1. MEIOSIS and Sexual Reproduction

2. Vocabulary MeoisisDiploid Gametefertilization Somatic cellCrossing Over Autosomal chromosomeIndependent Sex chromosome assortment Homologous chromosomes Sperm Meiosis I EggMeiosis II Haploidtetrad

3. Mitosis Vs. Meiosis What can you tell me about both?

4. Organisms that reproduce Sexually are made up of two different types of cells. 1.Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … 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)….. Sperm cells and ova are gametes.

5. Meiosis Forms Gametes!!!!! 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.

6. 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.

7. Sperm are small because they contain no organelles. Must be able to move. One egg produced that holds all of the organelles. fertilization

8. Spermatogenesis – Formation of sperm 2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II

9. Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte. One egg produced. Large holds all organelles

10. Egg Cell

11. Sexual Reproduction: Fusion of two gametes that results in the combo of the genetic material from the two parents.

12. Fertilization Sperm + Ovum (egg) ----  Zygote spermeggzygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

14. Sex Chromosomes OR Sex cells? There are sex cells and then there are sex chromosomes!! Don’t get them confused! Sex cells – sperm and egg Sex chromosomes XX and XY

15. Autosomal Chromosomes 1 – 22 ( In Humans the “Autosome s” are sets 1 - 22

16. Homologous Chromosomes ( because a homologous pair consists of 4 chromatids it is called a “Tetrad”) PaternalMaternal eye color locus eye color locus hair color locus hair color locus

17. Sex Chromosomes “Sex Chromosomes” …….the 23 rd set 23 This person has 2 “X” chromosomes… and is a female.

18. Types of Chromosomes chromosomes maternalpaternalHomologous Chromosomes: Pair of chromosomes (maternal and paternal) that are similar in shape and size. They carry the genes from parents to offspring. locus(position of a gene)Each locus (position of a gene) is in the same position on homologues. homologous chromosomes.Humans have 23 pairs of homologous chromosomes. autosomes – not related to the sex of the organism. 1 - 22 pairs of autosomes – not related to the sex of the organism. sex chromosomes – control the development of sexual characteristics. 1 pair of sex chromosomes – control the development of sexual characteristics.

19. Types of chromosomes Autosomal chromosomes– chromosome 1 -22 set Sex chromosome – set 23 Egg – sex chromosome X Sperm – sex chromosome X or Y Male – XYFemale - XX

20. Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

21. All Organisms have a characteristic number of chromosomes! The number has nothing to do with complexity! Fruit fly - 8 Horse - 64 Fern - 1200

22. Chromosome numbers All are even numbers – diploid (2n) sets of homologous chromosomes! Ploidy = number of copies of each chromosome. Diploidy

23. Meiosis Cell division – ‘ reduction division’ Production of sex cells – gametes Meiosis errors

29. Mitosis vs. Meiosis Most cells in the body produced by mitosis. Only gametes are produced by meiosis.

30. Why do we need meiosis? Meiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? At fertilisation the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

31. Meiosis Meiosis is Two cell divisions meiosis I meiosis II (called meiosis I and meiosis II) with only one duplication of chromosomes. Every gamete is different because of the mixing of genes during metaphase I. Crossing over of genes! Germ Cells - type of cells that for gametes. Females form all eggs before birth Males form sperm whole life

32. MitosisMeiosis Produce identical cells Unique cells Diploid cellsHaploid cells Takes place whole life of organism Takes place at only certain times Asexual reproduction Sexual reproduction Body cellsGerm cells

33. Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: –homologous chromosomes pair up form a tetrad. –crossing over of non-sister chromatids.In Metaphase I, homologous pairs line up at metaphase plate. This mixes up genes –In Anaphase I, sister chromatids do NOT separate. –End Product, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.

34. Homologous Chromosomes chromosomesmaternalpaternalPair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads)Homologous pairs (tetrads) carry genes controlling the same inherited traits. locus(position of a gene)Each locus (position of a gene) is in the same position on homologues. homologous chromosomes.Humans have 23 pairs of homologous chromosomes. autosomes a.22 pairs of autosomes sex chromosomes b.01 pair of sex chromosomes

35. How Does Sexual Reproduction creates genetic variation??? 1. Independent Assortment :Meiosis results in genetic variation by shuffling the genes that are placed in the gamete. No daughter cells formed during meiosis are genetically identical to either mother or father 2. Fertilization: During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. 3. Crossing over - During the prophase 1 of meiosis, chromosome segments will switch with homologous chromosomes.

36. Crossing Over - variation nonsister chromatids chiasmata: site of crossing over variation Tetrad

37. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

38. Interphase I - Like mitosis 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.

39. Interphase I NucleusnucleolusNucleus and nucleolus visible. nuclear membrane nucleolus cell membrane chromatin

40. Meiosis I (four phases) Cell division chromosome one-half.Cell division that reduces the chromosome number by one-half. four phasesfour phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

41. Prophase 1: DNA has been replicated. Spindle fibers and centrioles form – Nuclear membrane and nucleolus disappears. Chromosomes are tetrads!

42. Prophase I Longest and most complex phase (90%).Longest and most complex phase (90%). ChromosomesChromosomes condense. Synapsis homologous chromosomes tetradSynapsis occurs: homologous chromosomes come together to form a tetrad. Tetradchromosomes chromatidsTetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

43. Prophase I centrioles spindle fiber aster fibers

44. Metaphase 1: Homologous chromosomes align at the equatorial plate. Spindle fibers attach. Crossing over occurs with the homologous chromosomes

45. Crossing Over Crossing over (variation) chromatidschiasmataCrossing over (variation) may occur between nonsister chromatids at the chiasmata. Crossing over chromatids chromatidCrossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) crossing overChiasmata (chiasma) are the sites of crossing over.

46. Metaphase I metaphase plate OR metaphase plate

47. Metaphase I Shortest phaseShortest phase Tetradsmetaphase plateTetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. (mom and dad’s chromosomes randomly align on each side.) 2. Variation 3. Crossing over occurs

48. Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

49. Anaphase I Homologous chromosomesHomologous chromosomes separate and move towards the poles. Sister chromatids centromeresSister chromatids remain attached at their centromeres.

50. Anaphase I

51. . Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

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

53. Telophase I

54. Meiosis II No interphase IINo interphase II DNA replication (or very short - no more DNA replication) Remember: Meiosis II mitosisRemember: Meiosis II is similar to mitosis

55. Meiosis 2 Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Same as 1 Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

56. Prophase II prophasemitosissame as prophase in mitosis

57. Metaphase II metaphasemitosis. NO CROSSING OVER or HOMOLOGOUS CHROMOSOMES!same as metaphase in mitosis. NO CROSSING OVER or HOMOLOGOUS CHROMOSOMES! metaphase plate

58. Anaphase II anaphasemitosissame as anaphase in mitosis sister chromatids separatesister chromatids separate

59. Telophase II telophasemitosisSame as telophase in mitosis. Nuclei form. CytokinesisCytokinesis occurs. Remember:four haploid daughter cells produced.Remember:four haploid daughter cells produced. gametes = sperm or egg

60. Telophase II

61. Independent Assortment Process occurring in Metaphase I where it is random which one of the pair is placed in the germ cell.

63. MitosisMeiosis Produce identical cells Unique cells Diploid cellsHaploid cells Takes place whole life of organism Takes place at only certain times Asexual reproduction Sexual reproduction Body cellsGerm cells

64. Variation Important to population as the raw material for natural selection.Important to population as the raw material for natural selection. Question:Question: What are the three sexual sources of genetic variation?