Mitosis

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Mitosis

• Chapter 9
• (Starr Taggart 2003)

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Overview of Division Mechanisms

• Mitosis and meiosis are both nuclear division
  mechanisms.
• Both sort and package parental DNA into new
  nuclei for daughter cells.
• Mitosis used for growth, cell replacement, and
  tissue repair. Occurs in somatic cells.
• Meiosis occurs in germ cells set aside for gamete
  production and sexual reproduction.

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Chromosomes

• Chromosome DNA molecule with attached proteins
• Chromosomes duplicate before mitosis.
• As long as duplicated halves are attached they
  are called sister chromatids.
• Centromere is area with attachment sites for
  microtubules that will move chromosomes during
  division

Figure 9.2
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Mitosis and Chromosome Number

• Chromosome number the sum total of chromosomes
  of a given type
• Diploid condition in which a cell has two copies
  of each type of chromosome
• Mitosis keeps chromosome number constant,
  division after division, from one generation to
  the next.
• If a parent is diploid, then the daughter cells
  will be diploid as well.

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Mitosis is only one phase of the cell cycle.
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Overview of mitosis in cell of African blood lily
cell,Haemanthus. Spindle apparatus (two sets of
microtubules) moves the chromosomes.
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Animal Cell
Interphase cells duplicates its DNA and readies
for division
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Animal Cell
Early Prophase DNA and proteins start to
condense (purple-maternally inherited
blue-paternally inherited)
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Animal Cell
Late Prophase More condensing new microtubules
become assembled and move the two centrioles to
opposite ends of the cell nuclear envelope
starts to break up
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Animal Cell
Prometaphase New microtubules penetrate the
nuclear region to form a bipolar spindle
apparatus. Many microtubules become attached to
sister chromatids. Nuclear envelope breaks up
into tiny vesicles.
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Animal Cell
Metaphase all chromosomes have lined up at the
spindle equator they are their most condensed
during this phase
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Animal Cell
Anaphase attachments between sister chromatids
break microtubules move the two separate
chromosomes to opposite poles as the spindle
apparatus contracts
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Animal Cell
Telophase two clusters of chromosomes
decondense patches of new membrane fuse to form
new nuclear envelope mitosis completed
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cell wall
former
spindle
equator
light
micrograph
and transmission electron
micrograph
showing cell plate formation in a dividing plant
cell
vesicles
cell plate cellulose deposited between
new membranes
converging with wall precursers two new
membranes elongate
Fig. 9.6, p. 154
Slide 8
Cytoplasmic division (cytokinesis) for plants
involves cell plate formation
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Cytoplasmic division (cytokinesis) for animals
involves cytoplasmic pinching with indentation
called a cleavage furrow
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The Wonder of Interphase PRECISION
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Organization of Metaphase Chromosomes

• Histones proteins associated with DNA act as
  DNA spools
• Nucleosome DNA-histone spool
• Interactions between histones and DNA make
  chromosomes coil on itself repeatedly during
  mitosis (condensation)
• After condensation is nearly complete, sister
  chromatids has one constricted region called
  centromere
• Structure at centromere surface (kinetochore)
  where microtubles will attach

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Organization of Metaphase Chromosomes
Why dont the strands tangle? An enzyme called
DNA topoisomerase
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How does the distance between the microtubules
and the spindle poles change? Mechanism
(1) As a kinetochore slides over a microtubule,
it shortens by disassembling.
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How does the distance between the microtubules
and the spindle poles change? Mechanism
(2) As overlapping microtubules rachet past each
other, they move the spindle poles apart and
thereby increase the distance between the sister
chromatids of each chromosome.
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Cancer

• What controls the cell cycle (repeated pattern of
  growth, genetic duplication, and division)?
• What causes unrestrained growth or division of
  cells (i.e. cancer)?
• Genetics can play a large role.

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Unrestrained cell division

• Mechanisms that induce cell division can be come
  hyperactive (analogous to a stuck accelerator).
• Mechanisms that suppress cell division fail
  (analogous to failed brakes)

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Mechanism 1 Oncogenes

• Oncogene mutated gene that is associated with
  cancer and uncontrolled cell growth prompts
  excessive cell division

proto-oncogene (normal)
mutation
Example point mutation from G to C in ras-gene
on chromosome 11 associated with bladder cancer
oncogene
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Mechanism 2 Tumor Suppressor Genes

• Tumor suppressor genes normal gene that inhibits
  growth faulty tumor suppressor genes fail to
  turn off cell division

• Mutation of p53 gene on chromosome 17 associated
  with 50 of human cancers
• Normally prevents division of cells with damaged
  DNA

• Rb gene on chromosome 13 suppresses cancer in
  dominant form
• Mutation in both parental copies can cause
  retinoblastoma

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Characteristics of Cancer Cells

• Plasma membrane and cytoplasm dramatically
  altered.
• Membrane becomes more permeable and prone to
  leaking.
• Cytoskeleton becomes disorganized, shrinks, or
  both
• Cancer cells grow and divide abnormally.
• Cancer cells have weaker capacity for adhesion
• Cancer cells are lethal.

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Therapy

• Surgery
• Radiation therapy the use of radioisotopes
  (atoms with an uneven number of protons and
  electrons) to destroy or impair activity of
  targets cells examples include emissions of
  radium 226 and cobalt 60
• Chemotherapy the use of drugs to destroy cancer
  cells
• Gene therapy modification of genetic material of
  living cells to fight disease substitution of a
  working gene for a defective one (p53) or
  inserting a gene making cells more susceptible to
  chemo