Meiosis

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Meiosis
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What is Meiosis?

• Quick Review
• What organelle is responsible for holding the
  cells genetic code?
• Meiosis is the process of creating 4 haploid
  cells (n) from one diploid cell (2n) from a
  reproductive organ.
• ...Say What?

• Nucleus

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• Haploid cells (n) contain only one set of
  chromosomes from ONE parent
• one set of chromosomes (one set of genes).
• Diploid cells (2n) contains one set of
  chromosomes from EACH parent
• two sets of chromosomes (two sets of genes).
• Most cells are diploid
• Gametes (sex cells) produced by meiosis.
• Are gametes haploid or diploid cells?

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Phases of Meiosis

• Meiosis I
• Interphase
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Cytokenesis

• Meiosis II
• Prophase II
• MetaphaseII
• AnaphaseII
• Telophase II
• Cytokenesis

Why do you think Meiosis has two cycles while
mitosis only has one?
The purpose of meiosis is to produce haploid
gametes.
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1. Interphase

• Cell growth
• DNA replication
• Cell prepares for meiosis

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2. Prophase I

• Tetrad Chromosome pairs with homologous
  chromosome to form a tetrad. Each tetrad has 4
  chromatids.
• Crossing-over Exchange of alleles between
  homologous chromosomes during pairing.

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3. Metaphase I

• Tetrads line up at the equator.
• Spindle fibers attach to the chromosomes at the
  centromere.

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4. Anaphase I

• Tetrads break apart and move toward opposite
  poles. Differs from mitosis by sister chromatids
  stay together.

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5. Telophase I

• Spindle fiber breaks down.
• Chromosomes uncoil.
• Cytoplasm divides to form 2 new cells.
• Each cell has ½ the genetic information as the
  original cell. Cells are diploid because the DNA
  was replicated before meiosis I.

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Meiosis II

• The cell does not go into interphase between
  meiosis I and meiosis II.
• Meiosis II is the same as mitosis. The only
  difference is the end product 4 haploid cells.
• Recall What was the end product of mitosis?

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Critical Thinking

• Why do you think gametes (reproductive cells) are
  haploid when all other cells are diploid?

• When 2 haploid gametes join, they form one
  diploid cell (zygote)!