1. Meiosis Notes CP Biology Ms. Morrison

2. Chromosome Number  Every organism gets half of chromosomes from one parent and half from the other parent  Two sets of chromosomes = homologous chromosomes

3. Chromosome Number, pg 2  Cells with homologous chromosomes = diploid (2N)  Gametes (egg, sperm) – contain only one set of chromosomes = haploid (N)  Humans – 23 pairs (46 chromsomes)

4. Meiosis  Form of cell division to reduce the number of chromosomes in cell to half (2N  N)  Two stages to process: Meiosis I and Meiosis II  At end of Meiosis II – one diploid cell has become four haploid cells  Used to produce gametes (does not occur in regular body cells)

5. Meiosis I  Picture overview:

6. Meiosis I, pg 2  Interphase I – all chromosomes are replicated  Prophase I – each chromosome pairs with its homologous chromosome to form a tetrad (4 chromatids)

7. Meiosis I, pg 3  Prophase I – during this phase, chromosomes in tetrads may exchange portions of their chromatids in process = crossing over (gives new gene combinations

8. Meiosis I, pg 4  Metaphase I – spindle fibers attach to chromosomes along center of cell  Anaphase I – spindle fibers pull the homologous chromosomes toward opposite ends of the cell

9. Meiosis I, pg 5  Telophase I/Cytokinesis – nuclear membranes form and the cell separates into two cells  At end of Meiosis I – two daughter cells that are haploid because they contain one set of chromosomes (duplicated, though)

10. Meiosis II  Two haploid daugther cells go through another division similar to mitosis but does NOT have replication prior to prophase II

11. Meiosis II, pg 2  Prophase II – nuclear membranes disappear and spindle forms  Metaphase II – chromosomes line up along center of cell like those in mitosis

12. Meiosis II, pg 3  Anaphase II – sister chromatids separate and move towards opposite sides of the cell  Telophase II/Cytokinesis – nuclear membranes reform and the two cells separate into four cells – all are haploid

13. Gamete Formation  Males – haploid gametes = sperm, always produce four sperm through meiosis  Females – haploid gamete = egg, uneven divisions through Meiosis I & II so egg receives most of cytoplasm, remaining three cells = polar bodies (not used in reproduction)