Mitosis

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Mitosis

• Chapter 9

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Cell Division

• process by which new identical cells are produced
  from existing cells
• stomach and intestine cells every 2-3 days
• skin cells every 20 days
• liver cells every 200 days
• brain cells reproduce every 30-50 years

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Chromosomes

• short, stringy structures of DNA
• carriers of genetic material
• usually chromatin long strands of DNA which are
  wrapped around histone proteins
• just before cell division, chromatin compacts and
  reorganizes into chromosomes
• each chromosome internally duplicated
• consists of two identical DNA chains (sister
  chromatids) attached together at a single point
  (centromere)

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Chromosome Structure
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Mitosis in Animal Cells

• outside nucleus centrosome (microtubule
  organizing center)
• organizes mitotic spindle
• contains many fibers each composed of a bundle of
  microtubules
• in animals, contains two centrioles
• oriented at right angles to each other
• each with 9 triplets of microtubules arranged in
  a cylinder
• Centrosome was also replicated in S-phase, so now
  two centrosomes

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Mitosis Phase One Prophase

• longest phase of mitosis
• chromatin has condensed (visible in microscope)
• nuclear envelope and nucleolus disintegrate
• centrioles move to ends of cell (in animals)
• form microtubules in star-like arrays called
  asters
• spindle forms
• football-shaped, cagelike structure make up of
  microtubules
• in plants, spindle does not need centrioles

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Pictures of Prophase
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Prometaphase

• centromeres develop two kinetochores
• specialized protein complex
• 1 over each sister chromatid
• physically hook sister chromatids up with
  specialized microtubules (kinetochore fibers)
• connect sisters to opposite poles of mother cell

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Mitosis Phase Two Metaphase

• spindle fibers attach to chromosomes at
  kinetochores
• chromosomes are lined up across equatorial plane
• metaphase plate (looks like glass) shows where
  mother cell will be divided

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Mitosis Phase Three Anaphase

• centromeres dissolve
• daughter chromosomes separate and are pulled to
  opposite ends of cell by the shortening of the
  kinetochore fibers

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Mitosis Phase Four Telophase

• begins as daughter chromosomes reach opposite
  ends
• chromosomes unwind to become chromatin
• spindle breaks down
• nucleolus reappears nuclear envelope forms

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Cytokinesis

• cytoplasm divides to form two separate, identical
  cells
• animal cytokinesis
• A cleavage furrow appears between daughter nuclei
• Formed by a contractile ring of actin filaments
• Like pulling on a draw string
• Eventually pinches mother cell in two

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Cytokinesis in Animals
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Cytokinesis in Plants

• cell walls do not permit furrowing
• begins with formation of a cell plate
• many small membrane-bounded vesicles
• eventually fuse into one thin vesicle extending
  across the mother cell
• membranes of the cell plate become the plasma
  membrane between the daughter cells
• contents of vesicles become the middle lamella
  between the two daughter cells
• daughter cells later secrete primary cell walls
  on opposite sides of middle lamella

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Cytokinesis in Plants
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Animation of Stages of Mitosis
Mitosis takes 30 minutes to 1 ½ hours depending
on the type of cell Remember I P MAT C
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Results of Mitosis

• guarantees genetic continuity with two identical
  cells
• unicellular organisms simply multiply into two
  organisms they do not become multicellular
  organisms
• multicellular organisms form tissues, organs, and
  organ systems as a result of mitosis
• Identify which of the following examples in the
  video clip are actually mitosis

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The End