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Has 4 phases – G1, S, G2, and M
Driven by specific molecular signals present in the cytoplasm
Controlled by a number of checkpoints – at G1, G2, and M phases
Checkpoint = a critical control point where stop and go signals regulate the
Signals from within the cell tell it whether crucial cellular processes have been
completed correctly and whether or not the cell should proceed with division.
Signals from outside the cell are also registered at checkpoints.
In mammalian cells, G1 (the
“restriction point”) is the most
If “go,” the cell will usually
complete the rest of the cell cycle
and divide
If “stop,” the cell will switch into
G0 phase (non-dividing state)
Two main types: Kinases and Cyclins
Kinase – enzymes that activate or inactivate
other proteins by phosphorylating them
 Often called Cdks
Cyclins are proteins that attach to a kinase to activate them; they
fluctuate in concentration within the cell.
For cell division to proceed, two important factors must be present:
essential nutrients and growth factors.
 Growth Factor = a protein released by certain cells that stimulates other cell to divide
Density-dependent inhibition = a
phenomenon in which crowded
cells stop dividing
 Primarily determined by amount of
required growth factors and
nutrients available to each cell
Anchorage dependence = cells
must be attached to a substrate
to divide
What causes it?
 Cellular transformation always involves the alteration of genes that somehow
influence the cell cycle control system
Why does chemotherapy target cancer cells?
 Radiation damages DNA in cancer cells more because they have lost the
ability to repair such damage
 Drugs are toxic to actively dividing cells
Why are there side affects?
 Normal cells such as intestinal cells, hair follicle cells, and immune system cells
are affected too.
Benign tumor – when abnormal cells remain at the original
Malignant tumor – when abnormal cells become invasive
enough to impair the functions of one or more organs
Metastasis – when cancer cells spread to locations distant
from their original site
 Happens when some tumor cells separate from the original tumor, enter
blood vessels and lymph vessels, and travel to other parts of the body