Chapter 17

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

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

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

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