1. Meiosis

2. Where are genes? The chromosomes contain the genes
Remember that chromosomes are made up of DNA and protein
Genes are segments of DNA that code for a specific protein
the unit by which hereditary characteristics are transmitted

3. How are genes passed down?
We need to look at specialized cells that pass genetic information to offspring
Reproductive cells called gametes
Each organism contains two copies of every gene
ex: a gene for eye color may have the mothers blue eye form and also the father’s brown eye form

5. Therefore…
When a mother passes genetic info to her offspring through reproduction, she only passes one half or one copy.
The father does the same
The genetic information gets passed in the gamete or reproductive cell
Sperm and egg

6. Sperm and Egg

7. Gametes (sperm and egg)
Contain half the number of chromosomes found in other cells in the organism
Human cells contain 46 chromosomes
Gametes:
sperm and egg contain 23 chromosomes
haploid (n)
nonfunctional
When the sperm and egg unite, the 23 c-somes from the mother and father unite to create an offspring with 46 c-somes
This process is called fertilization and produces a zygote.
The resulting cell is diploid (2n) and functional

8. Meiosis referred to as the reduction division
number of chromosomes in a diploid cell is reduced by half
produces haploid cells called gametes (sex cells)
from 1 diploid cell you end up with 4 haploid cells (sex cells) that are genetically different
there are 2 stages:
Meiosis I and Meiosis II 8

9. Telophase I and Cytokinesis
Phases of Meiosis I
Meiosis I
Meiosis I
Interphase I
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis

10. Meiosis I I) Interphase I most common resting stage
preparation to divide 10

11. Meiosis I II) Prophase I chromosomes become visible
homologous chromosomes pair up (tetrad)
centrioles descend from equator to poles
nuclear membrane disappears 11

12. Prophase I
When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over or synapsis.
Crossing-over produces new combinations of alleles.

13. Meiosis I III) Metaphase I
homologous chromosomes line up in pairs along the equator
spindle fibers attach to the centromere 13

14. Anaphase I IV) Anaphase I
spindle fibers shorten pulling homologous chromosome pairs apart 14

15. Telophase I V) Telophase I 1) cell divides (cytokinesis)
2) nuclear membrane
reappears
3) centrioles ascend
back to the equator
4) end up with 2
distinct daughter
cells (diploid) 15

16. Meiosis 16

17. Meiosis II

18. Meiosis II I) Interphase II 1) skipped 2) chromosomes don’t replicate18

19. Meiosis II II) Prophase II (reduction phase)
1) chromosomes become visible
2) centrioles descend from
equator to poles
3) nuclear membrane
disappears 19

20. Meiosis II III) Metaphase II 1) chromosomes line up along the equator
2) spindle fibers attach to
the centromere 20

21. Meiosis II IV) Anaphase II
1) spindle fibers shorten pulling sister chromatids apart 21

22. Meiosis II V) Telophase II 1) cell divides
2) nuclear membrane reappears
3) centrioles ascend back to the
equator
4) end up with 4 haploid
daughter cells
Meiosis 22

23. Meiosis II 23

24. Meiosis Overview 24

26. Gamete Formation Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.

27. Gamete Formation
In many female animals, only one egg results from meiosis.
The other three cells, called polar bodies, are usually not involved in reproduction.
Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.

28. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.

29. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.

30. What type of cell does it occur with?
Mitosis
Meiosis
What type of cell does it occur with?
Number of daughter cells produced?
Are the cells identical or different?
How many divisions?
Type of reproduction (sexual or asexual)
Crossing over (synapsis)
Homologous chromosomes separate (name the phase)
Sister chromatids separate (name the phase)

31. 11-4
If the body cells of humans contain 46 chromosomes, a single sperm cell should have
46 chromosomes.
23 chromosomes.
92 chromosomes.
between 23 and 46 chromosomes.

32. 11-4
During meiosis, the number of chromosomes per cell is cut in half through the separation of
daughter cells.
homologous chromosomes.
gametes.
chromatids.

33. 11-4 The formation of a tetrad occurs during anaphase I. metaphase II.
prophase I.
prophase II.

34. 11-4 In many female animals, meiosis results in the production of
only 1 egg.
1 egg and 3 polar bodies.
4 eggs.
1 egg and 2 polar bodies.

35. 11-4
Compared to egg cells formed during meiosis, daughter cells formed during mitosis are
genetically different, while eggs are genetically identical.
genetically different, just as egg cells are.
genetically identical, just as egg cells are.
genetically identical, while egg cells are genetically different.