Meiosis is a type of cell division that produces gametes or sex cells with half the number of chromosomes as the original parent cell. It involves two divisions and results in four daughter cells each with a single set of chromosomes. This ensures that offspring will receive one set of chromosomes from each parent, maintaining the chromosome number from generation to generation. The key phases and events of meiosis include homologous chromosome pairing, crossing over, independent assortment of chromosomes into new cells, and two cell divisions that ultimately produce four haploid gametes.

1. GENETICS 2:
Meiosis
ppt. by Robin D. Seamon
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2. MEIOSIS
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3. sexual reproduction:
• eukaryotic reproductive adaptation
• advantage: mixing of genes from two
parents to create more genetic diversity in
the offspring
• not being identical to the parent is
advantageous for a species… if an outside
stimulus is introduced that the species is
unable to withstand, there is a greater
chance that at least some members of the
species will have a variation or gene
combination that can make it
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4. METHOD
meiosis-
a type of cell division where the cell divides twice
• this results in four daughter cells each with
half the number of chromosomes of the
parent cell
• produce gametes and plant spores.
WHY HALF?
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5. Consider humans:
Humans have 23 PAIRS of chromosomes
(or 46 total chromosomes)
If a female cell of 46 chromosomes combined
with 46 chromosomes of a male cell, the resulting
offspring would have TOO MANY- 92
chromosomes!
Therefore, a different cell division process must
occur for the production of sex cells: meiosis.
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6. XX – female
XY- male6
HUMAN CHROMOSOME Karyotype
one set from mom
one set from dad
autosomes: 22 pairs
of body cells
sex cells: X or Y
chromosomes
homologous
pair 1

7. gamete- sex cell
• egg or ova- female gamete
• sperm- male gamete
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8. 8
HAPLOID
sex cells are haploid- have half the number of
chromosomes
DIPLOID- two sets of
chromosomes

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10. 10
Meiosis 1 begins with ONE DIPLOID cell (2n)
Meiosis 2 ends with FOUR HAPLOID cells (n)
2n = 46
n = 23

11. 11
MEIOSIS PHASES
Interphase 1- chromosomes replicate
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Cytokinesis 1
Interphase 2 BRIEF
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Cytokinesis 2
*THIS ONLY
HAPPENS
ONCE!
“preparing”
“middle”
“away”
“two”
= 4 haploid cells

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15. 15
MEIOSIS 1:
Interphase 1- cell replicates DNA
• (This will not happen again in the second
division!)
• prepares for cell division

16. 16
MEIOSIS 1:
Prophase 1-
• homologous chromosomes pair up
• make a tetrad
• crossing over- non-sister chromosomes
exchange genetic pieces of chromosome
• results in NEW COMBINATIONS of genes on the
chromosomes

17. 17
Crossing over of Chromosomes

18. 18
Sister
chromatids
Non-sister
chromatids
Tetrad
Homologous
chromosomes
Gametes

19. 19
MEIOSIS 1:
Metaphase 1-
• new tetrads move to the middle along spindle
fibers

20. 20
MEIOSIS 1:
Anaphase 1-
• homologous chromosomes separate
• independent assortment: each new cell
randomly gets ONE chromosome from the
homologous pair
***Genetic variation***

21. 21
MEIOSIS 1:
Telophase 1-
• spindle breaks down
• cytoplasm divides
• each cell will have HALF the genetic information
because it only has ONE CHROMOSOME from
each homologous pair

22. 22
MEIOSIS 1:
Cytokinesis 1-
• 2 new cells

23. 23
MEIOSIS 2:
*NO COPYING or CROSS-OVER OF DNA
Prophase 2: preparing for division
Metaphase 2: moving to the middle
Prophase 2 Metaphase 2

24. 24
MEIOSIS 2:
Anaphase 2: moving away
Telophase 2: two cells
Anaphase 2 Telophase 2

25. MEIOSIS 2:
Cytokinesis 2: 4 haploid cells are produced
• 4 spermatids:
gametes produced in
the testes of males
• one egg and
3 polar bodies:
gametes produced in
the ovaries of females
(polar bodies
degenerated & are
not fertilized)
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