1. MEIOSIS!

2. What is Meiosis? We know what mitosis is Meiosis: A two-part cell division process used in sexually reproducing organisms that reduces the number of chromosomes in reproductive cells from diploid to haploid, leading to the production of gametes in animals and spores in plants

3. Key Terms Haploid (n) Cells that contain only one complete set of chromosomes Diploid (2N) A cell or organism containing two complete sets of chromosomes Polyploidy A cell/organism that contains more than two paired (homologous) sets of chromosomes (triplod/tetraploid) Gene A segment of DNA that provides information about a specific trait. (example: Hair Colour)

4. Key Terms cont’d Allele Alternative forms for a gene. (example: brown hair or black hair) Synapsis Pairing of two homologous chromosomes, which forms a tetrad Homologous chromosomes Homos in greek means same Have the same size, shape and location of the centromere and carry the information for the same trait. Not identical chromosomes (coding), but code for the same genes

5. Key Terms cont’d Sister Chromatids Are identical chromatids that were duplicated during the S phase of interphase. Sister chromatids are the exact same size, shape and contain identical information. Chiasma (plural chiasmata) The site where two homologous non-sister chromatids exchange genetic material during chromosomal crossover (crossing over) Crossing over The exchange of genetic material between homologous chromosomes that results in recombinant chromosomes

6. MOVIE TIME! http://www.youtube.com/watch?v=kVMb4Js99tA

7. Interphase! G1 (Gap 1): Produce RNA Synthesis protein Cell growth! S Phase DNA replicates Maternal and paternal DNA replicate Centrosomes replicate G2 (Gap 2): Cells prepare for meiosis

8. PROPHASE I Nuclear envelope and nucleoli disappear Chromatin condenses into chromosomes The centrioles (centrosomes) begin to migrate away from one another Synapsis occurs! This is when a pair of homologous chromosomes line up closely together forming a tetrad.

9. PROPHASE I cont’d During synapsis crossing over may occur! Occurs at a specific point The chiasmata Crossing over allows for the reception of different alleles for the same genes Extremely favourable for creating genetic variation Also, during this phase the tetrads begin to migrate to the metaphase plate

10. METAPHASE I Centrioles now at opposite poles Tetrads align at the metaphase plate Spindle fibres attach to the centromeres Specifically the kinetochore

11. ANAPHASE I Chromosomes have moved to the poles now Spindle fibres pull the chromosomes to the poles The sister chromatids DO NOT separate in Anaphase I Though the tetrads do!

12. TELOPHASE I Spindle fibres continue to move the homologous chromosomes to the poles Once movement is completed, each pole has a haploid number of chromosomes Nuclear envelope begins to reform In most cases cytokinesis occurs at the same time as telophase I (just as Telophase ends)

13. MEIOSIS I We began with a single diploid cell The maternal and paternal gametes both donated pairs of homologous chromosomes At the end of meiosis I we have 2 haploid cells. Each cell randomly got a set of chromosomes Leads to genetic variation!

15. ONWARD TO MEIOSIS II! Following Meiosis I we end up with 2 haploid cells Before going onto Meiosis II daughter cells sometimes enter Interphase II Meiosis II is much more like Mitosis! WOOHOO!

16. MEIOSIS II

17. PROPHASE II Once again nuclear envelope and nucleolus break up Spindle fibres attach on centrosomes On the kinetochores! Chromosomes don’t replicate any further in this phase of meiosis Chromosomes begin to migrate......to where? The metaphase (equatorial) plate

18. METAPHASE II & ANAPHASE II Chromosomes line up at the metaphase plate and the kinetochores point towards opposite poles In Anaphase II the sister chromatids now separate and move towards the opposite poles In Anaphase I, what was separated? The homologous pairs!!

19. TELOPHASE II & CYTOKINESIS Cell elongates Cleavage furrows Spindle fibres disintegrate Nuclear envelope reappears Cytokinesis also occurs towards the end of Telophase II Meiosis is now complete and we are left with 4 haploid cells called gametes