Meiosis

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Meiosis
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Need For Reproduction

• Reproduction is the life process in which living
  things produce other living things of the same
  species.
• Reproduction is not necessary for the life of one
  individual organism.
• It is necessary for the continued survival of a
  particular group of organisms.

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Cell Division

• All cells arise from other cells by cell
  division.
• When cells grow to a certain size they must
  either divide or die.
• If a cell continued to grow without dividing, the
  surface area of the cell would become too small
  to hold the cells contents.

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Cell Division

• Cell division is a complex series of changes in
  the nucleus of a cell that leads to the
  production of two new cells.
• The new cells are called daughter cells.
• The nuclei of the daughter cells are usually
  identical to each other and to that of the parent
  cell.
• The daughter cells grow and increase in size
  until they divide and produce two more daughter
  cells.
• This process continues, resulting in organism
  growth and reproduction.

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Mitosis

• All cells in the body (except sex cells) are
  produced by the process of mitotic cell division.
• Mitosis involves a complex series of changes in
  the nuclei of body cells that produce identical
  (same) daughter cells.
• They have the same number and type of chromosomes
  as the parent cells.

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The Cell Cycle Stages of Mitosis

• Although the events of mitosis are an ongoing
  process, they are generally described in terms of
  separate phases, or stages.
• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase

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Meiosis

• The process of meiosis involves two cell
  divisions and produces cells that are different
  from the parent cell.
• Meiosis produces cells that have one-half the
  number of chromosomes as the parent cells.
• If meiosis did not take place, the fertilized egg
  would have double the amount of chromosomes it
  needs.
• Meiosis takes place during sexual reproduction
  when sex cells, called the egg and sperm, are
  produced in sex organs.

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The Cell Cycle- Stages of Meiosis

• During meiosis, two divisions of the nucleus
  occur. These divisions are called meiosis I and
  meiosis II.
• Interphase
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

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Meiosis

• One-half the number of chromosomes is called the
  monoploid, (or n) number.
• In humans, the monoploid number (n) is 23.
• When the egg and sperm unite during
  fertilization, the species normal chromosome
  number called the diploid or (2n) number is
  restored (brought back).
• The diploid number in humans is 46.
• n (sperm) n (egg) 2n

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Summary of Meiosis

• Two cells form during meiosis I.
• In meiosis II, both of these cells form two
  cells.
• The result is four sex cells.
• Each sex cell has one-half the number of
  chromosomes in its nucleus that was in the
  original nucleus.
• From a human cell with 46 paired chromosomes,
  meiosis produces four sex cells each with 23
  unpaired chromosomes.

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Comparison of Mitosis and Meiosis
Characteristic Mitosis Meiosis
Number of daughter cells 2 4
Number of cell division 1 2
Daughter cells are diploid or haploid Diploid Haploid
Daughter cells are identical or different Identical Different
Parent cell is diploid or haploid Diploid Diploid