Download Meiosis

Meiosis
• The division of sex cells (gametes)
• Conserves the number of chromosomes
• Without meiosis, the number of chromosomes in a species would double with each generation
• Meoisis occurs in similar stages to mitosis, but has two distinct parts (2 divisions)
http://www.youtube.com/watch?v=D1_­mQS_FZ0
http://www.youtube.com/watch?v=uh7c8YbYGqo
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Meiosis I
Prophase I
Metaphase I
• homologous pairs of chromosomes join to form a tetrad, and to CROSSING OVER
exchange parts of chromosomes occurs
• tetrads line up along the middle of the cell
Anaphase I
• homologous pairs separate and move to the opposite ends of the cell. Sister chromatids remain together. This is called Independent Assortment
Telophase I
• two daughter cells are produced.
• each daughter cell has 1/2 of the number of chromatids in the original cell (e.g. mother cell had 4 copies of the chromatid for chromosome 1, each daughter cell has two copies of the chromosome) This is the Reduction Division
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Crossing­Over
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The exchange of parts of non­sister chromatids from homologous chromosomes.
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Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
• very brief
• sister chromatids line up along the equator
• sister chromatids separate
• four new cells are produced
• each new cell has half the number of chromosomes as a somatic cell
• each cell is haploid because it contains only one copy of each chromosome
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Independent Assortment
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• in meiosis I, the homologous pair separate, but with no pattern....
Daughter cells
like this,
This mother cell could divide to give....
...OR daughter cells
like this!
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Meiosis I: the Reduction Division
Mother cell has already replicated DNA, so each chromosome has a sister chromatid. Homologous pairs make tetrads. Daughter cells are haploid (n) because they have only one copy of each chromosome.
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It is diploid (2n) because it has two copies of each chromosome.
homologous chromosomes separate
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