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The Cell Cycle Control“to divide or not to divide, that is the

question”.

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Summary1. The role of cell division – reproduction and

growth (cell renewal and repair). 2. The mitotic cell cycle – consists of:

- interphase (I=G1+S+G2) – important steps for DNA duplication and initiation of mitosis;

- mitosis (P+PM+M+A+T) – separation of two daughter cells with equal amount of genetic material (chromosomes).

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The Cell Cycle ControlThe timing and rate of cell division differ between different organisms and also between different cells of an organism.

Compare skin cells with muscle or nerve cells.

What is controlling the rate of cell division, how cells “know” that it is time to divide?

Why cancer cells do not stop dividing?

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The cell cycle is regulated at the molecular levelExperimental evidences suggested that the cell cycle is driven by specific chemical signals present in the cytoplasm.

Most of the experiments were conducted with cell cultures.

Many types of animal and plant cells can be removed from an organism and cultured in an artificial environment.

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Cell cycle is controlled !

M M

M G1 S G2

Cultured mammalian cells can be induced to fuse, forming a single cell with two nuclei. The results of fusing cells at two different phases of the cell cycle suggested that particular chemicals control the progression of phases.

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Cell cycle is controlled !

M M

M G1 S G2

For example, when a cell in M phase was fused with one in any other phase, the nucleus from the latter cell immediately began mitosis. If the second cell was in G1, the condensed chromosomes that appeared had single chromatids.

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Cell-cycle control systemThese experiments suggested that events happening from one cell division to another are driven by cell-cycle control system,

a cyclically operating set of molecules in the cell that triggers and coordinates key events in the cell cycle

When and how is the cell cycle controlled?

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Cell cycle is controlled !

G1 nucleus S phase nucleus G2 nucleus

1 2

1 2

1 2+

+

+

1

G 1 c e l l

2

S -p h a s e c e l l

1

G 2 c e l l

2

S -p h a s e c e l l

1

G 1 c e l l

2

G 2 c e l l

G1 nucleus is competent to replicate. S-phase cells contain activator

G2 nuclei aren’t competent and do not re-replicate. G2 cells do not inhibit replication. S-phase nuclei retard mitosis in G2 nuclei.

G2 cells do not suppress S-phase entry of G1-phase nuclei.

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Cell cycle controlA checkpoint in the cell is a critical control point where stop and go signals can regulate the cycle

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Cell cycle controlAnimal cells have built-in “stop” signals that halt the cell cycle at checkpoints until overridden by “go” signals

To function properly checkpoint signals have to percept “reports” from crucial cellular processes:

have it been completed correctly and should the cell cycle proceed.

Checkpoint also register signals from outside the cell

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Cell cycle controlThe most important decision to make is: to continue the cell division after the exit from M phase or not.Cells that do not receive the “go” signal at the G1 checkpoint, switch into a nondividing state called the G0 phase.

A good example of quiescent cells are liver cells.They can be called back to the cell cycle by growth factors released during injury.

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Cyclins and cyclin-dependent kinases (Cdks)Regulatory molecules of the cycle transition are proteins

of two main types: protein kinases and cyclins.

Protein kinases are proteins that regulate the activity of the others by phosphorylating them.

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Cyclins and cyclin-dependent kinases (Cdks)“Go” signal at the G1 and G2 checkpoints is regulated by particular protein kinases.

To be active, such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell

This kinases are called cyclin-dependent kinases – Cdks.

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Control at the G2 checkpointThe stepwise processes of the cell cycle are timed by rhythmic fluctuations in the activity of protein kinases.

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Control at the G2 checkpointCdk-cyclin complex called MPF (maturation promotion factor), acts at the G2 checkpoint to trigger mitosis.

(a) The graph shows how MPF activity fluctuates with the level of cyclin in the cell.

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Control at the G2 checkpointThe cyclin level rises throughout interphase (G1, S, and G2 phases), then falls abruptly during mitosis (M phase). The Cdk itself is present at a constant level.

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Control at the G2 checkpoint(b) 1. By the G2 checkpoint

(red bar), enough cyclin is available to produce many molecules of MPF.

2. MPF promotes mitosis by phosphorylating various proteins, including other enzymes.

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Control at the G2 checkpoint

3. One effect of MPF is the initiation of a sequence of events leading to the breakdown of its own cyclin.

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Control at the G2 checkpoint 4. The Cdk component of MPF is recycled. Its kinase activity will be restored by association with new cyclin that accumulates during interphase.

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Internal regulationInternal signals: messages from kinetochores.

Anaphase, the separation of sister chromatids, does not begin until all the chromosomes are properly attached to the spindle at the metaphase plate.

Certain associated proteins trigger a signalling pathway that keeps an anaphase promoting complex (APC) in an inactive state.

M-phase checkpoint is the gatekeeper. Only when all the kinetochores are attached to the spindle does the “wait” signal cease.

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External regulationExternal signals: growth factors.

Most of mammalian cells divide in culture only if the growth medium includes specific growth factors.

PDGF – platelet-derived growth factor – is required for the division of fibroblasts.

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External regulationDensity-dependent inhibition of cell division, a phenomenon in which crowded cells stop dividing.

Cultured cells normally divide until they form a single layer of cells on the inner surface of the culture container.

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External regulationAnchorage dependence: to divide, cells must be attached to a substratum (extracellular matrix of a tissue).

Anchorage is signalled to the cell-cycle control system via plasma membrane proteins and elements of the cytoskeleton linked to them.

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Cell growth is controlled

S

G1

M

G2

G1 CdkC

cell surfacegrowth receptors

growth factorPlateletPDGF

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Cell cycle control

S

G1

M

G2

S-phase CdkC

1. DNA pre-replication complexes assemble at origins

2. Inactivates APC

3. Activates transcription of S-phase CdkC components 4. Phosphorylates S-phase CdkC inhibitor

Metaphase

Anaphase

G1 CdkC

Inhibitor

5 . C dc34 pathw ay degrades S -phase C dkC inhib ito r

C dc34pathw ay

D N A r e p l i c a t i o n

T e l o p h a s e a n d c y t o k i n e s i sAPC

pathway

10. Degrades mitotic CdkC cyclin subunit

S-phase CdkC6. Activates pre-replication complexes

7 . A c t i v a t e s c h r o m o s o m e c o n d e n s a t i o n , n u c l e a r e n v e l o p e b r e a k d o w n , a n d s p i n d l e a s s e m b l y

8 . A c t i v a t e s A P C a f t e r a l a g

M i t o t i c C d k C

A P Cp a th w a y

9 . D e g ra d e s a n a p h a s e in h ib ito r

Restriction points

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Cancer cellsCancer cells are living their own lives, they do not respond normally to the body’s control.

They divide excessively and invade other tissues.

When a single cell in a tissue undergoes transformation, immune system normally recognizes a transformed cell as an insurgent and destroys it.

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Density-dependent inhibition

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Cancer cellsIf the cell evades destruction, it may proliferate to form a tumor, a mass of abnormal cells.

Benign tumor is localised at original site, malignant tumor becomes invasive.

The spread of cancer cells beyond their original site is called metastasis.

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The growth and metastasis of a malignant breast tumorThe cells of malignant (cancerous) tumors grow in an uncontrolled way and can spread to neighboring tissues and, via the circulatory system, to other parts of the body. The spread of cancer cells beyond their original site is called metastasis.

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Summary• Cell cycle regulation

– everything is under the control of Cdk: association of the kinases with cyclines

support unidirectional cell cycle progression with the main influences at G1, G2 and M

phases.

ReadingCh 12. pp. 238-245

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Following steps are required for p34cdc2 (mitotic kinase) activation

• Phosphorylation of T-161 • Dephosphorylation of T-14 and Y-

15• Association with cyclin B• Release of the block by CKI

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Pathways which determine Cdk activity

• Phosphorylation of Cdk itself • Association with a specific cyclin• Association with CKIs• Association with other proteins related to

cell cycle