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MITOSIS CELL DIVISION - PowerPoint Powered By Docstoc
					The Cell
    The Cell Cycle

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

                           S Phase
Normal cell
                           (DNA synthesis)

                  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
 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
       has a single DNA molecule.             (including DNA
  Once duplicated, a chromosome                     Centromere
  consists of two sister chromatids
connected at the centromere. Each
 chromatid contains a copy of the
                     DNA molecule.
                                      Separation       chromatids
                                      of sister
  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

Normal cell                S Phase
functions                  (DNA synthesis)

               Preparing for Mitosis

Where the cell spends most of
its life

Occurs before mitosis

Chromosomes are copied in S

Nucleus is clearly visible

Chromosomes appear as
threadlike coils called chromatin

             1st step in Mitosis

             Chromatin condense
             and chromosomes
             become visible

Centrioles   Nuclear membrane

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

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

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

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
 Animal cells only have to divide a cell
 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

Normal cell functions                  S Phase
                                       (DNA synthesis)

                            Preparing for mitosis
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
                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.

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
 They also lose their specificity, are no longer
  differentiated based on their function.
 A benign tumor is localized and not invasive.
 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

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