Cellular Reproduction

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					Cellular Reproduction

      Chapter 9.1
              Why do cells divide?
• Cells divide to replace worn out cells
  – Some cells such as skin, blood, gastrointestinal cells are
    constantly being damaged by normal activity and are
    replaced regularly
• Cells divide to repair
  – Some cells will be replaced when an injury damages
    them
• Cells divide for growth
  – Multicellular organisms grow because they have more
    cells. Unicellular organisms divide to reproduce
            Why limits cell growth?
• Most cells are very small
• Cells are limited in size by their ratio of cell surface area
  to volume
• As cells get larger, their volume increases at a faster
  rate than their surface area
• All materials must enter and leave a cell through their
  cell membrane
• If the surface area to volume ratio is too great, there
  will not be sufficient area to supply nutrients to the
  increased volume
http://www.biologyjunction.com/images/clip01351.jpg
http://plaza.ufl.edu/alallen/pgl/modules/rio/stingarees/module/why.html
How do normal cells know when to divide?
• When a normal cell reaches it size limit it will
  function as a specialized cell
• If it receives a signal to divide it will begin the
  process of getting ready for cell division
• Cells reproduce by a cycle of growing and dividing
  called the cell cycle
• There are three main stages of the cell cycle
  – Interphase
  – Mitosis
  – Cytokinesis
         What happens in Interphase?
• Gap 1 (G1) is the part of Interphase when a cell grows
  to its normal size and carries out its normal functions
   –   Interphase immediately follows cell division
   –   Cells need to grow to their functioning size
   –   Cells stay in G1 while they are normally functioning
   –   Cells that are not going to divide stay in G1
   –   Cells leave G1 when they are getting ready to divide
• Synthesis (S) is when DNA is synthesized
   – Cells only go to S when they are getting ready to divide
   – A normal cell is obligated to divide if they progress to S
• Gap 2 (G2) is when the cell prepares to divide by
  growing
   – The cells make extra cytoplasm and organelles for the
     daughter cells in preparation for cell division
      What happens in mitosis?
• Mitosis is the division of the nucleus of a
  eukaryotic cell into two daughter nuclei
• Mitosis is an intricate “dance” of
  chromosomes which guarantees that the
  chromosomes are divided equally between
  the two daughter cells and each daughter cell
  gets one copy of each chromosome
  How does the cytoplasm divide?
• The division of the cytoplasm is called
  cytokinesis
• In animal cells the cell membrane pinches
  together forming a cleavage furrow
• When the two sides of the cleavage furrow
  meet, the two daughter cells separate
• In plant cells a new cell wall forms between
  the daughter cells
              Daughter cells
• The two daughter cells that are formed by
  mitosis and cytokinesis are genetically
  identical
http://www.cellsalive.com/cell_cycle.htm
   How is the cell cycle regulated?
• The cell cycle is regulated by two substances
  that signal the advance through the cell cycle
• Cyclin and cyclin-dependent kinases work
  together to start the different activities of the
  cycle
• There are check points in normal cells that
  determine whether the cell can move to the
  next stage
     What are the checkpoints?
• Check points monitor the progress though the
  cell cycle
• At the end of G1 a check point monitors for
  DNA damage
• During S phase a check point monitors
  whether the DNA has been copied correctly
• Spindle check points have also been identified
http://nobelprize.org/educational_games/medicine/2001/cellcycle.html
                What is cancer?
• Cancer is unregulated cell growth and division
• Cancer cells kill by crowding out normal cells
• Cancer is caused by genetic mutations that cause
  changes in the proteins that regulate the cell cycle
• It usually takes three mutations to cause cancer
• Some patients are born with one or two mutations
  and are therefore predisposed to cancer
http://science.education.nih.gov/supplements/nih1/cancer/activities/activity2_animati
ons.htm
http://www.pbs.org/wgbh/nova/cancer/grow_flash.html
http://www.cancerhelp.org.uk/cancer_images/cancer_cells_keep_repr
oducing.gif
          What causes cancer?
• Substances and agents that are known to
  cause the mutations that cause cancer are
  called carcinogens
  – Asbestos
  – Tobacco
  – Radiation, including UV from the sun
  – X-rays
• Multiples changes have to occur to cause
  cancer which is why older people get cancer
  more often than younger people
http://outreach.mcb.harvard.edu/animations/biochem.swf
http://media.hhmi.org/hl/03Lect1.html
              What is apoptosis?
• Some cells are programmed to live only for specific
  amounts of time
• Human hands and feet develop with cells occupying
  the space between the fingers and toes
  – These cells shrivel and die preventing the webbing of
    hands and feet
• When leaves drop from trees it is because of
  apoptosis between the leaves the tree branch
• When DNA is damaged beyond repair, cells self
  destruct
  – Protects against cancerous growth
http://www.insidecancer.org/
http://www.whfreeman.com/kuby/content/anm/kb04an01.htm
           What about stem cells?
• Eukaryotic cells become specialized after cell
  division
  – The process of a cell becoming specialized is called
    differentiation
• When a cell is specialized, part of its DNA is
  permanently turned off
• Cells that are not yet specialized (before
  differentiation) are called stem cells
  – Stem cells are thought to have the capability of
    becoming a wide variety of specialized cells
http://learn.genetics.utah.edu/content/tech/stemcells/scintro/
         How do prokaryotes divide?
• Prokaryotes divide by binary fission
• Because prokaryotes are unicellular, binary fission is also
  asexual reproduction
• During binary fission, the single, circular chromosome of
  the prokaryote replicates
• After replication, the two identical chromosomes attach to
  the cell membrane
• The cell membrane grows between the two chromosomes
  separating them
• When the chromosomes are separated, the cell membrane
  pinches until the two sides meet
• This divides the cell into two daughter cells
http://image.tutorvista.com/content/reproduction/bacteria-binary-fission-asexual-
reproduction.jpeg
http://www.cellsalive.com/qtmovs/ecoli_mov.htm
          What about mitosis?
• Mitosis is the division of the cell’s nucleus
• It occurs in steps, each step marked by specific
  events
Vocabulary associated with cell division
• Chromatin is DNA that is “relaxed”
• Chromosomes are condensed chromatin and only
  visible during cell division
• After DNA replication, there are two identical
  strands of DNA that are attached to each other at
  the centromere
• This structure with double the DNA is called a
  chromosome, each half is called a sister chromatid
  – Sister chromatids are genetically identical
• When the two chromatids divide, they are each
  called a chromosome
http://mcb.illinois.edu/faculty_research/images/mizzen/fig1.jpg
        Structure of a chromosome
• The center of each chromosome is tightly coiled and
  is called a centromere
• The centromere is where two sister chromatids are
  connected
• The ends of the chromosomes are also tightly coiled
  and are called telomeres
• Telomeres protect the ends of the chromosomes as
  they moved across the cytoplasm during mitosis
http://www.uic.edu/classes/bios/bios100/labs/realchromo.jpeg
   What other structures are involved
           in cell division?
• Animal cells have centrioles which organize the
  spindle fibers
• Spindle fibers are microtubules that guide the
  chromosomes
• Plant cells have cell walls which are constructed
  between the daughter cells during cytokinesis
• Animal cells would loose their shape during mitosis
  if microtubules would not be there to support the
  cell membrane
  – Spindle arrangement in animal cell is called an aster
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/Bio%20101/Bio%20101
%20Laboratory/Mitosis/Photographs/whitefish_mitosis_telophaseX400.jpg
    What are the phases of mitosis?
•   Prophase
•   Metaphase
•   Anaphase
•   Telophase
    – Cytokinesis follows and is concurrent with
      telophase

				
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