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Cell Cycle Regulation
Cell Cycle analogy to washer machine control
The cell cycle is driven by an internal clock regulated at certain
checkpoints by both external (ex: faucets) and internal (ex:
sensor) controls.
Major Checkpoints of the Cell Cycle Control System
Cell Cycle Clock: Cyclins and CyclinDependent Kinases
Cyclin fluctuates cyclically within the
cell.
Cyclin–dependent kinases (Cdks) such
as MPF must be attached to cyclin to be
active (MPF = mitosis promoting factor)
Activity of Cdks rise and fall with
changes in cyclin concentration.
MPF complex initiates mitosis by
phosphorylating a variety of proteins.
MPF switches itself off by initiating a
process that breaks down cyclin.
The effect of a Growth Factor
on Cell Division
PDGF = plateletderived growth factor
A growth factor is a protein released by certain body cells
that stimulates other cells to divide.
Density-Dependent Inhibition
• Crowded cells stop dividing.
• Cultured cells normally divide
until they form a monolayer.
• If some cells are removed, cell
division begins in cells bordering
the gap until it is filled.
• Most animal cells also exhibit
anchorage dependence
Cancer cells have escaped from cell
cycle controls
Cancer cells:
• do not exhibit density-dependent inhibition or
anchorage dependence.
• do not stop dividing when growth factors are
depleted.
• can go on dividing indefinitely in culture if
given a continual supply of nutrients (immortal).
• transformation is the process that converts
a normal cell to a cancer cell.