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					Cell Division


          What events take place during
            the three stages of the cell
                      cycle?
          How does the structure of DNA
           help account for the way in
             which DNA copies itself?
                     Cell Cycle
• The regular sequence of growth & division
  that cells undergo is known as the cell cycle.
• The cell cycle is divides into three main stages.
  – 1. Interphase
  – 2. Mitosis
  – 3. Cytokinesis
         First Stage-Interphase
• The first stage of the cell cycle is called
  interphase.
• During interphase, the cell grows, makes a
  copy of its DNA, & prepares to divide into
  two cells.
First Part of Interphase

             • During the first part
               of interphase, the
               cell grows to full
               size & produces all
               the structures it
               needs.
         2nd Part of Interphase
• In the next part of
  interphase, the cell
  makes an exact copy
  of the DNA in its
  nucleus in a process
  called replication.
• At the end of DNA
  replication, the cell
  contains two identical
  sets of DNA.
               Mitosis
     Second Stage of the Cell Cycle
• Mitosis is the stage during which the cell’s
  nucleus divides into two new nuclei.
• During mitosis, one copy of the DNA is
  distributed into each of the two daughter
  cells.
• Scientists divide mitosis into four parts, or
  phases:
• Prophase, metaphase, anaphase, & telophase.
                 Prophase
• The threadlike chromatin in the cell’s nucleus
  condenses to form double-rod structures
  called chromosomes.
• Each identical rod in a chromosome is called a
  chromatid.
• The two chromatids are held together by a
  structure called a centromere.
                     Prophase
                    1st step in Mitosis
                          •   Mitosis begins (cell begins to divide)
                          •   Centrioles (or poles) appear and begin to move
                              to opposite end of the cell.
                          •   Spindle fibers form between the poles.




                                                         Centrioles
Sister chromatids




                                                 Spindle fibers
                                 Prophase
                                                              Plant Cell
          Animal
          Cell
   Spindle fibers




                                                                      Centrioles



Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Metaphase
2nd step in Mitosis
  • Chromatids (or pairs of chromosomes) attach to
    the spindle fibers.




                                    Centrioles




                             Spindle fibers
                             Metaphase
          Animal Cell                                         Plant Cell




Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Anaphase
3rd step in Mitosis
         • Chromatids (or pairs of
           chromosomes) separate and
           begin to move to opposite
           ends of the cell.



                          Centrioles




                   Spindle fibers
                                Anaphase
          Animal Cell                                         Plant Cell




Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
            Telophase
            4th step in Mitosis
                   •   Two new nuclei form.
                   •   Chromosomes appear as chromatin
                       (threads rather than rods).
                   •   Mitosis ends.




  Nuclei                                       Nuclei




Chromatin
                               Telophase
          Animal Cell                                         Plant Cell




Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
                  Mitosis
• As the cell progresses through metaphase,
  anaphase, & telophase, the chromatids
  separate from each other & move to opposite
  ends of the cell.
• The two nuclei form around the chromatids at
  the two ends of the cell.
              Cytokinesis
                occurs after mitosis
• Cell membrane moves inward to create two
  daughter cells – each with its own nucleus with
  identical chromosomes.
• During cytokinesis, the cytoplasm divides,
  distributing organelles into each of the two new
  cells.
                 Cytokinesis
• Each daughter cell has the same number of
  chromosomes as the original parent cell.
• At the end of cytokinesis, each cell enters
  interphase, & the cycle begins again.
• The length of each stage & cell cycle varies,
  depending on the type of cell.
Cell Cycle




     21
      - Cell Division

The Cell Cycle




            22          22
             DNA Replication
• Ensures that each daughter cell will have all of
  the genetic information it needs to carry out
  its activities.
• The two sides of the DNA ladder are made up
  of alternating sugar & phosphate molecules.
• Each rung of DNA ladder is made up of a pair
  of molecules called nitrogen bases.
               Nitrogen Bases
• There are four kinds of
  nitrogen bases: adenine,
  thymine, guanine, &
  cytosine.
• Adenine only pairs with
  thymine.
• Guanine pairs only with
  cytosine.
            DNA Replication
• Begins when the two sides of the DNA
  molecule unwind & separate.
• DNA replication begins when the two sides of
  the DNA molecule unwind & separate.
• Next, nitrogen bases that are floating in the
  nucleus pair up with the bases on each half of
  the DNA molecule.
• Because of the way in which the nitrogen
  bases pair with one another, the order of the
  bases in each new DNA molecule exactly
  matches the order in the original DNA
  molecule.
• Once the new bases are attached, two new
  DNA molecules are formed.

				
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