MITOSIS CELL DIVISION - PowerPoint

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					The Cell
 Cycle
    The Cell Cycle
                    INTERPHASE




Normal cell functions             S Phase
                                  (DNA synthesis)




                        Preparing for mitosis
              The Cell Cycle
 Cells spend most of their lives not dividing
 Cells spend most of their lives in interphase
 This is when all normal cell functions occur

 Some cells are always in interphase, and never
  undergo cell division
 For example, it was thought that this was true of
  all brain cells, but in 1998, research showed that
  under certain conditions, some mature brain cells
  may regenerate
               Cell Division

 There are different signals that tell a cell it
  is time to divide
 There are several reasons that cells divide
     Reasons for Cell Division
 Reproduction
   Single-celled organisms
       Prokaryotes (bacteria)
       Amoeba

 Growth and development
   After fertilization, the zygote (fertilized egg) divides
    into 2, 4, 8, 16, etc. cells to form multi-celled organism
 Tissue and organ renewal
   Organisms with tissues must replace them
           Human Tissue Renewal
 Humans replace -
 stomach lining every 5 days
 liver every 2 months
 the entire body every 7 years
 skin (the largest organ) every 6 weeks
 The epidermis, the outermost layer of the skin, sheds itself at a
  rate of about a million cells every 40 minutes
 Much of what we call “dust” is actually human skin cells
            Cell Division
 Before cells can divide, they have to
  make sure that both daughter cells will
  have the correct amount of DNA
 DNA has to be doubled, so when it is
  split into two cells, each cell has
  enough DNA
 This happens in S Phase
The Cell Cycle
              INTERPHASE




                           S Phase
Normal cell
                           (DNA synthesis)
functions




                  Preparing for mitosis
      S Phase – DNA duplication
 A eukaryotic cell has multiple chromosomes
    Humans have 46 chromosomes
    Fruit flies have 4 chromosomes
 Before S phase, each chromosome has a single DNA
  molecule
 After S phase, each chromosome consists of two
  DNA molecules called sister chromatids
 Each chromatid is an exact DNA copy of its sister
 At the end of cell division, the sister chromatids
  separate and move to two new daughter cells
 This restores the correct amount of DNA
Below is a diagram of a chromosome
 after S phase
   Chromosome duplication and
   distribution during cell division
                                                                              0.5 µm
     A eukaryotic cell has multiple
    chromosomes, one of which is
        represented here. Before
    duplication, each chromosome              Chromosome
                                              duplication
       has a single DNA molecule.             (including DNA
                                              synthesis)
  Once duplicated, a chromosome                     Centromere
  consists of two sister chromatids
connected at the centromere. Each
 chromatid contains a copy of the
                     DNA molecule.
                                                       Sister
                                      Separation       chromatids
                                      of sister
                                      chromatids
  Mechanical processes separate
    the sister chromatids into two
     chromosomes and distribute
      them to two daughter cells.
                                      Centromeres                   Sister chromatids
              Cell Division
 There are two parts to cell division
 Mitosis is the division of the genetic
  material (chromosomes)
 Cytokinesis is the division of one cell into
  two cells
 At the end of mitosis and cytokinesis, there
  are two new cells that have all the
  necessary organelles and are genetically
  identical to each other
The Cell Cycle
              INTERPHASE




Normal cell                S Phase
functions                  (DNA synthesis)




               Preparing for Mitosis
          INTERPHASE

Where the cell spends most of
its life

Occurs before mitosis

Chromosomes are copied in S
Phase

Nucleus is clearly visible

Chromosomes appear as
threadlike coils called chromatin
                  PROPHASE

             1st step in Mitosis

             Chromatin condense
             and chromosomes
             become visible

Centrioles   Nuclear membrane
             disappears
   METAPHASE

2nd step in Mitosis
Chromosomes line
up along the equator
    ANAPHASE

3rd step in Mitosis
Each chromosome
separates and
chromatids move to
opposite ends of the
cell
   TELOPHASE

4th step in Mitosis
The nuclear
membrane reforms
– two new nuclei
are formed
Chromosomes
uncoil to become
chromatin
     Cytokinesis

Occurs after Mitosis
Cell membrane
moves inward to create
two new identical
daughter cells
Interphase   Prophase




Metaphase    Anaphase




Telophase    Interphase
Chromosomes in a dividing cell
              Cytokinesis
 Animal cells only have to divide a cell
  membrane
 A cleavage furrow is formed in animals,
  and cell division occurs along it
 Plant cells have a cell wall in addition to a
  plasma membrane
 A cell plate is formed between the two new
  plant cells
Cytokinesis in animals and plants




                               100 µm
 Cleavage furrow                           Vesicles     Wall of                            1 µm
                                           forming      patent cell   Cell plate
                                           cell plate                              New cell wall




Contractile ring of       Daughter cells
microfilaments                                                                      Daughter cells


 (a) Cleavage of an animal cell (SEM)      (b) Cell plate formation in a plant cell (SEM)
    The Cell Cycle
                        INTERPHASE




Normal cell functions                  S Phase
                                       (DNA synthesis)




                            Preparing for mitosis
         Interphase
         Prophase
         Metaphase
         Anaphase
         Telophase
         Cytokinesis
I Pay More Attention To Cats
                Cell Death
 At some point, all normal cells undergo
  programmed cell death (PCD) or apoptosis
 Between 50 and 70 billion cells die each day
  in an average human adult
 This process ensures that old cells - that
  might no longer function properly – don’t
  divide any more
 A structure on the chromosome helps
  control when apoptosis happens
                 Cell Death

 At the end of chromosomes are telomeres,
  zones of repeated chains of DNA
 Telomeres prevent chromosomes from fraying,
  and thus prevent genetic information from
  getting scrambled when cells divide.
 They are analogous to the plastic tips on
  shoelaces that keep shoelaces from fraying
 The telomere is like a cellular clock, because
  every time a cell divides, the telomere
  shortens.
                Cell Death
 After a cell has grown and divided a few
  dozen times, the telomeres turn on an
  alarm system that prevents further division.
 It prompts the cell to undergo apoptosis
 If this alarm doesn't function right, cells
  either end up with damaged chromosomes
  or they become "immortal" and continue
  dividing endlessly -- either way it's bad
  news and leads to cancer or disease.
CANCER CELLS

Uncontrollable, abnormal growth of cells.

Cancer is named based on where it begins
(e.g. lung cancer).

Cancer cells can invade other tissues.

Mutations in DNA cause cancer to occur –
cells do not die and form at the right times.
• Cancer cells rapidly divide and form a tumor.

• They can stop themselves from destructing or they
  destruct slower than normal cells.
    Normal controls of cell division
 When growing cells in a petri dish, the availability of
  nutrients, growth factors, and a surface for
  attachment limits normal cell density to a single layer.
 In a petri dish, cells anchor to dish surface and divide
  (anchorage dependence)
 When cells have formed a complete single layer, they
  stop dividing (density-dependent inhibition).
 If some cells are scraped away, the remaining cells
  divide to fill the gap and then stop.
      Abnormal growth: cancer
 Cancer cells usually continue to divide well
  beyond a single layer, forming a clump of
  overlapping cells.
 Cancer cells do not exhibit anchorage
  dependence or density-dependent inhibition.
 Cancer cells continue to grow and divide without
  stopping.
 They also lose their specificity, are no longer
  differentiated based on their function.
 A benign tumor is localized and not invasive.
                   Metastasis
 If these cells detach from the site of origin and
  move to other parts of the body, the cancer is
  said to have metastasized and is malignant.
    A tumor grows from a single cancer cell.
    Cancer cells invade neighboring tissue.
    Cancer cells spread through lymph and
     blood vessels to other parts of the body.
    A small percentage of cancer cells may survive
     and establish a new tumor in another part of the body.
    A person could thus have lung cells in the brain or skin
     cells in the stomach.
  Why don’t cancer cells stop dividing?
 In embryonic cells (and some stem cells), an enzyme
  called telomerase rebuilds the telomere so that the
  cells can keep dividing.
 Over time, this telomerase dwindles and eventually
  the telomere shortens and the cell becomes inactive.
 In cancer cells, the telomerase enzyme keeps
  rebuilding telomeres long past the cell's normal
  lifetime. The cells become "immortal", endlessly
  dividing, resulting in a tumor.
 Researchers estimate that excessive telomere
  maintenance activity occurs in about 90% of human
  cancers.

				
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posted:3/19/2012
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