Meiosis

1
Meiosis

• Making Sex Cells

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Chromosome Number

• Everyone has 2 sets of chromosomes
• A set from Mom and a set from Dad, these two sets
  are homologous chromosomes.
• A cell containing both sets of homologous
  chromosomes is said to be diploid, which means
  two sets.
• This number is usually represented by the symbol
  2N.
• Example humans have 46 chromosomes or 2 sets of
  23, so . . .
• 2N 46
• A cell containing only 1 set of chromosomes are
  said to be haploid, which means one set.
• This number is usually represented by the symbol
  N.
• Example humans 2N 46 then N 23

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Overview of Meiosis

• The function of meiosis is to produce eggs and
  sperm for sexual reproduction.
• Eggs and sperm cells must have ½ the number of
  chromosomes as the parent cell, so that when they
  are put together (during sexual reproduction)
  they will have the same number of chromosomes as
  the parent.
• Egg and sperm cells are NOT genetically identical
  to the parent cell.

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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I results in two haploid (N) daughter
cells, each with half the number of chromosomes
as the original.
The chromosomes line up in a similar way to the
metaphase stage of mitosis.
The sister chromatids separate and move toward
opposite ends of the cell.
Meiosis II results in four haploid (N) daughter
cells.
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Important Information about Meiosis

1. Genetic material must be replicated
2. Genetic material must be divided twice, so that
   each of the four daughter cells gets a set of
   information.
3. All other materials in the cell must be divided
   fairly between the four daughter cells, in
   cytokinesis.
4. Each daughter cell will end up with a set of
   chromosomes.

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Phases of Meiosis I

• Interphase I
• Cells undergo a round of DNA replication,
  forming duplicate chromosomes.

• Prophase I
• Each chromosome pairs with its corresponding
  homologous chromosome to form a tetrad.
• Crossing-Over can occur during the latter part of
  this stage.

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Phase of Meiosis I

• Metaphase I
• Spindle fibers attach to the chromosomes

• Anaphase I
• the fibers pull the homologous chromosomes
  toward oppposite ends of the cell

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Phases of Meiosis I

• Telophase I and Cytokinesis
• Nuclear membranes form
• The cell separates into two cells

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Phases of Meiosis II

• Prophase II
• meiosis I results in two diploid (2N) daughter
  cells, each with half the number of chromosomes
  as the original cell.
• No DNA replication

• Metaphase II
• the chromosomes line up in a similar way to the
  metaphase stage of mitosis.

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Phases of Meiosis II

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

• Telophase II and Cytokinesis
• Meiosis II results in four haploid (N) daughter
  cells.

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Summary of Meiosis
One parent cell produces four daughter cells.
Daughter cells have ½ the number of chromosomes
as the parent.
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Comparing Mitosis and Meiosis

• Meiosis
• produces 4 genetically different haploid cells.

• Mitosis
• results in the production of 2 genetically
  identical diploid cells.