1. Meiosis Gamete Production
Mr. Young

2. Cells do not increase in size. They increase in number.
Meiosis = process of nuclear division that reduces the chromosome number by half in new cells (gametes)
cells undergo interphase, grow, copy DNA just like mitosis, but in meiosis the cells divide twice
two Diploid (2n) cells divide twice to produce four Haploid (1n) cells
2 stages = Meiosis I and Meiosis II

3. Meiosis Interphase Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I & Cytokinesis (results in 2 haploid cells)
Goes directly into Meiosis II (no interphase, no dna copying)
Prophase II
Metaphase II
Anaphase II
Telophase II & Cytokinesis (results in 4 haploid cells)

4. Meiosis Creates four haploid cells (gametes)
Two nuclear divisions occur without DNA being copied in between

5. Crossing Over
Crossing over - portions of chromatids break off and attach to adjacent chromatids on homologous chromosome
Crossing over allows for the random mixing of genes (genetic recombination) which adds genetic variety to a species so that no two individuals are exactly the same.

6. Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

7. Crossing Over Prophase I: Metaphase I Anaphase I Telophase I
Tetrad formation/
crossing over
Metaphase I
Anaphase I
Telophase I
Because of crossing over, every gamete receives a unique set of genetic information.
Telophase II

8. Crossing over animation

9. Stages Of Meiosis
Meiosis resembles mitosis except that it is actually two divisions not one
These divisions are called Meiosis I and Meiosis II
Meiosis I results in haploid cells with chromosomes made up of two chromatids
Meiosis II is essentially mitosis on haploid cells

10. Stages Of Meiosis - Meiosis I
Prophase I - The beginning phase -
DNA which was unraveled and spread all over the nucleus is condensed and packaged
Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids)
Crossing over, in which chromatids within tetrads exchange genetic material, occurs
Metaphase I - Middle stage - Tetrads line up along the equator of the cell

11. Stages Of Meiosis - Meiosis I
Anaphase I - One copy of each chromosome still composed of two chromatids moves to each pole of the cell
Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells

13. NOTE: There is NO interphase between meiosis I and meiosis II.
Why is this important?
So that each new cell will only have half (haploid) the number of chromosomes as the original parent cell.
Meiosis II is not preceded by the copying of DNA

14. Stages Of Meiosis - Meiosis II
Prophase II - Cells do not go into interphase between meiosis I and II, thus chromosomes are already condensed
Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I
Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells
Telophase II - Cytokinesis and reformation of the nuclear membrane in four haploid cells each with one set of chromosomes made of one chromatid

16. Stages Of Meiosis: Meiosis I
Interphase
Mother cell
Prophase I:
Condensing Chromosomes
Prophase I:
Tetrad formation/
crossing over
Metaphase I
Anaphase I
Telophase I
Meiosis II

17. Stages Of Meiosis: Meiosis II
Telophase I
The products of meiosis are 4 haploid cells each with a unique set of chromosomes.
Metaphase II
Prophase II
The products of mitosis are 2 diploid cells with identical chromosomes.
Anaphase II
Telophase II

18. Let’s look at Meiosis Animations!!

19. Gametes Are Haploid
Gametes = haploid reproductive cells (sperm, egg cells) one set of chromosomes (1n)
Gametes must have half the genetic material of a normal cell
If the genetic material in the gametes was not halved, when they combined the zygote would have more genetic material than the parents
Meiosis is specialized cell division resulting in cells with half the genetic material of the parents

20. “Putting It All Together” - Fertilization

21. Spermatogenesis
Spermatogenesis - diploid reproductive cell divides through meiosis to form 4 sperm cells

22. Oogenesis Oogenesis - Production of mature egg cells through meiosis.
Only one of the four develops into a mature egg cell, the other 3 are called ‘polar bodies’ and degenerate

23. What Meiosis is About
Meiosis allows the creation of unique individuals through sexual reproduction.