Concept 12.3: Regulation of Cell Cycle by a Molecular

Control SystemWill S.

Esther P.Michelle K.

Alex G.Nick B.

Timing and the rate of cell division in plants and animals is crucial to normal growth, development, and maintenance

Some of the most specialized cells do not divide in a mature human—these cell cycle differences are from regulation at the molecular level

Evidence Of Cytoplasmic Streaming

One hypothesis—each event triggers the next; replication of chromosomes in S phase causes cell growth during G2 phase triggering onset of mitosis—not correct

Cell cycle is driven by specific molecular signals present in cytoplasm

Example: mammalian cells grown in culture

What drives the cell cycle?

Example

S and G1 –G1 went to the S phase and DNA was synthesized

M and G1 –G1 began mitosis- spindle formed and chromatin condensed even though chromosomes were not duplicated

Operating set of molecules that triggers and coordinates key events in the cell cycle.

Driven by a built-on clock Regulated at checkpoints by internal and

external controls. Checkpoint = critical point where stop and

go-ahead signals regulate the cycle

The Cell Cycle Control System

Signals come from surveillance inside the cell

Three major checkpoints are G1, G2, and M phases

G1 is the most important and the cell will most likely complete the cycle if given go-ahead by G1

Not given go-ahead then leaves into the G0

Continued

G0 State

G1

M G2

S

Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the sequential events of the cell cycle.

Protein kinases are enzymes that activate or inactivate other proteins by phosphorylating them.

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

The Cell Cycle Clock

Shuts down by cyclin degradation

Ensure mitosis properly carried out

“Stops” if parts not there

Stop and Go Signals

Cells examined for normal growth, space limitations, growth factor withdrawal, DNA damage, etc.

Look for DNA damage, mismatched bases that could have happened during S phase (DNA synthesis)

• metaphase ensures all chromosomes connected to mitotic spindle at kinetochore

• then can go to anaphase

Influences division

Physical or chemical

Growth Factors Ex. PDGF

Density-Dependent Inhibition

Anchorage Dependence

External Factors

Binding PDGF to receptor tyrosine kinases triggers transduction pathway, allows passage of G1

When cells divide excessively and invade other tissues and /or take up resources.

Apoptosis: process of programmed cell death Normal mammalian cells divide only about 20-50

times. Transformation: when normal cells transforms into a

cancer cell. Benign tumor: when cancer cells stay in original

site. Malignant tumor (metastasis): when the cancer

spreads and proliferates in various parts of the body.

Cancer

Tumors are treated with high powered. Chemotherapy: toxic drugs that impede the

cell cycle and stop cells from dividing. There is a cell line in culture that has been

reproducing since 1951

Treatment

http://youtu.be/1EB8q9aR8Hk

http://www.youtube.com/watch?v=QGx50C1w8YY

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