Cell Cycle

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Cell Cycle
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Key Roles of Cell Division

• Functions in asexual reproduction, growth,
  embryonic development, tissue repair.
• Distributes identical sets of chromosomes to
  daughter cells.
• Genetic information, packaged as DNA, is called
  a genome.
• Prokaryotes the genome is a single circular
  DNA molecule.
• Eukaryotes genome consists of several DNA
  molecules.

Bacterial cell
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DNA packaged as chromosomes

• Eukaryotic species have a characteristic number
  of chromosomes in the nucleus.
• Human somatic cells (body cells) have 46
  chromosomes.
• Human gametes (sperm or eggs) have 23
  chromosomes.
• Each chromosome has hundreds or thousands of
  genes, the units that specify an organism's
  inherited traits.
• Chromatin Eukaryotic DNA the associated
  proteins that maintain its structure and help
  to control gene activity.

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Chromosomes made of DNA

• Eukaryotic DNA is wrapped around proteins
  to form chromatin.

Chromatin
Chromatin condenses (super-coils) to become a
chromosome visible with a light microscope.
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Overview of mitosis

• Chromosomes are duplicated in interphase.
• Duplicated chromo-
  somes consist of 2
  sister chromatids which contain
  identical copies
  of DNA.
• Strands condense the region where they
  connect shrinks and be- comes the
  centromere.

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Overview of mitosis

• Then chromosomes are separated into
  daughter cells.
• Sister chromatids are later
  pulled apart, re- packaged
  into 2 new nuclei
  at opposite ends of the
  parent cell.
• This process is called mitosis. It is
  fol- lowed by division of the cytoplasm,
  called cytokinesis.

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Stages in the cell cycle

• The mitotic (M) phase of the cell cycle
  alternates with the much longer interphase.
• The M phase includes mitosis and cytokinesis.
• Interphase 90 of the cell cycle a very active
  period.
• During inter- phase the cell
  grows by
  pro- ducing
  proteins
  cytoplasmic organelles,
  cop-
  ies its chromo-
  somes, and pre-
  pares for cell division.

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Stages in the cell cycle

• Mitosis is broken into 4 sub-phases
• Interphase Prophase Metaphase
  Anaphase Telophase

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Stages in the cell cycle

• Prophase
• Late interphase
• Chromosomes duplicated but
  loosely packed.
• Centrosomes duplicated
  and organize microtubules into an aster ("star").
• Prophase
• Chromosomes tightly coil sister chromatids join.
• Nucleoli disappear mitotic spindle forms and
  appears to push the centrosomes away from each
  other toward opposite ends (poles) of the cell.

Late Interphase Prophase
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Stages in the cell cycle

• Late
• Prophase
• Late Prophase
• Nuclear envel- ope
  fragments,
  and microtub-
  ules from the spindles coming
  from each pole attach to 1 of 2 kinetochores,
  special regions of the centromere.
• Centrosomes are pushed to opposite ends of the
  cell due to the lengthening microtubules.

Prophase Late Prophase
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Stages in the cell cycle

• Metaphase
• Metaphase
• Spindle fibers push the sister
  chromatids until
  they are all arranged at
  the metaphase
  plate, an
  imaginary plane equi-
  distant between the
  poles.

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Stages in the cell cycle

• Anaphase
• Anaphase
• Centromeres divide, sep-
  arating sister
  chromatids into
  chromosomes again.
• As spindle fibers shorten,
  each chromosome is pulled
  toward the pole to
  which it
  is attached. The 2 poles will
  
  then have equivalent collec-
  tions of chromosomes.

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Stages in the cell cycle

• Telophase
• Telophase
• The cell continues to elongate
  as free spindle fibers from
  each
  centrosome push off
  each other.
• Two nuclei begin to form, sur-
  rounded by the fragments of
  the
  parent's nuclear envelope.
• Nucleoli begin to reappear as spindle apparatus
  disappears.
• Chromatin becomes less tightly coiled.
• Cytokinesis, division of the cytoplasm, begins.

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Cytokinesis divides the cytoplasm

• Cytokinesis, division of the cytoplasm, follows
  mitosis.
• In animals a cleavage furrow forms in the cell
  surface near the old metaphase plate.
• Contraction of the ring pinches the cell in two.
  
• Cytokinesis in plants involves building a cell
  wall.
• During telophase, Golgi vesicles form a cell
  plate.
• The plate membranes fuse with the plasma
  membrane contents of vesicles form new wall
  material.

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Genetically identical nuclei

• By this method, each
  daughter cell ends up
  with one copy of
  every chromosome.
• An abnormal number
  of chromosomes in
  a cell is usually fatal.

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Uncontrolled cell growth

• Cancer cells have escaped from cell cycle
  controls.
• They result from uncontrolled cell division.
• Cells divide excessively, invade other tissues.
• They can occur in any organ or tissue
• They may divide indefinitely if they have a
  constant food supply.
• In contrast, most mam- malian cells
  divide 20 to 50 times
  under culture
  conditions before they
  stop, age, or die.
• Cancer cells may be
  "immortal".

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Uncontrolled cell growth

• Cancer results from cell mutations.

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Uncontrolled cell growth

• The immune system recognizes destroys
  transformed cells.
• Cells that evade destruction proliferate to form
  a tumor.
• If the abnormal cells remain at the originating
  site, the lump is called a benign tumor remove
  by surgery.
• If cells lose attachment to nearby cells, they
  are carried by blood lymph system to other
  tissues (metastasis).
• Malignant tumor cells leave original site and
  form new tumors that impair the functions of one
  or more organs.

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Uncontrolled cell growth

• Breast cancer treatment
• 
  
• DCIS ductal carcinoma in situ

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Uncontrolled cell growth

• Skin cancer
• from excessive UV light

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Uncontrolled cell growth

• Lung cancer
• from smoking

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Uncontrolled cell growth

• Brain cancer