Download Control of the Cell Cycle - Southington Public Schools

Control of the Cell Cycle
Human digestive system cells have a cycle of 24-48 hours.
Yet human nerve cells normally do not divide at all once
they have matured. (Preserve those brain cells!) Cells of
embryos divide once every few hours and bacteria cells
divide so often that one cell can turn into (cue Dr. Evil
voice) one million cells in 24 hours!
Cell cycle control
Proteins called cyclins (mostly produced during the G1 or
G2 phase of interphase) control how fast a eukaryotic cell
completes its cell cycle. These enzymes are produced
when a gene on a chromosome in the cell is expressed (is
copied and then made by ribosomes).
Normal cell cycle
There are cyclins that control the timing of a cell moving
from phase to phase, and different cyclins that tell a cell to
stop dividing. When the cell has stopped dividing, it is in
the G0 phase.
Internal regulators—are cyclins produced based on
changes within the cell.
External regulators—are factors outside the cell. Some
examples are growth factors and hormones. There are
also proteins on the surface of the cell membrane that
act as a stop signal when cells grow in contact with
other cells.
Control of the Cell Cycle (cont.)
Abnormal cell cycle
When cyclins are not made, or their signal is not
“understood,” cells grow without stopping. This results in
Cancer. Generally cancer occurs after a gene that is
supposed to control cell growth is damaged (changed).
Damage to a gene called p53 appears to have a role in
many types of cancer.
How do genes get damaged?
Mutagens—anything that can mutate (change) a gene.
Some well known mutagens are:
 cigarette smoke
 air and water pollution
 many chemicals
 any strong source of radiation including UV radiation
from the sun.
Most cancers are NOT strictly genetic (inherited
from parents).
When do genes get damaged?
Genes are most likely to get damaged if they are exposed
to a mutagen while they are copying their chromosomes or
actively dividing.
What does cancer do to the body?
Cancerous cells divide so often that they use up nutrients
that other cells need. The body can’t support the
uncontrolled growth. Cancer tumors also destroy the
function of the tissues they are in.