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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