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Meiosis

Sexual Reproduction

 Animals that reproduce sexually need to create special sex cells called gametes by the process of meiosis.

  In females, these cells are called eggs and in males these cells are called sperm.

 These cells will contain half of the chromosome number of the parent and will fuse to form a new individual with a full set of chromosomes.

  Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father—and produces four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, ensuring that offspring are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations, which enables them to adapt during the course of evolution.

Gamete Production

  In males, these cells are produced in the testes and in females these cells are produced in the ovaries.

 This allows for genetic diversity in a population.

46 Chromosomes (23 Pairs)

  22 pairs of homologous (autosomal) chromosomes. 1-22

 Homologous means you received one from mom an one from dad

  1 pair of sex chromosomes. 23

Sister Chromatids vs Homologous Chromosomes

  Sister chromatids are exactly alike.

 Homologous chromosomes are not.

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Diploid (2n) to Haploid (n) 23 + 23 = New Zygote

Stages of Meiosis

  Meiosis 1

  Prophase 1

  Metaphase 1

  Anaphase 1

  Telophase 1

  Results are two cells with half of the chromosomes of the parent cell

  Meiosis 2

  Prophase 2

  Metaphase 2

  Anaphase 2

  Telophase 2

  Results are four cells with half of the chromosomes of the parent.

Products of Meiosis

  In males 4 sperm cells are poduced.

  In females, three polar bodies and one egg are produced.

What Would Happen if Meiosis Did Not Occur?

 46 +46 = 92

Meiosis 1

 Chromosome number is halved.

  Prophase 1

 Metaphase 1

 Anaphase 1

 Telophase 1

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Interphase

 Chromosomes/DNA

replicate

Prophase 1

 Chromosomes condense and become visible

 Nucleus disappears

  Spindle fibers form

Prophase 1 (cont.)

 The homologous pairs of sister chromatids (chromosomes) will form a tetrad. This is when genetic material is swapped in a process called crossing over.

Chiasma

 The site of crossing over

  Increases the chance of genetic diversity

Metaphase 1

 Tetrads line up on the metaphase plate

 Spindle fibers attach to the tetrad

Anaphase 1

 Tetrads are pulled apart. The resulting cell has half of the original chromosomes of the parent cell.

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Telophase I/Cytokinesis

 The cell divides, the nuclear envelope reforms and the DNA unwinds. Each cell has half of the chromosomes of the parent.

The Whole Story of Meiosis 1

Meiosis 2

 Chromosomes are NOT duplicated again.

 They line up in the middle of the cell and the spindle fibers will pull apart the chromosomes (sister chromatids) making four cells that are NOT identical to the parent.

Prophase II

 Chromosomes condense and become visible

 Nucleus disappears

  Spindle fibers form

Metaphase II

 Chromosomes line up down the middle in a row like in mitosis and the spindle fibers attach.

Anaphase II

 Chromosomes are pulled apart to each side of the cell. This step ensures that each cell gets the correct number of chromosomes.

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Telophase II/Cytokinesis

  4 cells form each one is different from the next.

The Whole Story of Meiosis 2

What Happens When Cells Do Not Separate Correctly

 Non-Disjunction

 Chromosomes fail to separate in meiosis 2. Some of the cells formed will have extra chromosomes while others will have less.

Possible Outcomes of Non-Disjunction

  Animation

2N

N N

0 2N N N

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Possible Disorders

 Down Syndrome- 3 copies of chromosome 21

 Klinefelter Syndrome- 3 copies of chromosome 23 (XXY)

 Turner’s Syndrome-1 copy of chromosome 23 (X0)

Meiosis vs. Mitosis