1. Stages of Meiosis Meiosis 1 ~
1)Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes that are in the parent cell.
2) During meiosis, a diploid cell goes through two divisions to form four haploid cells.
Meiosis 1 ~
Meiosis begins with a diploid cell that has copied its chromosomes.

2. Prophase 1
3) During prophase I, #Homologous chromosomes pair up, and #2 crossing over occurs
Nuclear membrane breaks up and spindles appear.
4) crossing-over.- exchange genetic material between Homologous chromosomes during meiosis.

3. Metaphase 1 5) In metaphase I,
1. the spindle moves the pairs of homologous chromosomes to the equator of the cell.
2. The homologous chromosomes remain together.

4. Anaphase 1 6) In anaphase I, 1. the homologous chromosomes separate.
2. The spindle fibers pull the chromosomes of each pair to opposite poles of the cell.
7) But the chromatids do not separate at their centromeres. Each chromosome is still made of two chromatids.
The genetic material, however, has recombined.

5. Telophase 1 During telophase I,
8) the cytoplasm divides (cytokinesis), and two new cells are formed.
Both cells have one chromosome from each pair of homologous chromosomes.

6. #8 part 2) Meiosis II begins with the two cells formed at the end of telophase I of meiosis I.
The chromosomes are not copied between meiosis I and meiosis II.
Meiosis II ~
9) Prophase II ~
New spindles form
Nuclear membrane dissolves

7. Metaphase II 10) During metaphase II,
the chromosomes line up along the equators
and are attached at their centromeres to spindle fibers.

8. Anaphase II ` 11) In anaphase II,
Centromere splits chromosome in half when spindle shortens
Sister chromatids go to opposite poles .

9. 13) The end result of meiosis is four
Telophase II
12) During telophase II,
1. a nuclear envelope forms around each set of chromosomes.
The spindle breaks down, and then the cell goes through
cytokinesis.
13) The end result of meiosis is four
haploid (1/2 # of chromosomes) cells!!

10. The Stages of Meiosis https://www.youtube.com/watch?v=nMEyeKQClqI
Meiosis animation

11. Comparing Mitosis and Meiosis
The processes of mitosis and meiosis are similar but meet different needs and have different results.
14) Mitosis makes new cells that are used during growth, development, repair, and asexual reproduction.
15) Meiosis makes cells that enable an organism to reproduce sexually and happens only in reproductive structures.
16) Meiosis produces 4 haploid cells while mitosis makes 2 diploid cells

12. 17) Main differences between meiosis and mitosis
1) Two daughter cells
genetically identical to parent cell
2) two full sets of chromosomes (diploid)
3) makes organism bigger
Meiosis
1) Four daughter cells
genetically different from parent cell (and each other)
2) one full set of chromosomes (haploid)
helps makes another organism (sexual reproduction)
Chap summary

13. Visual Concept: Comparing Meiosis and Mitosis
(copies)
(copies)
No DNA copying here

14. Genetic Variation Genetic variation is good for a population.
18) Genetic variation can help a population survive a major environmental change.
19) Genetic variation is made possible by sexual reproduction.

15. Genetic Variation, continued
In sexual reproduction, existing genes are rearranged. Meiosis is the process that makes the rearranging of genes possible.
Fusion of haploid cells from two different individuals adds further variation.
20) Three key contributions to genetic variation are 1) crossing-over, 2) independent assortment, and 3) random fertilization.

16. Genetic material from the homologous chromosomes is randomly swapped
This creates four unique chromatids
Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased

17. Independent Assortment
During metaphase I, homologous pairs of chromosomes line up at the equator of the cell.
The two pairs of chromosomes can line up in either of two equally probable ways ~ either on the right side or left. These 2 ways produce different results.
21) This random distribution of homologous chromosomes during meiosis is called independent assortment.

18. Independent assortment

19. Random Fertilization
Fertilization is a random process that adds genetic variation.
The zygote that forms is made by the random joining of two gametes.
22) Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared.
23) In humans, the possibility is 223 X 223 or about 70 trillion different possible outcomes.

20. Summary
During meiosis, a diploid cell goes through two divisions to form four haploid cells.
Mitosis produces cells that are used during growth, development, repair, and asexual reproduction. Meiosis makes cells that enable an organism to reproduce sexually and it only happens in reproductive structures.
Three key contributions to genetic variation are crossing-over, independent assortment, and random fertilization.