Cell Cycle Control System

Chapter 12.3

Its Time to

Play

Simon Says!!

Molecular Control system

• Timing and rate of cell division in different parts of the plant and animal are crucial to normal growth, development, and maintenance.

• Frequency varies among different types of cells– Ex: Frequent divisions of skin cells

Liver cells reserve as need arisesNerve and muscle cells don’t even divide

Cytoplasmic signals

• 1970s experiments revealed that molecular signals from the cytoplasm were driving the cell cycle.

Cell Cycle Control System

A cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle.

Other Examples

• Take a minute. Can you think of more examples of control systems?

Cycle Control System

• Checkpoints are critical control points where stop and go ahead signals regulate the cycle.

• Some cells have built in stop signals that halt the cycle until another signal overrides and starts cycle.

• Checkpoints can register signals from both inside and outside the cell

Checkpoints

• Three major checkpoints:G1 – the restriction pointG2 –MPF facilitates to cell dividing stageM – mitotic division

Regulatory Molecules

• Protein kinases enzymes that activate or inactivate proteins by phosphorylating them

• Cyclins are proteins that cyclically fluctuate their concentration in the cell

*determine active or inactive state of kinases.

• CdK – cyclin-dependent kinases in varying numbers control all stages of cell cycle

Internal Checkpoints

• G1 phase – activator CdK will cause cycle to complete itself; whereas lack of cyclins will halt in G0 phase

• G2 phase – MPF “ M-phase promoting factor” will trigger passage to mitosis or cell dividing phase.

• M phase – kinetochores send signal that delays anaphase until all chromosomes are attached to spindle

External CheckpointsGrowth factors – protein released by cells that stimulate

other cells to divide*mitogen – mitosis promoter

PDGF – made by platelets; required for division of fibroblasts to clot

(G1 phase)Density dependent inhibition – occurs when crowded cells

stop dividing*results from physical contact, and lack of

adequate growth factors and nutrients Anchorage dependence – cells must be attached to a

substratum such as extracelluar matrix; cytoskeleton attachment

Cancer Cells• Cancer cells divide

excessively and invade other tissues.

• Cancer cell do not respond to most of the body’s control mechanisms.

• Cancer cell still grow even when growth factors have been depleted.

• Cancer cells stop dividing at random points of the cycle.

Cancer CellsTransformation – process that converts a normal cell to a cancer cellTumors:

benign – remains at original sitemalignant – invades other organs and impairs

function

Cancer Cells

Metastasis- spread of cancer cells to locations distant from original site.

*cells can secrete signal molecules that cause blood vessels to grow toward tumor*cells can also separate and travel in blood and lymph to other parts of the body

TreatmentsRadiation – localized tumors or proliferation

*targets and damages DNA of cancer cell

Chemotherapy – malignant or metastasis*most target and prevent cells from proceeding past metaphase

*side effects – exerts same targets on normal cells Ex: intestinal cells - nausea hair follicle cells – hair loss immune cells - increased infections