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Meiosis

Meiosis . Biology 11 Chapter 12. Part I. Meiosis In Males & Females. Meiosis Overview. Meiosis is the process of going from a diploid cell to a haploid cell N = haploid number of any genome For humans n = 23 Your diploid chromosome count is 2n = 46.

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Meiosis

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  1. Meiosis Biology 11 Chapter 12

  2. Part I Meiosis In Males & Females

  3. Meiosis Overview • Meiosis is the process of going from a diploid cell to a haploid cell • N = haploid number of any genome • For humans n = 23 • Your diploid chromosome count is • 2n = 46

  4. Homologous pair of chromosomes in diploid cell One diploid cell with 2 chromosomes Duplication Sister Chromatids Separation of Homologous Chromosomes Meiosis I Separation of Sister Chromatids Meiosis II

  5. Meiosis In Males • meiosis occurs in the testis • Mature sperm begin as spermatogonium diploid cells • When a male reaches puberty hormones in the brain signal the testis to make testosterone • Testosterone then prompts • Maturation of sperm • Growth of muscles • Increased bone density • Deepening of voice • Increased hair growth

  6. How Hormone Signals Work

  7. Testosterone Levels Over Time • In the uterus there is production of testosterone • Female development is the default setting • When testosterone is present this causes the gonads to drop outside and form the testis

  8. Meiosis In Females • Meiosis begins in the ovaries • Mature oocytes (eggs) develop from diploid oogonium cells • When a female reaches puberty hormones in the brain signal the ovaries to develop mature eggs • As a follicle develops two major hormones are released • Estrogen • Progesterone

  9. Female Sex Hormones • Progesterone • Readies the uterus for implantation • Estrogen prompts • Development of breasts • Appearance of pubic hair • Increase in fat beneath the skin • Widening and lightening of the pelvis

  10. Follicle maturing every month Middle Stage Female Ovary Follicle maturing every month Beginning Stage This whole structure is a follicle Egg Immature Eggs Corpus luteum After follicle releases egg Releases estrogen & progesterone

  11. Histology of An Ovary • This is a tissue slide from an ovary • Can you find the • Corpus luteum • Oogonia • Maturing follicles • Eggs arrested in metaphase II • Where is estrogen released from

  12. Part II Mechanisms of Meiosis

  13. Color Scheme for Chromosomes • We will begin with a hypothetical cell which contains 6 chromosomes • Homologous chromosomes are the same size, but not directly attached to one another • One homolog will be solid color the other homolog will be the same color with a pattern • Sister chromatids are of course attached and the same color

  14. Hypothetical Spermatogonium or Oogonium Cell How many chromosome does this cell have? What is n for this cell? When this cell goes through Meiosis I and II how many chromosomes will it have?

  15. Step 1 Duplication of the chromosomes is S Phase

  16. Meiosis I Separation of Homologs

  17. Homologs Separate in Ananphase I

  18. Anaphase I • After a cell completes anaphase I the cell is haploid • Now you only have one copy of every gene, it is a duplicated copy but still just one copy B b B b Hypothetically – the black dash represents a gene for eye color. Since the separation of the homologs your cell now has only one gene for eye color and not two as found in the diploid cell

  19. Anaphase I • Also during anaphase I in males the Y chromosome pairs with the X chromosome • After these separate the cell is haploid and has either one duplicated copy of X or one duplicated copy of Y

  20. Sister Chromatids Separate in Anaphase II

  21. Meiosis II Separation of Sisters

  22. Meiosis Overview

  23. Meiosis Overview

  24. Meiosis Overview

  25. Variation In Traits • Two mechanism by which variation in genetic traits is produced during meiosis • Crossing over • Random metaphase alignments

  26. Crossing Over • Crossing over is an exchange of genetic material between two homologous chromosomes • It can only take place in prophase I of meiosis I • Produces variation in traits by mixing the maternal and paternal chromosomes

  27. Crossing Over • Homologous chromosomes are 90 to 95% identical • Sister chromatids are 100% identical • Crossing over allows and exchange of information between non sisters

  28. Crossing Over • The black or white squares is a gene for eye color When these homologs separate during _____________ this chromosome will end up in a gamete. Now it is a mixture of maternal and paternal DNA Blue = Paternal Purple = Maternal

  29. Random Alignments • There is no particular pattern to the metaphase I positions of maternal and paternal chromosomes • Possible Combinations • The rule is 2n possible combinations of chromosomes • For our hypothetical cell with 6 chromosomes how many combinations of chromosomes are possible

  30. Mistakes In Meiosis • Nondisjunction during Meisosis I • When one set of homologs does not separate during anaphase I • Certain gametes will have too many or too few chromosomes • Aneuploid cells • 2n + 1 = trisomy • 2n – 1 = monosomy

  31. Mistakes In Meiosis • Nondisjunction during Meisosis II • Sister chromatids do not separate during anaphase II • Relatively rare event leads to the production of two normal gametes and two abnormal gametes

  32. Mistakes In Meiosis • Mistakes in meiosis are a major cause of spontaneous abortion in humans • These seem to be random errors • The sex chromosomes and chromosome 21 are more prevalent in the population because they are not embryonic lethal • Down syndrome • Turners syndrome XO • Klinefelter syndrome XXY • Triple X syndrome XXX

  33. Meiosis In Males Many Complex and physiological and morphological changes happen during sperm development • Sperm formation occurs in specialized cells in the testis • It takes approximately 30 days for a mature sperm cell to form • Sperm production begins in puberty and continues until death

  34. Meiosis In Males • One spermatogonium should result in the production of 4 functional haploid cells

  35. Meiosis in Females • Females produce eggs from puberty until menopause • Female oogonia undergo meiosis but do not complete meiosis II unless the egg becomes fertilized • Arrested in metaphase II

  36. Meiosis In Females • In Females the oogonium undergo two cellular division to yield four haploid cells • However, the division of the cytoplasm is not equal • Why?

  37. Meiosis In Females • Polar bodies are not functional • They only contain genetic material and are reabsorbed by the body

  38. Unequal Division of Cytoplasm Proteins – instruct neural tube development • The female egg must have • Proteins ready to direct development • Mitochondria for energy • Many rRNA molecules ready to make proteins • Biologically sperm is motile and small • Its purpose is to carry half to the genetic information to the egg Nucleus Mitochondria Ribosomes Proteins – instruct formation of heart Proteins – instruct the formation of gut

  39. Comparing Meiosis to Mitosis • Mitosis • PMAT • 1 2n cell to 2 2n cells • Meiosis • 2 divisions • PMAT I & II • 1 2n cell to 4 n cells • Not all haploid cells functional

  40. Why Sex • All things considered equal asexual reproduction confers a large numerical advantage Asexual reproduction is more efficient because everyone is capable of reproduction

  41. Purifying Selection Hypothesis • What if a critical gene is damaged in an asexually producing organism • This gene would be transferred to offspring and may hinder “fitness” • In sexual reproduction an individual will probably have only one damaged allele • If they mate with an unaffected individual then the probability of inheriting the damaged allele decreases

  42. Purifying Selection • Natural selection against deleterious alleles is called purifying selection • Sexual reproduction reduces the impact of purifying selection

  43. Changing Environment Hypothesis • If environmental conditions change parents may be poorly adapted • Temperature • Moisture • Predation/parasites • Competitors/food resource • But the genetically different offspring may have acquired alleles in different combinations that give them an advantage

  44. Changing Environment Hypothesis • Study using a snail species that reproduces sexually and asexually • This snail is susceptible to parasitic worm infections • The variation in offspring via sexual reproduction should have the chance of a higher “fitness” in the parasitic worm environment • Sexual reproduction should increase as parasitism rates increase

  45. Changing Environment Hypothesis

  46. Sexual Reproduction • Is an adaptation that reduces the impact of purifying selection • Is an adaptation that increases the fitness of individuals in an environment where disease causing organisms are common

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