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Control of Cell Division


Control of Cell Division Cell division is highly controlled. Cell growth and division depend on protein signals and other environmental signals. – PowerPoint PPT presentation

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Title: Control of Cell Division

Control of Cell Division
  • Cell division is highly controlled.
  • Cell growth and division depend on protein
    signals and other environmental signals. Many
    proteins within the cell control the phases of
    the cell cycle.
  • Signals from surrounding cells or even from other
    organs can also regulate cell growth and
  • Environmental conditions, including the
    availability of nutrients, also affect the cell

  • During the cell cycle, a cell undergoes an
    inspection process to ensure that the cell is
    ready for the next phase in the cell cycle.
  • Feedback signals at key checkpoints in the cell
    cycle can delay or trigger the next phase of the
    cell cycle.
  • There are three main checkpoints in the cell
    cycleG1 Checkpoint, G2 checkpoint, mitosis

  • G1 Checkpoint
  • Before the cell copies its DNA, the cell checks
    its surroundings. If conditions are favorable and
    the cell is healthy and large enough, the cell
    enters the synthesis phase.
  • If conditions are not favorable, the cell goes
    into a resting period.
  • Certain cells, such as some nerve and muscle
    cells, remain in this resting period for a long
    time. They do not divide very often.

  • G2 Checkpoint
  • Before mitosis begins, the cell checks for any
    mistakes in the copied DNA. Enzymes correct any
  • This checkpoint ensures that the DNA of the
    daughter cells will be identical to the DNA of
    the original cell.
  • Proteins also double-check that the cell is large
    enough to divide.
  • If the cell passes the G2 checkpoint, then the
    cell may begin to divide. Once past this
    checkpoint, proteins help to trigger mitosis.

  • Mitosis Checkpoint
  • During the metaphase stage of mitosis,
    chromosomes line up at the equator. At this
    point, the cell checks that the chromosomes are
    properly attached to the spindle fibers.
  • Without this point, the sister chromatids of one
    or more chromosomes may not separate properly.
  • This checkpoint ensures that the genetic material
    is distributed equally between the daughter cells.

  • Loss of Control
  • Normally, a cell responds properly to signals and
  • However, damage to a cells DNA can cause the
    cell to respond improperly or to stop responding
    leaving the cell cycle uncontrolled.
  • The defective cell divides and produces more
    defective cells. Eventually, these cells can form
    a mass called a tumor.

The Cell Cycle and Cancer
  • Characteristics of Cancer Cells
  • Cancer cells lack differentiation.
  • Cancer cells have abnormal nuclei.
  • Cancer cells form tumors.

Cancer cells lack differentiation
  • Unlike normal cells that differentiate into
    muscle or nerves cells, cancer cells have an
    abnormal form and are nonspecialized.
  • Normal cells enter the cell cycle only about 50
    times cancer cells are immortal in that they can
    enter the cell cycle repeatedly.

Cancer cells have abnormal nuclei
  • The nuclei may be enlarged and may have an
    abnormal number of chromosomes.
  • The chromosomes have mutated some chromosomes
    may be duplicated or deleted.
  • Gene amplification, extra copies of genes, is
    more frequent in cancerous cells.
  • Whereas ordinary cells with DNA damage undergo
    apoptosis, cancer cells do not.

Cancer cells form tumors
  • Normal cells are anchored and stop dividing when
    in contact with other cells i.e., they exhibit
    contact inhibition.
  • Cancer cells invade and destroy normal tissue and
    their growth is not inhibited.
  • Cancer cells pile on top of each other to form a

(No Transcript)
Chemical Signals tell a cell when to start and
stop dividing.
(Target cells animation click on go sign)
  • Cancer Cells
  • 1. Mutations occur in the
  • DNA when it is replicated.
  • 2. Chemical signals that start and stop the cell
    cycle are ignored.
  • 3. Cells do not communicate with each other and
    tumors form.
  • Normal Cell Division
  • 1. DNA is replicated
  • properly.
  • 2. Chemical signals start and stop the cell
  • 3. Cells communicate with each other so they
    dont become overcrowded.