MEIOSIS Critical Questions: What phase of meiosis creates genetic variety?

1. MEIOSISCritical Questions: What phase of meiosis creates genetic variety?

2. 2n n n • Mitosis occurs in Somatic Cells, “body” cells • Creates 2 Diploid cells- contain the normal number of chromosomes (2n). • Examples - skin cells, brain cells, etc. • Meiosis occurs in Gametes, “sex” cells • Creates 4 Haploid cells– contain ½ the normal number of chromosomes (n) • Examples Sperm cells and eggs

4. sperm n=23 2n=46 n=23 Body cells egg Meiosisis the process by which ”gametes” (sperm and egg cells) , withhalfthe number ofchromosomes,are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n)

5. MEIOSIShttps://www.youtube.com/watch?v=Ba9LXKH2ztU

6. Homologous Chromosomes • Homologous chromosomes are similar is shape and size and carry genes controlling the same inherited traits (1 from Mom, 1 from Dad). 22 pairs of autosomes – code for traits 1 pair of sex chromosomes – code for the sex of the off pring

7. Meiosis is Two cell divisions Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I Crossing-Over occurs in Prophase I which creates genetic variation(this doesn’t occur in mitosis) Meiosis IIProphase II, Metaphase II, Anaphase II, Telophase II

9. nonsister chromatids Crossing Oversharing genetic material between identical chromatids. Which creates variation (diversity) in the offspring’s traits. variation

10. Meiosis Drawings

11. STOP

12. spindle fiber centrioles aster fibers Prophase I

13. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

14. OR metaphase plate metaphase plate Metaphase I

15. Anaphase I • Homologous chromosomesseparate and move towards the poles. • Sister chromatids remain attached at their centromeres.

16. Anaphase I

17. Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed.

18. Telophase I

19. Meiosis II • No interphase II (or very short - no more DNA replication) • Remember:Meiosis IIis similar to mitosis

20. Prophase II • same as prophase in mitosis

21. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

22. Anaphase II • same as anaphase in mitosis • sister chromatids separate

23. Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

24. Telophase II

25. Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over) • Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. • Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring.

26. Non-disjunctions usually occur in one of two fashions. • The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set.

27. Common Non-disjunction Disorders • Down’s Syndrome – Trisomy 21 • Turner’s Syndrome – Monosomy 23 (X) • Kleinfelter’s Syndrome – Trisomy 23 (XXY) • Edward’s Syndrome – Trisomy 18

28. Amniocentesis • An Amniocentesis is a prrocedure a pregnant woman can have in order to detect some genetics disorders…..such as non-disjunction.

29. Amniocentesis Amniotic fluid withdrawn

30. Karyotype(picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome