Mitosis Cell Division

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MitosisCell Division

• E. McIntyre

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In The Beginning One

• Most of the organisms we see started out as one
  cell
• Humans start out as a single cell, the zygote,
  formed by uniting a sperm and egg
• The zygote divides to make approximately one
  trillion cells
• During the process of dividing, cells become
  specialized to function in the various tissues
  and organs of the body
• Mitosis is the process of cell division in
  eukaryotic cells

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Why Cells Must Divide

• In multi-celled organisms (like humans) cells
  specialize for specific functions thus the
  original cells must divide to produce different
  kinds of cells
• Cells can only take in nutrients and excrete
  waste products over the surface of the membrane
  that surrounds them. The surface to volume ratio
  decreases with the square of the volume (unless
  special accommodations are made)

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The Cell Lifecycle

• The cell lifecycle is well defined and can be
  divided into four stages
• Gap 1 (G1) - The growth phase in which most cells
  are found most of the time
• Synthesis (S) - During which new DNA is
  synthesized
• Gap 2 (G2) - The period during which no
  transcription or translation occurs and final
  preparations for division are made
• Mitosis - Cell division

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The Cell Life Cycle
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Stages Of Mitosis

• During mitosis an exact copy of the genetic
  material in the mother cell must be distributed
  to each daughter cell
• Each stage of mitosis is designed to achieve
  equal and exact distribution of the genetic
  material which has been copied during the S phase
  of the cell cycle

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Stages Of Mitosis

• Interphase - The in between stage - Originally
  metaphase was thought to be a resting stage now
  we know that this is the stage most cells spend
  their time in doing the things that cells do and,
  if they are preparing to divide, growing and
  replicating their DNA

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Stages Of Mitosis

• Prophase - The beginning phase - DNA which was
  unraveled and spread all over the nucleus is
  condensed and packaged
• Metaphase - Middle stage - Condensed chromosomes
  line up along the equator of the cell
• Anaphase - One copy of each chromosome moves to
  each pole of the cell
• Telophase - End stage - New nuclear membranes are
  formed around the chromosomes and cytokinesis
  (cytoplasm division) occurs resulting in two
  daughter cells

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Stages Of Mitosis
Mother cell
Nucleus with un-condensed chromosomes
Condensed chromosomes
Equator of the cell
Telophase
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Packaging DNA
B DNA Helix
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Packaging DNA
T
G
A
Histone octomer
C
G C
TA
G C
C G
T A
A T
A T
C G
B DNA Helix
G C
T A
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Packaging DNA
T
G
A
Histone octomer
C
G C
TA
G C
C G
T A
A T
A T
C G
B DNA Helix
G C
T A
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Packaging DNA
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Packaging DNA
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Packaging DNA
Beads on a string
Looped Domains
Tight helical fiber
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Packaging DNA
Metaphase Chromosome
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Chromosomes, Chromatids and Centromeres
Chromosome arm
Two identical chromosomes
Centromere
Chromosome arm
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Important Link to DNA Coiling

• http//www.biostudio.com/demo_freeman_dna_coiling.
  htm

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Cancer

• Cancer is a disease of uncontrolled cell
  division. It starts with a single cell that
  loses its control mechanisms due to a genetic
  mutation. That cell starts dividing without
  limit, and eventually kills the host.
• Normal cells are controlled by several factors.
• - Normal cells stay in the G1 stage of the cell
  cycle until they are given a specific signal to
  enter the S phase, in which the DNA replicates
  and the cell prepares for division. Cancer
  cells enter the S phase without waiting for a
  signal.
• - Normal cells are mortal. This means that
  they can divide about 50 times and then they
  lose the ability to divide, and eventually die.
  This clock gets re-set during the formation of
  the gametes. Cancer cells escape this process
  of mortality they are immortal and can divide
  endlessly.
• - Normal cells that suffer significant
  chromosome damage destroy themselves due to the
  action of a gene called p53. Cancer cells
  either lose the p53 gene or ignore its message
  and fail to kill themselves.

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Cancer Progression

• There are many different forms of cancer,
  affecting different cell types and working in
  different ways. All start out with mutations in
  specific genes called oncogenes. The normal,
  unmutated versions of the oncogenes provide the
  control mechanisms for the cell. The mutations
  are caused by radiation, certain chemicals
  (carcinogens), and various random events during
  DNA replication.
• Once a single cell starts growing uncontrollably,
  it forms a tumor, a small mass of cells. No
  further progress can occur unless the cancerous
  mass gets its own blood supply. Angiogenesis
  is the process of developing a system of small
  arteries and veins to supply the tumor. Most
  tumors dont reach this stage.
• A tumor with a blood supply will grow into a
  large mass. Eventually some of the cancer cells
  will break loose and move through the blood
  supply to other parts of the body, where they
  start to multiply. This process is called
  metastasis. It occurs because the tumor cells
  lose the proteins on their surface that hold them
  to other cells.

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Cancer Treatment

• Two basic treatments surgery to remove the
  tumor, and radiation or chemicals to kill
  actively dividing cells.
• It is hard to remove all the tumor cells. Tumors
  often lack sharp boundaries for easy removal, and
  metastatic tumors can be very small and anywhere
  in the body.
• Radiation and chemotherapy are aimed at killing
  actively dividing cells, but killing all dividing
  cells is lethal you must make new blood cells,
  skin cells, etc. So treatment must be carefully
  balanced to avoid killing the patient.
• Chemotherapy also has the problem of natural
  selection within the tumor. If any of the tumor
  cells are resistant to the chemical, they will
  survive and multiply. The cancer seems to have
  disappeared, but it comes back a few years later
  in a form that is resistant to chemotherapy.
  Using multiple drugs can decrease the risk of
  relapse its hard for a cell to develop
  resistance to several drugs at the same time.