1. Think: We have discussed how somatic cells (body cells other than the gametes) are formed…what process forms the sex cells (the gametes- sperm/egg)? Take out paper for notes. This process is called MEIOSIS.

2. The Basics Genes are located on the chromosomes. Each organism must inherit one copy of every gene from both parents. Some organisms have 2 complete sets of genes. Those two sets must be separated so that each gamete produced contains just one set of genes.

3. Chromosome Number Humans have 46 chromosomes. –23 came from your mother –23 came from your father Each pair of chromosomes are referred to as homologous chromosomes All 46 chromosomes are present in every human body cell (a.k.a. somatic cell). This is the diploid chromosome number (2 sets ). “Di” means 2, so 2 copies of each chromosome are present 23 pairs

4. Gametes Gametes are sex cells —sperm or egg. Gametes contain only 1 copy of each chromosome. These cells are haploid (1 set) –sounds like “half”, so they only have half of the number of chromosomes Humans have 23 chromosomes in every gamete.  Not 23 pairs, just 23!

5. Diploid & Haploid Diploid = a cell with two sets of chromosomes, 2n Haploid = a cell with one set of chromosomes, gametes, 1n or n n = number of chromosomes in the organism (23 in humans)

6. Purpose of Meiosis Meiosis forms the gametes (sperm and eggs) that are necessary for sexual reproduction. Gametes need to be haploid so that when they meet for fertilization, the zygote has 2 copies of the genetic material (diploid) Meiosis has 2 stages –Meiosis I (different than mitosis) –Meiosis II (almost identical to mitosis)

7.  MEIOSIS I Main events: Separation of homologous pairs Homologous Pairs: Sets of same chromosome from different parents

8.  1. Prophase I DNA coils into chromosomes Nuclear envelope & nucleolus break down Synapsis = homologous chromosomes pair up into tetrads (What does the prefix “tetra” mean? Think tetrahedron) Crossing-over = homologous pairs switch pieces of their chromatids Genetic Recombination = maternal & paternal genetic info get shuffled during crossing-over

10.  2. Metaphase I Tetrads line up in the middle of the cell Spindle fibers attach at the centromeres of each homologous chromosome (one member of the pair)

12.  3. Anaphase I Each homologous chromosome moves to the opposite end of the cell (one member of the pair) Independent Assortment = random separation of chromosomes, allows for genetic variation

14.  4. Telophase I & Cytokinesis I Two newly formed cells are haploid Each chromosome is still made up of sister chromatids (two copies of chromosome)

16.  MEIOSIS II Separation of sister chromatids Sister Chromatids = identical halves of a chromosome that has been duplicated

17.  5. Prophase II 2 newly created cells from Meiosis I start to divide

18.  6. Metaphase II Chromosomes line up in the middle

19.  7. Anaphase II Chromatids separate

20.  8. Telophase II & Cytokinesis II 4 new cells, each with half the number of chromosomes of the original cell

21. Note Quiz 2/22 Put your name and period on the small sheet of paper, answer the following questions: 1. Are gametes produced through mitosis or meiosis? 2. What is separated during meiosis II? 3. What is separated during meiosis I? 4. What is one way that genetic diversity of daughter cells is increased during meiosis?

22. Agenda for today: Finish notes (3 or less slides) Using available resources, complete worksheet. If you are seen not working, points will be deducted. –Due tomorrow at the beginning of class. Will be looking at CONTENT, not COMPLETION

23. Development of Gametes Spermatogenesis = production of sperm cells Oogenesis = production of mature egg cells

24. Nondisjunction Nondisjunction = failure of homologous pairs to separate in Meiosis I If an organism survives, it may have a genetic disorder

25. Polyploidy Polyploid = organisms with more than the usual number of chromosome sets Occurs frequently in plants, results in plants that are larger than normal

26. http://cellsalive.com/meiosis.htm

27. Mitosis vs. Meiosis Mitosis Body cells 2 cells produced Diploid (2 of each chromosome) Genetically identical to each other AND to the original cell Meiosis Sex cells (sperm/egg) 4 cells produced Haploid (1 of each chromosome) Genetically different