosis by
phosphorylating the Cdc6 and ORC proteins
With all the control elements preventing rereplication,
how does DNA replication take place in the next cell
cycle?
At the end of mitosis, APC/C activation leads to the inactivation of
Cdk activity and the destruction of geminin. Pre-RC components are
dephosphorylated and Cdt1 is activated allowing pre-RC assembly
to initiate a new round of replication
Mitosis
1. M-Cdk drives entry into mitosis
2. Dephosphorylation activates M-Cdk at the onset of
mitosis
3. Condensin helps configure duplicated chromosomes for
separation
4. The mitotic spindle is a microtubule-based machine
5. Centrosome duplication occurs early in the cell cycle
6. M-Cdk initiates spindle assembly in prophase
7. The completion of spindle assembly in animal cells
requires nuclear envelope breakdown
8. The APC/C triggers sister-chromatid separation and the
completion of mitosis
9. Unattached chromosomes block sister-chromatid
separation: The spindle assembly checkpoint
Activation of M-Cdk drives entry into mitosis
The APC/C triggers sister-chromatid separation and the
completion of mitosis
Control of cell division and cell growth
Mechanism controlling
cell-cycle entry and
S-phase initiation
in animal cells
How DNA damage
arrests the cell cycle
in G1