Meiosis

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Meiosis
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What is Meiosis?

• The process that produces haploid gametes in
  diploid organisms. Cells produced only contain ½
  number of chromosomes.
• Haploid cell n
• Diploid 2n

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What are gametes?

• Sex cell a haploid egg or sperm.

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Describe haploid

• Contains only a single set of chromosomes or
  single set of genes.
• Remember you get one set of genes from your
  mother the other set from your father.
• Diploid-contains both sets of chromosomes.

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What is difference between Meiosis and Mitosis?

• In meiosis, a cell undergoes two consecutive
  divisions, called meiosis I and meiosis II. Four
  genetically different daughter cells result from
  these divisions. Meiosis produces sex cells.
• In mitosis, a cell undergoes one cell division
  and two genetically identical cells result.
  Mitosis produces somatic (body) cells.

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What are the stages of Meiosis?

• Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

• Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

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Prophase I

• Each chromosome pairs with its corresponding
  homologous chromosome to form a tetrad.
• During prophase I homologous chromosomes may
  cross over and exchange portions of their
  chromatids. This process is called crossing over.

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What is a homologous chromosome?

• Two chromosomes that make up a matched pair in a
  diploid cell. One homologous chromosome is
  inherited from the organisms mother, the other
  from the organisms father.

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What is a tetrad?

• A paired set of homologous chromosomes, each
  composed of two sister chromatids.

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Metaphase I

• Spindle fibers attach to the chromosomes and
  tetrads line up in the center of the cell.

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Anaphase I

• The fibers pull the homologous chromosomes toward
  opposite ends of the cell.

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Telophase I

• Nuclear membranes form. The cell separates into
  two haploid cells.

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Cytokinesis I

• Meiosis I results in two haploid (N) daughter
  cells, each with half the number of chromosomes
  as the original cell.

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Prophase II

• The DNA coils back up around the proteins forming
  chromosomes.
• The nuclear membrane dissolves
• The spindle fibers reform

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Metaphase II

• The chromosome line up in a similar way to the
  metaphase stage of mitosis.

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Anaphase II

• The sister chromatids separate and move toward
  opposite ends of the cell.

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Telophase II

• Meiosis II results in four genetically different
  haploid (N) cells.

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Let s see what happens in each!

• mitosis cell division
• meiosis double cell division

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Accidents that can occur during Meiosis

• Nondisjunction Members of a chromosome pair
  fail to separate during anaphase. This results
  in diploid gametes. Crossing over, that
  sometimes occur during Prophase I, can cause
  this.
• Examples Trisomy 21(Downs syndrome) and
  Klinefelter syndrome.

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What alterations of chromosome structure
(mutations) could lead to birth defects and
cancer?

• Deletion- When a fragment of a chromosome is
  lost.
• Duplication- When a fragment from one chromosome
  joins to a homologous chromosome.
• Inversion- When a fragment reattaches to the
  original chromosome but in the reverse direction.
• Translocation- the attachment of a chromosomal
  fragment to a nonhomologous chromosome.

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Quiz

• Name 2 ways in which meiosis differ from mitosis?
• What is a gamete?
• What does haploid mean?
• If a human skin cell contains 46 chromosomes, how
  many chromosomes would a human sperm cell contain?

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Quiz

• If a human skin cell contained 46 chromosomes,
  how many chromosomes would a human liver cell
  contain?
• What types of accidents can occur in Meiosis?
• What is crossing over? When does it occur?
• Name and describe one alteration of chromosome
  structure.