1. MEIOSIS & Genetic Variation Goal: Divide up chromosomes & cytoplasm to sex cells so that when they unite the zygote has the correct # of chromosomes. Only in eukaryotic cells Only is gametes

2. 2 Cell Divisions (Meiosis I & II) 1 Duplication of chromosomes If Meiosis did not occur the chromosome # in each new generation would double…. The offspring would die.

3. Organisms that reproduce sexually are made up of two different types of cells. 1.Somatic Cells –“body” cells –“Normal” # of chromosomes (46 for humans) –Diploid # (the symbol is 2n). –Humans have 23 pairs of chromosomes for a total of 46 chromosomes. Diploid #: 46 2.Gametes –“sex” cells (sperm & eggs) –½ the normal number of chromosomes – Haploid # (the symbol is n) –Humans: n = 23 (23 pairs of chromosomes)

4. Making Gametes Making Gametes Meiosis in males = spermatogenesis = make sperm in testes Meiosis in females = oogenesis = make ova in ovaries Ovum+ Sperm  Zygote (fertilized egg) 2n=46 zygote n=23 egg Sperm n = 23

5. 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

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

7. Homologous Chromosomes 2n = 46 n = 23 … pairs of chromosomes 1 set from mom; 1 set from dad Homologous Chromosomes = Matching pairs of chromosomes cusposition of a gene Locus: position of a gene eye color locus eye color locus hair color locus hair color locus PaternalMaternal

8. Humans have 23 Sets of Homologous Chromosomes Homologue Sex Chromosomes (Pair 23) XY (male) XX (female) Autosomes Chromosomes # 1 – 22; code for most traits 23

9. Homologous Chromosomes

10. Homologous Chromosomes Tetrad = homologous pairs when they line up and touch consists of 4 chromatids PaternalMaternal eye color locus eye color locus hair color locus hair color locus

11. Stages of Meiosis 2 cell divisionswith only 1 duplication of chromosomes. 2 cell divisions with only 1 duplication of chromosomes.

12. Interphase I mitosisSimilar to mitosis interphase. Chromosomes (S phase).Chromosomes replicate (S phase). chromosome chromatids centromeresEach duplicated chromosome consist of two identical sister chromatids attached at their centromeres. CentrioleCentriole pairs also replicate. nuclear membrane nucleus nucleolus

13. Meiosis I Cell division chromosome one-half (2n  n)Cell division that reduces the chromosome number by one-half (2n  n) Stages:Stages: a.prophase I b.metaphase I c.anaphase I d.telophase I

14. Prophase I Longest; most complex phase.Longest; most complex phase. 90% of meiosis90% of meiosis Chromosomes condense. Synapsis tetradSynapsis occurs: homologous chromosomes come together to form a tetrad. TetradTetrad is two duplicated chromosomes (4 chromatids) CROSSING OVER occurs! Also: Nuclear membrane disappears, spindle fibers form, etc.

15. Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

16. Prophase I : “Crossing Over” chromatids chromatidSegments of nonsister chromatids break and reattach to the adjacent chromatid. RESULTS IN GENETIC RECOMBINATION! (New mixture of genetic material; Creates diversity in gene pool)

17. Crossing Over nonsister chromatids chiasmata: site of crossing over variation Tetrad Humans: 1 – 3 crossover events per chromosome pair in meiosis

18. Prophase I centrioles spindle fiber aster fibers

19. Metaphase I Shortest phaseShortest phase TetradsTetrads align in middle of cell. INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Results in Variation Formula for genetic combinations: 2 n Example: 2n = 4 then n = 2; thus 2 2 = 4 combinations

20. Metaphase I metaphase plate OR metaphase plate

21. Anaphase I Homologous chromosomes separate and move towards the poles. SEGREGATION of chromosomes. Sister chromatids remain attached at their centromeres. Sister chromatids attached Homologous chromosomes separate

22. Telophase I haploid chromosomesEach pole now has haploid set of chromosomes. CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

23. Meiosis II: Sister chromatids separateSister chromatids separate No interphase II DNA replicationNo interphase II (or very short - no more DNA replication) mitosissimilar to mitosis

24. Meiosis Meiosis I: Homologous chromosomes divide (2n  n) Meiosis II: Sister chromatids separate

25. Mitosis vs. Meiosis

26. MitosisMeiosis 1 parent produces 2 DIPLOID daughter cells (2n  2n) 1 parent produces 4 HAPLOID daughter cells (2n  n) Somatic cellsGametes Parent & daughter are genetically identical Daughter cells are not genetically identical (They have different chromosomes) Prophase: Homologous chromosomes do not line up Prophase 1: Homologous chromosomes SYNAPSE (Tetrads); Cross-over occurs Metaphase: Chomosomes line up in middle of cell Metaphase 1: Homologous chromosomes line up together. Anaphase: Sister chromatids splitAnaphase 1: Homologous chromosomes split; sister chromatids still attached.

27. Sources of Genetic Variation in Sexual Reproduction. 1. Mutation = original source of variation (in asexual & sexual) 2 Random fertilization = offspring are a combination of paternal + maternal chromosomes. There different versions of genes (alleles) for each trait (BB, Bb, bb; B = brown fur; b = black fur) Different versions = diff. nucleotide sequences on chromosomes.

28. Sources of Genetic Variation in Sexual Reproduction. 3. Independent arrangement of homologous chromosomes at metaphase plate in Metaphase I of meiosis. (Do you get maternal or paternal chromosome?) –2n = # combinations n=23 chromosomes –Over 8 million combinations of chromosomes 4. Crossing over: Exchange of corresponding segments b/w 2 homologous chromosomes (Prophase I)

29. Sources of Genetic Variation: Crossing Over Produces new combinations of traits that did not exist in maternal or paternal genome. New combinations of genes = Genetic Recombination

30. Genetic Variation: Who Cares? Variations of traits w/in populations is what natural selection acts on in the process of evolution. What traits are favorable? Unfavorable?

31. Nondisjunction in Meiosis Failure of chromosomes to separate during Meiosis I or II Leads to abnormal # of chromosomes in offspring (usually fatal) Ex: Downs syndrome

32. Nondisjunction in Meiosis I vs. II Nondisjunction in meiosis I  all resulting gametes have incorrect chromosome # (fertilization will result in trisomy or monosomy in zygote) Nondisjunction in meiosis II  Half of resulting gametes have incorrect chromosome #

33. Monosomies Having 45 Chromosomes Lethal if autosomal Turner Syndrome (XO) - infertile females, short stature, “webbed” neck, possible cognitive impairments http://en.wikipedia.org/wiki/Monosomy http://web.udl.es/usuaris/e4650869/d ocencia/segoncicle/genclin98/recurso s_classe_(pdf)/revisionsPDF/chromo syndromes.pdf

34. Trisomies: 47 chromosomes Autosomes Down’s Syndrome (#21) – most common Edwards (#18) Patau (#13) Sex Chromsomes (Y determines maleness) XXY Klinefelter males (infertile, tall, long limbs, some secondary female characteristics, possible cognitive issues) XYY normal male XXX normal female http://en.wikipedia.org/wiki/Monosomy http://web.udl.es/usuaris/e4650869/d ocencia/segoncicle/genclin98/recurso s_classe_(pdf)/revisionsPDF/chromo syndromes.pdf

35. Nondisjunction in Meiosis

36. Chromosomal mutations : change in an entire chromosome; may involve loss or duplication of multiple genes Types of Chromosomal Mutations 1.Deletion 2.Duplication 3.Inversion 4.Translocation (balanced & unbalanced)

37. Types of Chromosomal Mutations

38. Partial deletions, duplications, translocations http://en.wikipedia.org/wiki/Monosomy

39. Nondisjunction in Meiosis

40. Links http://www.youtube.com/watch?v=Ba9LXKH2ztU&feature=related http://www.youtube.com/watch?v=sJCWVTnFf5o&feature=related http://www.youtube.com/watch?v=QwmpD0OB3AQ&NR=1&feature=endscreen http://www.youtube.com/watch?v=kVMb4Js99tA&feature=related