1. 4.2- Sexual Reproduction: Meiosis
SBI3U1

2. Recall: Sexual vs. Asexual Reproduction
-Involves two parents and leads to the production of genetically distinct offspring
- Involves only one parent and leads to the production of genetically identical offspring

3. Sexual Reproduction
The fusion of male and female reproductive cells ( gametes) which form a zygote. The process is called fertilization

4. Haploid (single): half the # of chromosomes as the parent cell
Gametes are haploid cells
In humans, 23 chromosomes (n)

5. Diploid (double): all the pairs of homologous chromosomes
Zygotes are diploid
Somatic cells are diploid
In humans, 46 chromosomes (2n)

6. MEIOSIS
Meiosis occurs in diploid cells and produces 4 haploid cells (gametes)
There are two main phases:
Meiosis I
Meiosis II

7. Key Outcomes of Meiosis:
1. The formation of gametes
Genetic reduction: reduces the number of chromosomes to half (23)
Genetic Recombination: different combinations of alleles are formed. This increases genetic variation in a population

8. Begins as a diploid cell (2n) and becomes a haploid cell (n)

9. Interphase
The cell goes through the G1, S and G2 stages (similar to mitosis)
DNA replicates
The cell is diploid (46 chromosomes)

10. Meiosis I: Prophase I
Nuclear envelope disappears and chromatin condenses to chromosomes
Homologous chromosomes pair (tetrads)
Centrosomes move to the poles and spindle fibres form
Synapsis occurs- the aligning of homologous chromosomes
Crossing over occurs

11. 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

12. Meiosis I: Metaphase I Tetrads line up at equator of cell
One chromosome of each pair is facing each pole
Spindle fibers attach to each homologous chromosome at the centromere

13. Meiosis I: Anaphase I
Homologous chromosomes (tetrads) separate and go to opposite poles
Centromeres do not split b/c sister chromatids do not separate
Chromosome number is now haploid (n)

14. Meiosis I: Telophase I
Homologous chromosomes uncoil (forming chromatin)
Spindle fibres disappear
Nuclear membrane forms around each nuclei
Cytokinesis occurs simultaneously
Formation of haploid daughter cells (each with 23 chromosomes)

15. Meiosis II (similar to mitosis)

16. Meiosis II: Prophase II
Nuclear envelope disappears
Spindle apparatus forms
Chromatin condenses to chromosomes

17. Meiosis II: Metaphase II
Chromosomes line up at equator of cell
Sister chromatids are no longer identical
Due to crossing over
The spindle fibers are attached to the centromeres

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

19. Meiosis II: Telophase II
Chromosomes uncondense into chromatin
Two nuclei form
Cytokinesis occurs simultaneously
End result= 4 haploid cells in total (n)

20. Video Overview

21. Meiosis vs. Mitosis
Meiosis
Mitosis

22. Meiosis vs. Mitosis

23. Gamete Formation
Spermatogenesis: production of male gametes (sperm) which takes place in the testes
Four haploid sperm cells form from one diploid cell.

24. Oogenesis: production of female gametes (eggs) which takes place in the ovaries
Only ONE haploid mature egg cell forms.
Polar bodies form, but degenerate.

25. Multiple Births
Twins can occur when more than one egg is released and fertilized.
Fraternal Twins-when 2 eggs are released and both are fertilized
Identical Twins-when a single zygote divides into 2 separate bodies

26. Important things to know…
Difference b/t haploid and diploid cells
All stages of meiosis I , meoisis II
Difference b/t meiosis and mitosis
Process of spermatogenesis and oogenesis

27. Homework Read and make notes pgs. 169-174
Complete pg. 172 Q # 7, 8, 11
pg.176 Q #13,