MEIOSIS 1. What is Meiosis?? 2 Every cell in the body makes more cells through mitosis We call...
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Transcript of MEIOSIS 1. What is Meiosis?? 2 Every cell in the body makes more cells through mitosis We call...
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MEIOSIS
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What is Meiosis??2
Every cell in the body makes more cells through mitosis We call these cells somatic
cells The only cells in your body
that are not produced by mitosis are gametes Gametes are sperm and egg
cells They are made through
meiosis
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Meiosis (finally)3
Meiosis, is how cells reduce their chromosome number by half As a bonus, they use it to shuffle DNA
around Start with 1 diploid (2N) cell End with 4 haploid (1N) cells
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What is Meiosis?4
WHY WOULD WE WANT TO CUT OUR CHROMOSOME NUMBER IN HALF?!
So that when moms gamete and dad’s gamete join together you get a zygote with 46 chromosomes!
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Mitosis Review5
Stages of mitosis Prophase
Chromosomes condense Nuclear membrane
disappears Metaphase
Chromosomes line up Anaphase
Chromosomes pulled apart
Telophase Nuclear membrane re-
forms and cell pinches Chromosomes “dissolve”
Makes 2 genetically identical, diploid (2N=46) cells
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Stages of Meiosis6
It happens in two steps: Meiosis 1 and Meiosis 2
Meiosis I – similar to mitosis Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis
Meiosis II – identical to mitosis!!!! Prophase II Metaphase II Anaphase II Telophase II & Cytokinesis
Makes 4 similar, but genetically Makes 4 similar, but genetically different haploid different haploid gametesgametes
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Meiosis I (reduction Meiosis I (reduction division)division)
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Goal: reduce the number of chromosomes in the cells
Prophase I Replicated chromosomes
condense (this means we can see each individual X)
Homologous chromosomes pair up Mom’s #1 and Dad’s #1, all the way
through Moms #46 and Dad’s #46 “Crossing-over” happens
An extra shuffling of the DNA
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Prophase IProphase I8
Crossing over Chromatids of homologous
chromosomes exchange material Mom’s #1 and Dad’s #1 switch
some pieces Leads to new genetic
combinations; that way you don’t look like your older or younger siblings!
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Prophase IProphase I9
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Metaphase I10
Paired homologous chromosomes line up along equator
Moms #1
Dads #1
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Anaphase I11
Homologous chromosomes separate & move to opposite poles
Moms #1
Dads #1
**Remember, at this point, mom and dad’s chromosomes are replicated, that is why they look like X’s. Each part of the X is called a sister chromatid
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Telophase I12
Nuclear membranes partially reform around two nuclei
Cytokinesis occurs between meiosis I & II
46 Chromosomes
46 Chromosomes
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Meiosis II13
We have now achieved haploid cells This means, each of the two cells have 23
X’s each. However, sister chromatids need to be
separated A second round of division pulls the
chromosomes apart Just like in mitosis
Meiosis II is virtually identical to mitosis
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Meiosis II14
Prophase II Chromosomes condense and nucleus/nucleolus
disappear Metaphase II
Chromosomes line up in the middle of the cell Anaphase II
Sister chromatids separate to opposite sides of the cell
Telophase II and Cytokinesis 4 new cells, each with 23 chromosomes are
made
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Prophase II Metaphase II Anaphase II Telophase IIFour non-
identical haploid
cells
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Spermatogenesis in humans(Meiosis in the male)
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Comparison of spermatogenesis and oogenesis
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Why understanding meiosis is important19
A stage of human development where things can go very wrong: Down syndrome
Trisomy 21 Learning disability/MR
Patau syndrome Trisomy 13
Klinefelter’s syndrome Turner syndrome Edwards syndrome
Trisomy 18 5-10% survive first year