The Cell Cycle

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The Cell Cycle
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When do cells divide?

• Reproduction
• Replacement of damaged cells
• Growth of new cells
• In replacement and growth cell divisions how
  should daughter cells compare to parent cell?
• The daughter cells should be identical copies of
  the parent cell.

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How can identical daughter cells form?

• The genome must be copied and then divided such
  that each daughter cell gets one of the copies.
• Genome all the genes in an organism

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Bacterial Reproduction

• How do bacterial cells reproduce?
• Binary fission

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Important terms in eukaryotic cell division

• Chromosome

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Important terms in eukaryotic cell division

• Chromosome threadlike structures that are
  composed of DNA protein
• replication

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Important terms in eukaryotic cell division

• Chromosome threadlike structures that are
  composed of DNA protein
• replication process whereby DNA is identically
  copied (before cell division)
• mitosis

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Important terms in eukaryotic cell division

• Chromosome threadlike structures that are
  composed of DNA protein
• replication process whereby DNA is identically
  copied (before cell division)
• mitosis division of the nucleus
• cytokinesis

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Important terms in eukaryotic cell division

• Chromosome threadlike structures that are
  composed of DNA protein
• replication process whereby DNA is identically
  copied (before cell division)
• mitosis division of the nucleus
• cytokinesis division of the cytoplasm
• chromatin

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Important terms in eukaryotic cell division

• Chromosome threadlike structures that are
  composed of DNA protein
• replication process whereby DNA is identically
  copied (before cell division)
• mitosis division of the nucleus
• cytokinesis division of the cytoplasm
• chromatin DNA protein complex that is thin
  and fibrous it will condense into distinct
  chromosomes during cell division

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• Chromatid

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• Chromatid after replication the chromosome
  consists of 2 sister chromatids joined at the
  centromere.
• Centromere

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• Chromatid after replication the chromosome
  consists of 2 sister chromatids joined at the
  centromere.
• Centromere specialized region of the
  chromosome, where chromatids are joined. Each
  chromosome has one centromere.

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The Cell Cycle
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Interphase

• 90 of cell cycle is spent in this phase
• G1 first growth phase
• S synthesis phase, DNA synthesis (replication)
  occurs here
• G2 second growth phase

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G2 phase of Interphase in animal cells

• Nuclear envelope is visible
• One or more nucleoli are present
• Centrioles are replicated and the 2 pairs are
  near nucleus
• aster forms around each pair of centrioles
• chromosomes are loosely packed into chromatin
  fiber, not distinguishable

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General Overview of Mitosis
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Late Interphase
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Prophase

• In the Nucleus
• Nucleoli disappear
• chromosome fibers condense into discrete
  chromosomes
• each chromosome consists of 2 sister chromatids
  joined at the centromere

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• In the Cytoplasm
• mitotic spindle begins to form
• spindle consists of microtubules arranged between
  the centrosomes
• centrosomes move apart due to lengthening of
  microtubules

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Prometaphase

• Nuclear envelope breaks apart
• each chromatid has specialized structure called
  kinetochore located at the centromere region
• kinetochore microtubules (km) interact with
  chromosomes at the kinetochore region
• The kms cause the chromosomes to move
• nonkinetochore microtubules radiate from each pole

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Metaphase

• Chromosomes move to the metaphase plate and line
  up there
• the centromeres of the chromosomes are all
  aligned on the metaphase plate
• each sister chromatid of one chromosome, has a
  kinetochore microtubule attached to it from
  opposite poles
• kinetochore microtubules nonkinetochore
  microtubules spindle fiber

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Anaphase

• Kinetochore microtubules shorten and
  non-kinetochore microtubules lengthen
• Centromeres divide and each chromosome has no
  sister chromatid component
• the shape of the cell elongates into an elipse
• chromosomes are pulled to the opposite poles

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Gary Borisy UW Madison 1987
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Telophase

• Nonkinetochore microtubules continue to elongate
  the cell
• new daughter nuclei form at the two poles
• new nuclear envelopes are formed around the
  chromosomes
• nucleoli reappear
• chromosomes uncoil into chromatin fiber
• last phase of mitosis

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Cytokinesis

• Begins before telophase has completed
• evidenced by cleavage furrow in animal cells and
  cell plate in plant cells

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Evolution of mitosis
Bacterial cells
Dinoflagelates chromosomes attach to nuclear
envelope.
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Diatoms nuclear envelope stays, microtubules
inside nucleus
Most other eukaryotes spindle forms outside of
nucleus, and nuclear envelope breaks apart
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Cytoplasmic Cell Signals in Regulation of Cell
Cycle
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Checkpoints in the cell cycle If it passes the
G1 checkpoint cell divides if not enters G0 phase
and does not divide
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Cyclin protein levels fluctuate according to cell
cycle stage. When cyclin is high the Cdk
attaches and phosphorylation leads to breakdown
of nuclear envelope. Later MPF initiates cyclin
breakdown
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M phase checkpoint

• Kinetochores that are not attached to
  microtubules secrete a wait protein that delays
  anaphase
• Only when all kinetochores are attached does APC
  (anaphase promoting complex) become active
• Ensures that daughter cells do not end up with
  missing or extra chromosomes

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External signaling

• Growth factors such as platelet derived growth
  factor (PDGF) are secreted from areas of injury
• Acts as the ligand in a tyrosine kinase pathway
  that triggers cell division

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Cancer cells

• How does abnormal cell division of cancer cells
  differ from normal cell division?
• Cancer cells are not under density dependent
  inhibition
• Continue to grow until all nutrients are used up
• Cancer cells are immortal, do not shorten
  telomeres. (HeLa since 1951)

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Cancer cells

• If they stop dividing, they do so at random
  points in the cell cycle
• Have unusual chromosome
• may not function in a constructive way
• Abnormal cell surface allows them to move
• Secrete signal molecules promoting blood vessel
  growth

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