Cell Cycle Stages in growth & division G1 Phase S Phase G2
Phase M Phase Cytokinesis copyright cmassengale
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G1 Phase First growth stage Cell increases in size Cell
prepares to copy its DNA copyright cmassengale
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Synthesis Phase Copying of all of DNAs instructions Chromosomes
duplicated copyright cmassengale
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G2 Phase Time between DNA synthesis & mitosis Cell
continues growing Needed proteins produced copyright
cmassengale
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M Phase Cell growth & protein production stop Cells energy
used to make 2 daughter cells Called mitosis or karyokinesis
(nuclear division) copyright cmassengale
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The Cell Cycle Most of the cell's life is spent doing its
regular function.Most of the cell's life is spent doing its regular
function. Cells divide along a rough time frame called its Cell
Cycle.Cells divide along a rough time frame called its Cell Cycle.
The Cell cycle consists of the folowing steps:The Cell cycle
consists of the folowing steps: G1 (Gap 1) Phase - Cell performs
its normal function (cells which do not divide stay in this stage
for their entire life span)G1 (Gap 1) Phase - Cell performs its
normal function (cells which do not divide stay in this stage for
their entire life span) S (Synthesis) Phase - Here the cell
actively duplicates its DNA in preparation for divisionS
(Synthesis) Phase - Here the cell actively duplicates its DNA in
preparation for division G2 (Gap 2) Phase - Amount of cytoplasm
(including organelles) increases in preparation for division.G2
(Gap 2) Phase - Amount of cytoplasm (including organelles)
increases in preparation for division. Mitosis - Actual division
occursMitosis - Actual division occurs
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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: G 1 /S can DNA
synthesis begin? G 2 /M has DNA synthesis been completed correctly?
commitment to mitosis spindle checkpoint are all chromosomes
attached to spindle? can sister chromatids separate correctly?
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Growth Factors and Cancer Growth factors can create cancers
proto-oncogenes normally activates cell division 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 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
G 1 /S restriction point p53 protein halts cell division if it
detects damaged DNA options: stimulates repair enzymes to fix DNA
forces cell into G 0 resting stage keeps cell in G 1 arrest causes
apoptosis of damaged cell ALL cancers have to shut down p53
activity p53 discovered at Stony Brook by Dr. Arnold Levine p53 is
the Cell Cycle Enforcer
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DNA damage is caused by heat, radiation, or chemicals. p53
allows cells with repaired DNA to divide. Step 1 DNA damage is
caused by heat, radiation, or chemicals. Step 1 Step 2 Damaged
cells continue to divide. If other damage accumulates, the cell can
turn cancerous. Step 3 p53 triggers the destruction of cells
damaged beyond repair. ABNORMAL p53 NORMAL p53 abnormal p53 protein
cancer cell Step 3 The p53 protein fails to stop cell division and
repair DNA. Cell divides without repair to damaged DNA. Cell
division stops, and p53 triggers enzymes to repair damaged region.
Step 2 DNA repair enzyme p53 protein p53 protein p53 master
regulator gene
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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 It s
like an out-of-control car with many systems 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 without Gleevec with Gleevec
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Cell Division Mitosis
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Mitosis Eukaryotes divide by a more complicated system called
MitosisEukaryotes divide by a more complicated system called
Mitosis This is because:This is because: 1.They have a nucleus
which must be broken up and then reformed 2.They have their DNA
packaged in the form of Chromosomes 3.Chromosomes are composed of
Chromatin 1.Made of DNA Strands & Proteins 4.Also contain
Nucleosomes containing Histones - Proteins the DNA is wrapped
around Name for the DNA/Protein complex is Chromatin 5.They usually
have more than 1 chromosome (Humans have 23 pairs) 6.They have
numerous organelles to equally share
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Chromatin / Chromosomes
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Interphase Cell Replicates its DNA/Chromosomes in preparation
of upcoming divisionCell Replicates its DNA/Chromosomes in
preparation of upcoming division Animal Cell Plant cell
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Mitosis is a continuum of changes. For description, mitosis is
usually broken into 4 subphases: prophase, metaphase, anaphase, and
telophase. Copyright 2002 Pearson Education, Inc., publishing as
Benjamin Cummings
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By late interphase, the chromosomes have been duplicated. The
centrosomes begin to organize microtubules into an aster (star).
Copyright 2002 Pearson Education, Inc., publishing as Benjamin
Cummings Fig. 12.5a
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In prophase, the chromatin condenses into chromosomes. The
chromosomes are two identical threads (sister chromatids) joined at
the centromere. Copyright 2002 Pearson Education, Inc., publishing
as Benjamin Cummings Fig. 12.5b
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During late prophase, the nuclear envelope fragments.
Microtubules from the spindle interact with the chromosomes.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin
Cummings Fig. 12.5c
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Metaphase chromosomes gather at the midline of the cell with
their centromeres aligned on the metaphase plate. Enzymes separate
the Sister chromatids. Copyright 2002 Pearson Education, Inc.,
publishing as Benjamin Cummings Fig. 12.5d
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At anaphase, the centromeres divide, separating the sister
chromatids. Each is now pulled toward the pole to which it is
attached by spindle fibers. By the end, the two poles have
equivalent collections of chromosomes. Copyright 2002 Pearson
Education, Inc., publishing as Benjamin Cummings Fig. 12.5e
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At telophase, the cell continues to elongate. Two nuclei begin
to form, surrounded by the fragments of the parents nuclear
envelope. Chromatin becomes less tightly coiled. Cytokinesis,
division of the cytoplasm, begins. Copyright 2002 Pearson
Education, Inc., publishing as Benjamin Cummings Fig. 12.5f
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin
Cummings Fig. 12.5 left
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin
Cummings Fig. 12.5 right
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Prophase Animal Cell Plant Cell
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Metaphase Animal Cell
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Anaphase
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Telophase (cytokenesis) Animal Cell
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Animal Cytokeneisis With animals, the membranes pinch together
to form a Cleavage Furrow, which eventually fuses to form two
daughter cells
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Overview of Mitosis
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Copyright 2002 Pearson Education, Inc., publishing as Benjamin
Cummings Fig. 12.9