1. Sexual Reproduction
SBI3U

3. Meiosis allows:
1. The formation of gametes 2. Genetic reduction: reduces the number of chromosomes to half (n=23) 3. Genetic Recombination: different combinations of alleles are formed. Contributes to genetic variation in population.Due to independent assortment of chromosomes and crossing over.

4. MEIOSIS
Meiosis occurs in progenitor diploid cells (cells with 46 chromosomes) and produces 4 gametes; haploid cells.
Unlike mitosis, these gametes do not divide further, but instead fuse with another gamete to make a diploid zygote. (i.e. sperm fuses with the egg)
There are two main phases: Meiosis I and Meiosis II

5. Begins as a diploid cell (2n) and becomes a haploid(n) cell.)
Meiosis is subdivided into 2 main phases: Meiosis I, Meiosis II

6. Phases of meiosis
Meiosis I: Involves the separation of homologous chromosomes, One of each pair is given to two new daughter cells. REDUCTIVE DIVISION
Meiosis II: Involves the separation of sister chromatids into 4 new daughter cells. (Each of the previous daughter cells separate into 2) Similar to mitosis.

7. Stage 1: Interphase
The cell goes through the G1, S and G2 stages. (similarly to Mitosis)
DNA replicates and is in the form of chromatin
Centrosome replicates (2 centrioles))
The cell is diploid (46 chromosomes)

8. Meiosis i Stage 1: Prophase I
Nuclear envelope disapears and chromatin condenses and becomes chromosomes
Homologous chromosomes pair (tetrads)*****
Synapsis occurs, a protein holds homologous chromosomes together
Crossing Over, non sister chromatids exchange genetic information. *******
Centrioles move to opposite poles and spindle fibers form

9. Crossing over
Nonsister chromatids
Each gene of one homologue is aligned with the other homologue
The non sister chromatids – one maternal and one paternal chromatid of a homologous pair – are broken at the same place then rejoined to each others DNA
Genetic information is exchanged

10. Meiosis i Stage 2: Metaphase I
Homologous chromosomes (tetrads) line up on the metaphase plate of the cell
One chromosome of each pair is facing each pole
Spindle fibers attach to each homologue at the centromere

11. Meiosis i Stage 3: Anaphase I
Homologous chromosomes (tetrads) separate and go to opposite poles
Cenromeres stay intact because the sister chromatids do not separate
Chromosome number is now haploid (n) .

12. Meiosis i Stage 4: Telophase I
Homologous chromosomes uncoil (forming chromatin)
Spindle fibers disappear
Nuclear membrane forms around each nuclei
Cytokenesis occurs simultaneously
Formation of haploid daughter cells (each with 23 chromosomes)

13. Meiosis II

14. Meiosis ii Stage 6: Prophase II Nuclear envelope disappears
Spindle apparatus forms

15. Meiosis ii Stage 7: Metaphase II
Chromosomes are positioned on metaphase(equatorial) plate
Due to crossing over in meiosis I, the sister chromatids are no longer identical
The spindle fibers are attached to the centromeres

16. Meiosis ii Stage 8: Anaphase II
The centromere of each chromosome separates
Sister chromatids come apart
Individual chromosomes move to opposite poles of the cell

17. Meiosis ii Stage 9: Telophase II Chromosomes uncoil into chromatin
Nuclei form (nuclear membrane reforms)
Cytokenesis occurs simultaneously amd separates the cytoplasm and forms two new,variable haploid daughter cells