THE CELL CYCLE

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THE CELL CYCLE
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What is the cell cycle?

• A 4-phase cycle in which cells grow and divide
• In the first 3 phases, the cell is growing and
  undergoing metabolic processes (called
  INTERPHASE)
• In the other phase (called MITOSIS), the cell is
  dividing

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Steps of Interphase

• G1 Phase ( 1st growth phase)
• metabolic rate is high
• Many proteins are made
• Cellular organelles increase in size/number
• S Phase (synthesis phase)
• DNA within the cell duplicates
• G2 Phase (2nd growth phase)
• Proteins needed for cell division are made
• These proteins are moved to place in cell where
  they will be needed
• Centrioles duplicate

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MITOSIS Cellular division

• 2 distinct phases
• M Phase (mitosis) which involves the division
  of the nuclear material (also called
  KARYOKINESIS)
• CYTOKINESIS in which the cytoplasm of the cell is
  divided evenly between the new cells

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PROPHASE

• Chromatin within the cells coils up into two
  identical strands called SISTER CHROMATIDS
• Sister chromatids are joined together by the
  CENTROMERE
• Centrioles begin to move to opposite poles of the
  cell
• Spindle fibres (made of microtubules) begin two
  form between the centrioles
• At the end of prophase, the nuclear
  envelope/membrane begins to disappear, as does
  the nucleolus

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METAPHASE

• The sister chromatids line up in the middle of
  the cell along the equatorial plate
• Each sister chromatid attaches itself to a
  spindle fibre through a KINETOCHORE

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ANAPHASE

• The sister chromatids separate into individual
  chromosomes
• The chromosomes are pulled to opposite ends of
  the cell by the spindle fibres
• This occurs when the spindle fibres shorten and
  contract
• Equal numbers of chromosomes move to opposite
  ends of the cell (I.e. in humans, this number is
  46)

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TELOPHASE

• Chromosomes arrive at opposite sides of the cell
• Chromosomes uncoil and reform the chromatin
• Spindle fibres begin to break down
• The nuclear envelope reforms around the chromatin
  and the nucleolus reappears

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CYTOKINESIS

• Occurs right after mitosis is complete
• Cytoplasm (cytosol and other organelles) are
  divided evenly between the two new cells
• In animal cells, begins when the cleavage furrow
  forms as the cell membrane pinches in from both
  sides
• The furrow continues to move inward until the two
  new cells are formed

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CONSEQUENCES OF MITOSIS

• End up with two daughter cells that are
  genetically IDENTICAL to the parent cell
• Ensures that the amount of nuclear material in
  each new cell is constant
• What would happen to the cell if mitosis did not
  occur???

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Why do cells undergo mitosis?

• Development/growth of the organism (why dont
  cells just increase in size?)
• Replace dead cells
• Repair damaged cells (regeneration)

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What happens when the process malfunctions?

• Aging some scientists believe that the cell
  cycle slows down as we age, thus, when cells die,
  they are either not replaced, or are replaced at
  a much slower rate

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• Cancer the cell cycle speeds up and the cells
  begin to divide uncontrollably
• These abnormal cells interfere with the
  surrounding healthy cells by taking away vital
  nutrients and oxygen
• This prevents the healthy cells from functioning
  normally
• Cancer cells can either lump together to form a
  tumour or spread to other parts of the body to
  continue their growth