1. Meiosis

2. Why do we need meiosis?

3. Why do we need meiosis? Meiosis reduces the chromosome number by half
Fertilization then restores the full number of chromosomes
These half-chromosome cells are called haploid
After fertilization, the cells with two sets of each chromosome are called diploid

4. Why do we need meiosis?
Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
2n = 6
1n =3

5. Meiosis: Two-Part Cell Division
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Haploid
Haploid

6. Meiosis I: Homologous chromosomes separate
Nucleus
Spindle
fibers
Nuclear
envelope
Early Prophase I
(Chromosome number doubled)
Late Prophase I
Metaphase I
Anaphase I
Telophase I (haploid)

7. Meiosis II: Sister Chromatids Separate
Prophase II
Metaphase II
Telophase II
Anaphase II
4 Genetically Different haploid cells

8. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome

9. Prophase 1 Late prophase Early prophase Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Early prophase
Homologs pair.
Crossing over occurs.

10. Tetrads Form in Prophase 1
Homologous chromosomes (each with sister chromatids)  
Join to form a TETRAD
Called Synapsis

11. Crossing Over Homologous chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces Genetic recombination in the offspring

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

13. Actual, Real-Life Picture of Crossing Over

14. Homologous pairs of chromosomes align along the equator of the cell
Metaphase 1
Homologous pairs of chromosomes align along the equator of the cell

15. Anaphase 1 Homologs separate and move to opposite poles.
Sister chromatids remain
attached at their centromeres.

16. Telophase 1 Nuclear envelopes reassemble. Spindle disappears.
Cytokinesis divides cell into two.

17. Meiosis II Sister chromatids carry identical genetic information
So they must be split up
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.

18. Prophase 2
Nuclear envelope fragments.
Spindle forms.

19. Metaphase 2
Chromosomes align
along equator of cell.

20. Anaphase 2 Sister chromatids separate and move to opposite poles.
Equator
Pole
Sister chromatids separate and move to opposite poles.

21. Telophase 2 Nuclear envelope assembles. Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.

22. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome