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Meiosis The Formation of
Gametes
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Meiosis
Meiosis- a type of cellular reproduction that produces gametes.
Like mitosis, meiosis only occurs in eukaryotic cells.
Biggest difference between meiosis and mitosis is the two cell division process.
Meiosis produces more cells total
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Things that occur during meiosis
Produce cells that contain half the number of chromosomes as the original cell
The daughter cells are not all alike
The number of daughter cells produced is 4 rather than 2 for mitosis
Meiosis Meiosis produces gametes.
Gametes are egg and sperm (animals)
Gametes are pollen and ovum (plants)
Meiosis
There are 2 types of Meiosis. Spermatogenesis: Occurs in the testes of males. (sperm)
Oogenesis: Occurs in the ovaries of females. (eggs)
Spermatogenesis (in the testes of males)
Oogenesis (in the ovaries of females)
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Every gamete, contains the genetic information for the organism to function properly. In humans, each gamete contains 23 chromosomes. In humans all the body cells contain two sets of 23 chromosomes (46 chromosomes)
Chromosome Number
Chromosome number
Any cell that contains any two complete sets is called a diploid cell
The diploid number can also be written as 2N.
A higher diploid number does not mean a more complex organism.
Lettuce =14 Goldfish =94 Human =46 Dog = 78
Chromosome number A cell with only one complete set of chromosomes is call a haploid cell.
The haploid number can also be written as N.
Each human sperm and egg has 23 chromosomes.
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, Meiosis is the fundamental basis of sexual reproduction
Egg (1N) and sperm (1N) unite through fertilization to restore the original chromosome number (2N) for the organism.
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Fertilization of a 6 chromosome organism
1n =3
2n = 6
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Meiosis: Two Part Cell Division
Homologs separate
Sister chromatids separate
Diploid
Meiosis I
Meiosis II
Diploid
Haploid
Homolog
Homologs are “like” chromosomes or the same sized chromosomes that code for the same traits.
Homologs
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Prophase I
Early prophase Crossing over occurs. Homologs pair up.
Late prophase Spindle forms. Nuclear envelope fragments.
Crossing Over The exchange of genetic material between homologous chromosomes.
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Metaphase I
Homologous pairs of chromosomes align along the equator of the
cell (aka:Tetrads)
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Anaphase I
Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres.
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Telophase I
Nuclear envelopes reassemble.
Spindle disappears. Cytokinesis divides cell into two.
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Meiosis II: Reducing Chromosome Number
Prophase II
Metaphase II
Anaphase II Telophase
II 4 Genetically Different
haploid cells
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Prophase II
Nuclear envelope fragments. Spindle forms.
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Metaphase II
Chromosomes align along equator of cell.
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Anaphase II
Sister chromatids separate and move to opposite poles.
Equator
Pole
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Telophase II
Nuclear envelope assembles. Chromosomes de-condense. Spindle disappears. Cytokinesis divides cell into two.
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Results of Meiosis
Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome Different combinations of genes
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The 2 ways the gametes can become genetically diverse
1.When the sister chromatids separate
Meiosis I
Meiosis II
2. When crossing over occurs
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Mitosis Meiosis
Number of divisions
1 2
Number of daughter cells
2 4
Genetically identical?
Yes No
Chromosome # Same as parent Half of parent
When Throughout life At sexual maturity
Role Growth and
repair Sexual
reproduction
Comparison of Divisions