1. Meiosis • VIDEO http://www.youtube.com/watch?v=kVMb4Js99tA

2. Learning the Basics… • Chromosomes contain genetic information • Human somatic cells have 46 chromosomes • Divided into 23 pairs of homologous chromosomes, or homologs; • one set of 23 from mother, one set of 23 from father. • Specific traits are coded for in genes • Example: eye colour, hair colour, etc

3. Each of your parents contribute to genes towards a specific trait, but they are different forms of the same gene, called alleles. • Mom’s gene codes for blue eyes • Dad’s gene codes for green eyes • Both are alleles for the “eye colour gene”

4. Recall: • Mitosis involves cell division that ensures both daughter cells receive a full set of chromosomes (diploid cells) • Sexual reproduction involves the “merging” of two gametes (sex cells) together (eg. sperm and egg)

5. Meiosis: • Division process that prevents the “doubling” of genetic material from occurring. • Creates gametes with half the number of chromosomes (haploid cells) • Fertilization is the fusion of 2 gametes (male + female) to get back to the diploid number

6. Stages of Meiosis • Prior to the start of meiosis, cell undergoes S Phase (chromosome replication) • Meiosis has two rounds of cell division: • Meiosis I • Meiosis II • Each round is divided into 4 sub phases: • Prophase, Metaphase, Anaphase, Telophase

7. Meiosis I

8. Interphase (S-phase) • DNA replication occurs • • Each chromosome is made up of a pair of identical sister chromatids held together at the centromere (X-shape)

9. Prophase I Tetrad of a homologous pair • Chromosomes condense & shorten, visible • Spindle fibresform • Centrioles moving towards poles of the cell • Homologous chromosomes form a tetrad made of 4 chromatids • Process of forming a tetrad is called synapsis

10. Prophase I • In the tetrad, chromatids often break at the end and swap places with the sister chromatid • This process is called recombination or crossing overand allows for genetic variation • The location of recombination is called the chiasma

11. Recombination/Crossing Over: Homologous pair of chromosomes(tetrad)

12. Metaphase I • Tetrads line up along equator randomly = Independent Assortment • Spindle fibres attach to the pair of sister chromatids

13. Anaphase I • The sister chromatids do NOT separate here (different than mitosis) • Instead, the pairs of chromosomes move apart to opposite poles

14. Anaphase I • The chromosome number is reduced from 2n (diploid) to n (haploid) 46 23 23

15. Telophase I • Chromosomes condense slightly, nuclear membrane may form • Cytokinesis occurs forming two genetically different daughter cells • Each daughter cell has half the number of chromosomes as the parent cell (haploid - n)

16. Spindle fibres Nucleus Nuclear envelope Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (haploid) Telophase I (haploid) Meiosis I

17. Meiosis II

18. Meiosis II • Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase) • Each chromosome (made of two "mixed" chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles • End result – four haploid cells

19. Prophase II • Nuclear envelope begins to break down • Spindle fibres begin to form • Centrioles begin to move to poles

20. Metaphase II • Chromosomes align along equator of cell. • Spindle fibres attach to centromeres of sister chromatids

21. Anaphase II • Spindle fibres contract and pull sister chromatids apart

22. Telophase II • Nuclear envelope assembles • Chromosomes decondense • Spindle disappears • Cytokinesis divides each cell into two

23. Results of Meiosis: • 4 haploid gamete cells • 1 copy of each chromosome • 1 allele of each gene • Different combinations of alleles for different genes along the chromosome

24. Four Non-identical haploid daughter cells Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Meiosis II

25. Cell Division: A Comparison Homework