1. MEIOSISMEIOSIS & SEXUAL REPRODUCTION MEIOSIS Genetics Lesson 4

2. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes called the “Diploid” number (the symbol is 2n) called the “Diploid” number (the symbol is 2n) e.g. skin cells, brain cells, etc. e.g. skin cells, brain cells, etc. 2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes called the “Haploid” number (the symbol is n) called the “Haploid” number (the symbol is n) E.g. sperm and egg cell E.g. sperm and egg cell Types of Cells

3. Gametes The Male Gamete is the Sperm and is produced in the male gonad the Testes. The Male Gamete is the Sperm and is produced in the male gonad the Testes. The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries.

4. The fusion of a sperm and egg to form a zygote. The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Fertilization

5. Meiosis: involves 2 parents (usually) involves 2 parents (usually) hereditary material is exchanged (mixed and transferred) hereditary material is exchanged (mixed and transferred) Each species has a specific number of chromosomes For example humans total number of chromosomes is 46 For example humans total number of chromosomes is 46 diploid number is 46 or 2n = 46 diploid number is 46 or 2n = 46 the number of chromosomes in the gametes is the haploid number or n = 23 the number of chromosomes in the gametes is the haploid number or n = 23 Other examples:crayfish 2n = 126fruit flies 2n = 4 Other examples:crayfish 2n = 126fruit flies 2n = 4

6. Meiosis Meiosis animation (1:49 min): Meiosis animation (1:49 min): http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1

7. Each resulting cell still has chromosomes from mom & dad

8. Interphase Interphase Meiosis is preceded by interphase chromosomes have not yet condensed http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

9. Interphase chromosomes have replicated, and the chromatin begins to condense 2n=46 http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

10. Meiosis Prophase I begins with two complete sets of duplicated chromosomes homologous chromosomes pair up or synapse to form tetrads homologous chromosomes then exchange DNA http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

11. Homologous chromosomes sister chromatids Tetrad Prophase I – Synapsis

12. During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) During Prophase I “Crossing Over” occurs.

13. non-sister chromatids chiasmata: site of crossing over variation Tetrad Crossing Over Crossing Over creates variation (diversity) in the offspring’s traits.

14. Synapsis in Prophase I of Meiosis

15. Meiosis Metaphase I tetrads line up across the middle of the cell tetrads line up across the middle of the cell Independent assortment Independent assortment centromeres of chromosomes attach to spindle fibres centromeres of chromosomes attach to spindle fibres http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

16. Meiosis Anaphase I homologous chromosomes separate from the tetrad homologous chromosomes separate from the tetrad move along spindle fibres towards opposite poles of the cell move along spindle fibres towards opposite poles of the cell Sister chromatids remain attached at centromeres Sister chromatids remain attached at centromeres http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

17. Meiosis Telophase I & Cytokinesis Nuclear membrane reforms around each cluster of chromosomes Nuclear membrane reforms around each cluster of chromosomes Chromosomes in each cell are present as duplicate Chromosomes in each cell are present as duplicate Each cell considered haploid Each cell considered haploid http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

18. Meiosis Prophase II no DNA replication prior to Prophase II no DNA replication prior to Prophase II nuclear membrane dissolves and the spindle fibres begin to form nuclear membrane dissolves and the spindle fibres begin to form http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

19. Meiosis Metaphase II as in Meiosis I, the chromosomes line up on the spindle fibers as in Meiosis I, the chromosomes line up on the spindle fibers sister chromatids remain attached at their centromeres sister chromatids remain attached at their centromeres http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

20. Meiosis Anaphase II centromeres split and sister chromatids separate and move to opposite poles of cell centromeres split and sister chromatids separate and move to opposite poles of cell chromatids are now called chromosomes chromatids are now called chromosomes http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

21. Telophase II & Cytokinesis with the formation of four cells, meiosis is over. with the formation of four cells, meiosis is over. each of these prospective germ cells carries half the number of chromosomes of somatic cells. each of these prospective germ cells carries half the number of chromosomes of somatic cells. http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

22. Comparing Mitosis and Meiosis

23. Comparing Chromosome Numbers in Mitosis and Meiosis

24. Gametogenesis: the production of gametes (sex cells) through the process of meiosis. GAMETOGENESIS

25. Meiosis in males Meiosis in males occurs in the testes occurs in the testes Starts with a diploid cell called a spermatogonium Starts with a diploid cell called a spermatogonium produces four non- identical haploid sperm cells produces four non- identical haploid sperm cells SPERMATOGENESIS

26. SPERMATOGENESIS Occurs all the time from puberty until death. Occurs all the time from puberty until death. It takes approximately 68-74 hours for a sperm to be created. It takes approximately 68-74 hours for a sperm to be created. Meiosis produces approximately 250 000 000 sperm every day in males! Meiosis produces approximately 250 000 000 sperm every day in males!

27. SPERMATOGENESIS

28. Meiosis in females Meiosis in females Occurs in the ovaries and oviducts Occurs in the ovaries and oviducts Starts with a diploid cell called an oogonium. Starts with a diploid cell called an oogonium. Egg production starts before a female is born, but pauses in Meiosis I before the cells (primary oocyte) divide. Egg production starts before a female is born, but pauses in Meiosis I before the cells (primary oocyte) divide. The meiotic process resumes at puberty with ovulation (and fertilization), for 1 cell every month. The meiotic process resumes at puberty with ovulation (and fertilization), for 1 cell every month. OOGENESIS

29. OOGENESIS After Telophase I and II only one of the cells receives the majority of the cytoplasm, resulting in one egg cell and three polar bodies. After Telophase I and II only one of the cells receives the majority of the cytoplasm, resulting in one egg cell and three polar bodies. The purpose of the unequal division is to allow the egg cell to have sufficient nutrients to support a zygote immediately after fertilization. The purpose of the unequal division is to allow the egg cell to have sufficient nutrients to support a zygote immediately after fertilization.