the cell cycle
DESCRIPTION
The Cell CycleTRANSCRIPT
The Cell Cycle
Molecular Biology of the Cell
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
An Overview of the Cell Cycle
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
room temperature 36°C
Behavior of a temperature-sensitive Cdc mutant
Morphology of budding yeast cells arrested by a Cdc mutation
Normal yeast cells – buds vary in sizeaccording to the cell-cycle stage
In a Cdc15 mutant, grown at the restrictive temperature, cells completeanaphase but cannot complete the exitfrom mitosis and cytokinesis. Theyarrest 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
Cell-cycle progression studied in various ways
Labeling S-phase cells
Analysis of DNA content with a flow cytometer
1. Cell-cycle control system triggers the major events of the cell cycle
2. The cell-cycle control system depends on cyclically activated cyclin-dependent 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
The cell-cycle control system
Control of the cell cycle
Two key components of the cell-cycle control system
Cyclin-Cdk complexes of the cell-cycle control system
The structural basis of Cdk activation
Inhibitory phosphorylation and Cdk inhibitory proteins (CKIs) can suppress Cdk activity
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 mRNAswhose 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
1. S-Cdk initiates DNA replication once per cycle
2. Chromosome duplication requires duplication of chromatin structure
3. Cohesins help hold sister chromatids together
S phase
Control of chromosome duplication
Control of the initiation of DNA replication
The ORC remains associated with the ori sitethroughout the cell cycle.
In early G1, Cdc6 and Cdt1 (helicase loadingproteins) 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 assemblyof several additional proteins to form the preinitiation complex. Other proteins arerecruited to the origin and replication begins.
S-Cdk blocks rereplication by triggering thedestruction of Cdc6 and the inactivation ofthe ORC.
The cell is able to assemble the pre-RC onlyafter 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 preventsrereplication from occurring after the S phase
M-Cdk also prevents rereplication from occurring during mitosis byphosphorylating 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 assemblyto initiate a new round of replication
1. M-Cdk drives entry into mitosis2. Dephosphorylation activates M-Cdk at the onset of
mitosis3. Condensin helps configure duplicated chromosomes for
separation4. The mitotic spindle is a microtubule-based machine 5. Centrosome duplication occurs early in the cell cycle6. M-Cdk initiates spindle assembly in prophase7. The completion of spindle assembly in animal cells
requires nuclear envelope breakdown8. The APC/C triggers sister-chromatid separation and the
completion of mitosis9. Unattached chromosomes block sister-chromatid
separation: The spindle assembly checkpoint
Mitosis
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