Cell Division 2: Meiosis

1. Cell Division 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
3

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

9. 9

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

14. 14

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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26. 26

27. 27

28. 28