MEIOSIS and Sexual Reproduction

1. MEIOSIS and Sexual Reproduction

2. Vocabulary Meoisis Diploid Gamete fertilization Somatic cell Crossing Over Autosomal chromosome Independent Sex chromosome assortment Homologous chromosomes Sperm Meiosis I Egg Meiosis II Haploid tetrad

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

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

10. Egg Cell

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

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

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 “Autosomes” are sets 1 - 22

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

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

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

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

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 (called meiosis Iand 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

33. Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cellsdiffer 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 • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus(position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes 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. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Over - variation variation

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 • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate.

39. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

40. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four 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%). • Chromosomes condense. • Synapsis occurs:homologous chromosomes come together to form a tetrad. • Tetrad is two chromosomes or four chromatids(sister and nonsister chromatids).

43. spindle fiber centrioles aster fibers Prophase I

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) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over.

46. OR metaphase plate metaphase plate Metaphase I

47. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • 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 chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres.

50. Anaphase I