Wed 11/27

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Wed 11/27

• Collect HW Bozeman Biology Mitosis Video
• Cancer Video/Powerpoint
• Finish Cell Cycle/Mitosis Control Booklet
• HOMEWORK But Im Too Young Cancer Case Study
  follow link on website, complete 8 questions
• DUE MONDAY!!!
• Cell Cycle, Mitosis, Control (Chp.12) Quiz will
  be TUESDAY (short Quiz 12 questions)

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(No Transcript)
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Review

• Cells must either reproduce or they die.
• Cells that can not reproduce and are destined to
  die are terminal cells (red blood, nerve cells,
  muscles cells etc.).
• The "life of a cell" is termed the cell cycle as
  there are distinct phases.
• They are G1, S, G2, M

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Frequency of cell division

• Frequency of cell division varies by cell type
• embryo
• cell cycle lt 20 minute
• skin cells
• divide frequently throughout life
• 12-24 hours cycle
• liver cells
• retain ability to divide, but keep it in reserve
• divide once every year or two
• mature nerve cells muscle cells
• do not divide at all after maturity
• permanently in G0

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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

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Checkpoint control system

• 3 major checkpoints
• G1/S
• can DNA synthesis begin?
• G2/M
• has DNA synthesis been completed correctly?
• commitment to mitosis
• MPF (mitosis promoting factor)
• spindle checkpoint
• are all chromosomes attached to spindle?
• can sister chromatids separate correctly?
• APC (anaphase promoting complex)

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G0 phase if cells do not pass G1/S checkpoint

• non-dividing, differentiated state
• most human cells in G0 phase

• liver cells
• in G0, but can be called back to cell cycle by
  external cues
• nerve muscle cells
• highly specialized
• arrested in G0 can never divide

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• Internal control of the cell cycle
• Controlled by signal molecules.
• must be phosphorylated in order to work.
• Below is a simple model of how this could occur.

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Kinases are proteins (enzymes) that phosphorylate
these chemical signals trigger the cell cycle
phases.

• ?This kinase represents the inactive form.
• This kinase has two active forms
• S-form or the M-form
• phosphorylate different chemical signals.

OR
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Cell cycle kinases must be activated by molecules
called cyclins.
? inactive kinase (no cyclin attached) The
kinases are called cyclin-dependent-kinases (Cdk)
because it needs cyclin to be phosphorylated. ?
two different cyclins
This represents what happens when ? cyclins are
present.
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• As the cell goes through the cell cycle
• Different cyclins are made to activate the
  various Cdks.
• Once the kinase is activated, the cyclin is
  destroyed which deactivates the kinase.
• Kinases are not destroyed, they are only
  activated or deactivated.

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The cell cycle begins. The cell has a certain
amount of cyclin-dependent kinases (Cdks). The
cell begins to make the S cyclin.
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The S-cyclin activates the Cdk.
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The Cdk complex phosphorylates the S-signal which
initiates the S-phase to start once it gets to a
critical level.
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• Once the S-signal is phosphorylated, it leaves.
• the S-cyclin is destroyed
• the kinase returns to the inactive state.
• When there is enough S-signal, then the S-phase
  will begin.

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Now the Cdk is inactive, and the cell begins to
make the M-cyclin.
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The M-cyclin activates the Cdk.
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• Cdk complex phosphorylates the M-signal
• initiates the M-phase to start once it gets to a
  critical level.
• This complex is called the mitosis-promoting-facto
  r (MPF).

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• Once the M-signal is phosphorylated, it leaves.
• M-cyclin is destroyed
• kinase returns to the inactive state.
• When there is enough M-signal, then the M-phase
  will begin.

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Various cyclins are made destroyed throughout
the cell cycle whereas the level of cell division
kinases remain constant. Kinases however are
activated by various cyclins and the activity are
mirrored by the rise and fall of cyclins.
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Fluctuations in concentration of cyclins allow
for cell cycle checkpoints. The three major
check points are G1/S , G2/M and Spindle
checkpoints. These checkpoints have
build-in-stop signals that hold the cell cycle at
the checkpoint until overridden by go-ahead
signals.
This is a textbooks diagram of how cyclins and
kinases in the cell cycle work.
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Often the G1 check point or "restriction point"
in mammalian cells seems to be the most important
one. If a cell receives a go-ahead signal at
this check-point, it will complete the cell cycle
and divide. However, if the cell does not
receive the go-ahead signal in G1, the switches
to a nondividing state called G0.
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How Cdks actually work is not well understood but
the Cdks seem to activate other proteins and
enzymes that affect particular steps in the
cycle.
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External Signals-This include certain chemical
and physical factors that affect cell division.
Mammalian cells need certain nutrients and
regulatory proteins or growth factors are needed
for cell division. For example, when the skin
has been damage (wound), platelets release a
substance called platelet-derived growth factor
(PDGF). This growth factor stimulate fibroblast
cells to start to reproduce and make scar tissue.
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External signals can effect how cells grow in
culture. Density-dependent inhibition- cells in
culture stop dividing when they become crowded
forming a single layer of cells. It seems that
when crowded, there is insufficient growth factor
produced and nutrients for cell division to
continue. Anchorage dependence- mammalian cells
need to be attached to substratum like the inside
of a culture jar or other tissue in order to
reproduce. This phenomenon is linked to a
control system attached to the plasma membrane
proteins and the cytoskeleton. These phenomenon
keep the growth of tissue in check. Cancer cells
do not exhibit density-dependent inhibition or
anchorage dependence.
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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
• each cell binds a bit of growth factor
• not enough activator left to trigger division in
  any one cell
• anchorage dependence
• to divide cells must be attached to a substrate
• touch sensor receptors

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Growth factor signals
growth factor
nuclear pore
nuclear membrane
P
P
cell division
cell surface receptor
Cdk
E2F
protein kinase cascade
P
chromosome
P
Rb
P
E2F
Rb
nucleus
cytoplasm
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Example of a Growth Factor

• Platelet Derived Growth Factor (PDGF)
• made by platelets in blood clots
• binding of PDGF to cell receptors stimulates cell
  division in connective tissue
• heal wounds

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Growth Factors and Cancer

• Growth factors can create cancers
• proto-oncogenes
• normally activates cell division (positive
  control)
• growth factor genes
• become oncogenes (cancer-causing) when mutated
• if switched ON can cause cancer
• example RAS (activates cyclins)
• tumor-suppressor genes
• normally inhibits cell division (negative
  control)
• if switched OFF can cause cancer
• example p53

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Cancer Cell Growth

• Cancer is essentially a failure of cell division
  control
• unrestrained, uncontrolled cell growth
• What control is lost?
• lose checkpoint stops
• gene p53 plays a key role in G1/S restriction
  point
• p53 protein halts cell division if it detects
  damaged DNA
• options
• 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

p53 is theCell CycleEnforcer
p53 discovered at Stony Brook by Dr. Arnold Levine
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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 2
Step 1
Step 3
DNA damage is caused by heat, radiation, or
chemicals.
p53 triggers the destruction of cells damaged
beyond repair.
Cell division stops, and p53 triggers enzymes to
repair damaged region.
ABNORMAL p53
abnormal p53 protein
cancer cell
Step 2
Step 1
Step 3
The p53 protein fails to stop cell division and
repair DNA. Cell divides without repair
to damaged DNA.
DNA damage is caused by heat, radiation, or
chemicals.
Damaged cells continue to divide. If other damage
accumulates, the cell can turn cancerous.
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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

Its like anout-of-controlcar with manysystems
failing!
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What causes these hits?

• Mutations in cells can be triggered by

• UV radiation
• chemical exposure
• radiation exposure
• heat

• cigarette smoke
• pollution
• age
• genetics

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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 tumor
• 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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New miracle drugs

• Drugs targeting proteins (enzymes) found only in
  cancer cells
• Gleevec
• treatment for adult leukemia (CML) stomach
  cancer (GIST)
• 1st successful drug targeting only cancer cells

Novartes
withoutGleevec
withGleevec
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Any Questions?? Cell Cycle Cancer Movie
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Extra slides
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• Three major checkpoints
• G1/S (R point) checkpoint is the primary
  determining factor for cell division to take
  place. Growth factors are affecting the cell
  cycle, and cells are growing. Once the R point
  is passed the DNA is going to be replicated. If a
  cell receives a go-ahead signal at this
  check-point, it will complete the cell cycle and
  divide. However, if the cell does not receive
  the go-ahead signal in G1, the switches to a
  nondividing state called G0.

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2. This checkpoint represents the commitment for
starting the process of mitosis. This checkpoint
also ensures that the DNA has been replicated
correctly. If the DNA has been damaged, then the
cell does not continue to mitosis. Once the Cdk
and cyclin combine, it is called mitosis
promoting factor or MPF. 3. The M/ spindle check
point ensures that all the chromosomes are
attached to the spindle in preparation of
mitosis. The separation of the chromatids are
irreversible. Once chromatids are replicated they
are held together by a protein substance called
cohesion protein. Another protein called
seperase will destroy this protein. Seperase is
inhibits or unable to destroy cohesion because of
third protein called securin. So in effect the
APC (anaphase promoting complex) activates
securin, which actives an enzyme seperase to
destroy cohesion. In many cells this occurs
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during anaphase, however in vertebrates, all of
the cohesion is removed during chromatid
condensation except the cohesion at the
centromeres. Once the cohesion is completely
removed, then the tension of the microtubules
cause the separation of the chromatids. APC
also destroys cyclins in order to drive the cell
out of mitosis.