2. Regents Biology 2006-2007 Meiosis & Sexual Reproduction

3. Regents Biology Cell division / Asexual reproduction  Mitosis  produce cells with same information  identical daughter cells  exact copies  clones  same number of chromosomes  same genetic information Aaaargh! I ’ m seeing double!

4. Regents Biology Asexual reproduction  Single-celled eukaryotes  yeast  Paramecium  Amoeba  Simple multicellular eukaryotes  Hydra  budding What are the disadvantages of asexual reproduction? What are the advantages?

5. Regents Biology How about the rest of us?  What if a complex multicellular organism (like us) wants to reproduce?  joining of egg + sperm  Do we make egg & sperm by mitosis? 46 + 92 eggspermzygote What if we did, then…. Doesn’t work! No! 

6. Regents Biology Human female karyotype 46 chromosomes 23 pairs XX diploid = 2 copies 2n

7. Regents Biology 46 chromosomes 23 pairs XY Human male karyotype diploid = 2 copies 2n

8. Regents Biology gametes How do we make sperm & eggs?  Must reduce 46 chromosomes  23  must half the number of chromosomes  haploid 23 46 egg sperm 46 meiosis 46 fertilization 23 zygote

9. Regents Biology Meiosis makes sperm & eggs  46 chromosomes to 23 chromosomes  half the number of chromosomes 23 46 egg sperm 46 meiosis haploiddiploid

10. Regents Biology Paired chromosomes  Homologous chromosomes  both chromosomes of a pair carry “matching” genes  control same inherited characters  homologous = same information diploid 2n 2n = 4 homologous chromosomes double stranded homologous chromosomes eye color (brown?) eye color (blue?)

11. Regents Biology Meiosis 1 overview  1st division of meiosis  4 chromosomes  diploid  2n  2 chromosomes  haploid  1n double stranded Copy DNALine Up 1 Divide 1 gamete prophase 1metaphase 1 telophase 1

12. Regents Biology  2nd division of meiosis  looks like mitosis Meiosis 2 overview  2 chromosomes  haploid  1n 4 gametes Line Up 2 Bye Bye 2 telophase 1 metaphase 2 telophase 2

13. Regents Biology Meiosis = reduction division  Meiosis  special cell division in sexually reproducing organisms  reduce number of chromosomes  2n  1n  diploid  haploid  half  makes gametes  sperm, eggs

14. Regents Biology Meiosis & mitosis  Meiosis to make gametes  sperm & egg  Mitosis to make copies of cells  growth  repair  development

15. Regents Biology Sexual reproduction lifecycle  1 copy  haploid  1n  2 copies  diploid  2n  1 copy  haploid  1n meiosisfertilization We ’ re mixing things up here! A good thing?

16. Regents Biology mitosis zygote Putting it all together… 23 46 egg sperm 46 meiosis 46 23 fertilization mitosis & development meiosis  fertilization  mitosis + development 46 gametes

17. Regents Biology The value of meiosis 1  Consistency over time  meiosis keeps chromosome number same from generation to generation from Mom from Dad offspring DadMom

18. Regents Biology The value of meiosis 2  Change over time  meiosis introduces genetic variation  gametes of offspring do not have same genes as gametes from parents  new combinations of traits from Mom from Dad new gametes made by offspring offspring We ’ re mixing things up here! variation

19. Regents Biology How does this explain: family resemblance & differences! Why are the kids so similar to the parents but not exact? Martin & Charlie Sheen, Emilio Estevez Michael & Kirk Douglas Baldwin brothers

20. Regents Biology 2006-2007 Any Questions?? What are the DISadvantages of asexual reproduction? What are the DISadvantages of sexual reproduction? What are the advantages of sexual reproduction? What are the advantages of asexual reproduction?

21. Regents Biology Meiosis in males is called spermatogenesis and produces sperm. Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova. Meiosis in females is called oogenesis and produces ova.

22. Regents Biology Spermatogenesis 2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II 4 sperm cells are produced from each primary spermatocyte. Primary Spermatocyte Secondary Spermatocyte

23. Regents Biology Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

24. Regents Biology Interphase I mitosis  Similar to mitosis interphase.  Chromosomes(S phase).  Chromosomes replicate (S phase). chromosome chromatids centromeres  Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.  Centriole  Centriole pairs also replicate.

25. Regents Biology Prophase I  Longest and most complex phase.  90% of the meiotic process is spent in Prophase I  Chromosomes  Chromosomes condense.  Synapsis homologous chromosomes tetrad  Synapsis occurs: homologous chromosomes come together to form a tetrad.  Tetradchromosomeschromatids  Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

26. Regents Biology Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

27. Regents Biology During Prophase I “Crossing Over” occurs.  During Crossing over chromatids chromatidChiasmata (chiasma) crossing over  During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. 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)

28. Regents Biology Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids chiasmata: site of crossing over variation Tetrad

29. Regents Biology Question: 20 chromosomes (diploid) chromosomes  A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

30. Regents Biology Answer:  10 chromosomes (haploid)

31. Regents Biology Question: 40 chromatids chromosomes  A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

32. Regents Biology Answer:  10 chromosomes

33. Regents Biology Non-disjunction  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. Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over)

34. Regents Biology 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 18 th set, Trisomy 21…. Three chromosomes in the 21 st set. If an organism has Monosomy 23 it has only one chromosome in the 23 rd set.

35. Regents Biology 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