Cell Cycle Regulation

The cell cycle is driven by an internal clock regulated at certain checkpoints by both external (ex: faucets) and internal (ex: sensor) controls.

Cell Cycle analogy to washer machine control

Major Checkpoints of the Cell Cycle Control System

Cell Cycle Clock: Cyclins and Cyclin-Dependent 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

A growth factor is a protein released by certain body cells that stimulates other cells to divide.

PDGF = platelet-derived growth factor

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.