Download Chapter 12. Regulation of the Cell Cycle

Regulation of Cell Division
AP Biology
2006-2007
Activation of cell division
 How do cells know when to divide?

cell communication signals
 chemical signals in cytoplasm give cue
 signals usually are proteins
 activators or inhibitors
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Coordination of cell division
 Multicellular organisms coordinate cell
division across different tissues & organs

critical for normal growth,
development & maintenance
 coordinate
 timing of
cell division
 rates of
cell division
 not all cells may have the
same cell cycle
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Frequency of cell division
 Frequency of cell division varies by cell type

embryo
 cell cycle < 20 minute

skin cells
 divide frequently throughout life
 12-24 hours cycle

liver cells
 retain ability to divide, but keep it in reserve M
metaphase anaphase
 divide once every year or two
prophase

mature nerve cells & muscle cells
C
G2
 do not divide at all after maturity
 permanently in G0
S
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telophase
interphase (G1, S, G2 phases)
mitosis (M)
cytokinesis (C)
G1
There’s no
turning back,
now!
Overview of Cell Cycle Control
 Two irreversible points in cell cycle
1) replication of genetic material
 2) separation of sister chromatids

 Checkpoints

process is assessed & possibly halted
sister chromatids
centromere
single-stranded
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chromosomes

double-stranded
chromosomes

Checkpoint control system
 Checkpoints
cell cycle controlled by STOP & GO
chemical signals at critical points
 signals indicate if key cellular
processes have been
completed correctly

 3 major checkpoints:

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G1, G2 and M
Checkpoint control system
 3 major checkpoints:

G1
 Can DNA synthesis begin?

G2
 Has DNA synthesis been
completed correctly?
 Commitment to mitosis

M
 Are all chromosomes
attached to spindle?
 Can sister chromatids
separate correctly?
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Major Checkpoints
1. G1 checkpoint (Most important!)



Controlled by cell size, growth factors,
environment
“Go”  completes whole cell cycle
“Stop”  cell enters nondividing state (G0 Phase)
2. G2 checkpoint
 Controlled by DNA replication completion,
DNA mutations, cell size
3. M-spindle (Metaphase) checkpoint

Check spindle fiber (microtubule) attachment to
chromosomes at kinetochores (anchor sites)
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G1 Checkpoint is the most critical!
 Primary decision point
 “restriction point”
 If cell receives a “GO

ahead”signal, it will
divide
If cell does not receive
signal, it exits cycle &
switches to G0 phase
Apoptosis – cell death
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G1 Checkpoint
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G0 phase
 G0 phase
non-dividing, differentiated state
 many human cells in G0 phase

 liver cells
M
Mitosis
G2
Gap 2
S
Synthesis
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 in G0, but can be “called
G1
Gap 1
G0
Resting
back” to cell cycle by
external cues
 nerve & muscle cells
 highly specialized
 stopped in G0 & can
never divide
Cell Cycle Control System
 Checkpoint = control point where stop/go
signals regulate the cell cycle
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“Go-ahead” signals
 Protein molecules that promote cell
growth & division

internal signals
 “promoting factors”

external signals
 “growth factors”
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Where is
the P
attached?
“Go-ahead” signals

Protein molecules that promote cell growth & division


internal signals
 “promoting factors”
external signals
 “growth factors”
 Primary mechanism
of control: phosphorylation
 Use of kinase enzymes
which either activate or
inactivate cell signals by
adding a phosphate
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Where is
the P
attached?
Internal Regulatory Molecules
• Kinases : protein enzyme controls cell cycle; active when
connected to cyclin
• Cyclin-dependent kinase: Cdk
• Cyclins: proteins which attach to kinases to activate them;
levels fluctuate in the cell cycle
• When are cyclin levels highest?
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Cyclin & Cyclin-dependent kinases
 CDKs & cyclin drive cell from one
phase to the next in cell cycle

proper regulation of cell
cycle is so key to life
 the genes for these
regulatory proteins
have been highly
conserved through
evolution

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the genes are
basically the same
in yeast, insects,
plants & animals
(including humans)
inactivated Cdk
Cell cycle Chemical signals
 Cyclins
regulatory proteins
 levels cycle in the cell
 phosphorylates
cellular proteins
 activates or
inactivates proteins
Cdk-cyclin complex
 Forms MPF complex
 Triggers movement
into next phase


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activated Cdk
Internal Regulatory Molecules
MPF = maturation-promoting factor
•
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specific cyclin-Cdk complex which allows cells
to pass G2 and go to M phase
External Regulatory Factors
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External Regulatory Factors
 Growth Factor: proteins released by other


cells to stimulate cell division
Density-Dependent Inhibition: crowded cells
normally stop dividing; cell-surface protein
binds to adjoining cell to inhibit growth
Anchorage Dependence: cells must be
attached to another cell or ECM to divide
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External signals
 Growth factors


coordination between cells
protein signals released by
body cells that stimulate other
cells to divide
 density-dependent inhibition
 crowded cells stop dividing

When not enough growth factor
left to trigger division in any one
cell, division stops
 anchorage dependence
 to divide cells must be attached to a
substrate or tissue matrix
 “touch sensor” receptors
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Growth factor signals
growth factor
nuclear pore
nuclear membrane
P
P
cell division
cell surface
receptor
protein kinase
cascade
Cdk
P
P
E2F
chromosome
P
APcytoplasm
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nucleus
M-spindle Checkpoint:
Mitotic spindle at metaphase
Kinetochore =
proteins associated
with DNA at
centromere
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M checkpoint
G2 checkpoint
Chromosomes attached
at metaphase plate
• Replication completed
• DNA integrity
Active
Inactive
Inactive
Cdk / G2
cyclin (MPF)
M
Active
C
cytokinesis
mitosis
G2
G1
S
MPF = Mitosis
Promoting Factor
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Cdk / G1
cyclin
Active
G1 checkpoint
Inactive
• Growth factors
• Nutritional state of cell
• Size of cell
Cancer & Cell Growth
 Cancer is essentially a failure
of cell division control

unrestrained, uncontrolled cell growth
 What control is lost?

gene p53 plays a key role in G1 restriction point
 p53 protein halts cell division if it detects damaged DNA
 options:
p53 is the
Cell Cycle
Enforcer




stimulates repair enzymes to fix DNA
forces cell into G0 resting stage
keeps cell in G1 arrest
causes apoptosis of damaged cell
 ALL cancers have to shut down p53 activity
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p53 — master regulator gene
NORMAL p53
p53 allows cells
with repaired
DNA to divide.
p53
protein
DNA repair enzyme
p53
protein
Step 1
Step 2
Step 3
DNA damage is caused
by heat, radiation, or
chemicals.
Cell division stops, and
p53 triggers enzymes to
repair damaged region.
p53 triggers the destruction
of cells damaged beyond repair.
ABNORMAL p53
abnormal
p53 protein
Step 1
DNA damage is
caused by heat,
radiation, or
AP chemicals.
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cancer
cell
Step 2
The p53 protein fails to stop
cell division and repair DNA.
Cell divides without repair to
damaged DNA.
Step 3
Damaged cells continue to divide.
If other damage accumulates, the
cell can turn cancerous.
Development of Cancer
 Cancer develops only after a cell experiences ~6 key
mutations (“hits”)

unlimited growth
 turn on growth promoter genes

ignore checkpoints
 turn off tumor suppressor genes (p53)

escape apoptosis
 turn off suicide genes

immortality = unlimited divisions
 turn on chromosome maintenance genes

promotes blood vessel growth
 turn on blood vessel growth genes

overcome anchor & density dependence
 turn off touch-sensor gene
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It’s like an
out of control
car!
What causes these “hits”?
 Mutations in cells can be triggered by




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UV radiation
chemical exposure
radiation exposure
heat




cigarette smoke
pollution
age
genetics
Tumors
 Mass of abnormal cells

Benign tumor
 abnormal cells remain at original site as a
lump
 p53 has halted cell divisions
 most do not cause serious problems &
can be removed by surgery

Malignant tumors
 cells leave original site
 lose attachment to nearby cells
 carried by blood & lymph system to other tissues
 start more tumors = metastasis
 impair functions of organs throughout body
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Traditional treatments for cancers
 Treatments target rapidly dividing cells

high-energy radiation
 kills rapidly dividing cells

chemotherapy
 stop DNA replication
 stop mitosis & cytokinesis
 stop blood vessel growth
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