Transcript Concept 12.3

Concept 12.3

The cell cycle is driven by specific chemical signals that
act as controls
 Internal
controls – mostly in the cytoplasm
 External controls – from an outside source

Events of the cell cycle are directed by a distinct cell
cycle control system
 driven
by a built in clock - timing
 regulated at certain checkpoints
12.3 – 3 major checkpoints

For most cells, the G1 checkpoint seems to be the
most important one
 Cell
receives a go-ahead signal at the G1 checkpoint, it
will usually complete the S, G2, and M phases and
divide
 Cell does not receive the go-ahead signal, it will exit
the cycle, switching into a non-dividing state called the
G0 phase
A
lot of cells found in G0
Fig. 12-15
G0
G1 checkpoint
G1
(a) Cell receives a go-ahead
signal
G1
(b) Cell does not receive a
go-ahead signal
Signal molecules in cell cycle

The activity of cyclins and Cdks fluctuates during the
cell cycle
 Cdks
(cyclin dependent kinases) are present inside the
cell in constant concentration and inactive
 Cyclins build up in the cell during certain phases and
activate Cdks
 Cyclins
activate Cdks
 When cells are dividing Cdk activity (not concentration)
lowers; waiting for the concentration of cyclins to build again
Fig. 12-16
RESULTS
5
30
4
20
3
2
10
1
0
100
200
300
Time (min)
400
0
500

When a cyclin and Cdk bind they form a cyclinCdk complex
 Ex:
MPF (M phase-promoting factor) triggers a cell’s
passage past the G2 checkpoint into the M phase
 Highest
levels of MPF are when a cell goes into mitosis
Fig. 12-17
M
S
G1
G2
M
G1
S
G2
M
G1
MPF activity
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration during
the cell cycle
Degraded
cyclin
G2
checkpoint
Cyclin is
degraded
MPF
Cdk
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle
Cyclin accumulation
Cdk
Other Internal and External Signals


internal signals - kinetochores not attached to spindle
fibers correctly in metaphase delays anaphase
external signals - growth factors, proteins released by
certain cells that stimulate other cells to divide
 platelet-derived
growth factor (PDGF) stimulates the
division of human fibroblast cells in culture – make
collagen a connective tissue
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-18
Scalpels
Petri
plate
Without PDGF
cells fail to divide
With PDGF
cells proliferate
Cultured fibroblasts
10 µm

external signal - is density-dependent inhibition, in
which crowded cells stop dividing
 If
part of sample removed, signal to bordering cells to
divide to fill in the gap

external signal - anchorage dependence, in which
they must be attached to a substrate in order to divide
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
25 µm
25 µm
(a) Normal mammalian cells
(b) Cancer cells
Cancer cells ignore signals to stop
dividing

have escaped from normal cell cycle controls and
external signals
 they
do not respond to the control mechanisms –
anchorage dependence, density dependent inhibition,
or checkpoints
 if
cancer cells stop dividing, it is at random points and not at
the checkpoints
 They
even secret their own growth factors – will ignore
healthy normal cell growth factors
Cancer cells…

Form when a normal cell undergoes a
transformation – becomes cancer cell
 no
longer responding to control mechanisms- would
normally be destroyed by immune system
 if
the cell evades destruction by the immune system it may
form a tumor – mass of abnormal cells
Tumors…



remain at the original site, it is called a benign
tumor
becomes invasive enough to impair the functions of
surrounding tissue and organs now have a
malignant tumor
cells spread from the original site is called
metastasis
 Cells
spread through blood stream and lymphatic
system
Fig. 12-20
Lymph
vessel
Tumor
Blood
vessel
Cancer
cell
Metastatic
tumor
Glandular
tissue
1 A tumor grows
from a single
cancer cell.
2 Cancer cells
invade neighboring tissue.
3 Cancer cells spread
to other parts of
the body.
4 Cancer cells may
survive and
establish a new
tumor in another
part of the body.
Concept 12.3
Treating cancer…


Localized tumors are treated through high does of
radiation
Metastasized tumors are treated with
chemotherapy – combo of toxic drugs
 Both
treatments also affect healthy cells that rapidly
divide
 Hair
cells
 GI tract – mouth, esophagus, stomach lining