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Ch.11.4 - Meiosis

Ch.11.4 - Meiosis. Formation of Gametes (egg & sperm). Our Chromosomes. 46 Chromosomes ( 23 pairs ) #1-22 Pairs are Autosomes (body cell chromosomes): Determine all traits except gender #23 pair are Sex chromosomes referred to as X & Y male (XY) female (XX). X - Chromosome.

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Ch.11.4 - Meiosis

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  1. Ch.11.4 - Meiosis Formation of Gametes (egg & sperm)

  2. Our Chromosomes • 46 Chromosomes (23 pairs) • #1-22 Pairs are Autosomes (body cell chromosomes): • Determine all traits except gender • #23 pair are Sex chromosomes referred to asX & Y • male (XY) • female (XX) X - Chromosome Y-Chromosome

  3. Chromosome Numbers Vary in organisms

  4. A closer look at Chromosome Pairs

  5. Karyotype • Picture of chromosomes arranged by size • Pairs 1-22 = autosomes • Pair 23 = sex chromo • Detects abnormalities & • XX female or XY male

  6. Homologous Chromosomes (Buddy-Buddy) • Matching pair = homologous chromosomes • Homologous chromosomes: 2 chromosomes (one from mom and one from dad) that are alike in: • size, • location of centromere, • dark/light banding pattern of genesRemember: only non matching pair are sex chromosomes

  7. Homologous Chromosome Pairs Mom’s Blue eye gene Dad’s Brown eye gene *Allele – different versions of the same gene (traits)*

  8. If a cell has all 46 chromo (23 pairs), it’s called a diploid cell • Shorthand: 2N Homologous Pair 

  9. Somatic Cell Gamete • Body Cell • Skin cell, etc. • Mitosis • 2 sets of DNA • 46 total chromo • 1 set from each parent • DIPLOID (2N) • Sex Cell • Egg/sperm • Meiosis • 1 set of DNA • 23 total chromo • ½ set from each parent • HAPLOID (N)

  10. Important Vocab • Somatic Cell vs. Gamete • Body cell vs. Sex cell • Diploid (2N) vs. Haploid (1N) • 2 sets of DNA vs. 1 set of DNA • Homologous Chromosomes • Matching pairs of chromo in 2N cell • Alleles • Different version of the same trait • Fertilization • When sperm meets egg and combines DNA • Zygote • Cell in growth state following fertilization • Tetrad • Structure containing 4 chromatids

  11. Meiosis Info… Similar but different from Mitosis: • Sex cell division only • Involves 2 cell divisions • Results in 4 cells with half the normal genetic info • Produces gametes (egg/sperm) • Male Testes (spermatogenesis) • Female Ovaries (oogenesis)

  12. 1N Fertilization 1N 2N Why Do we Need Meiosis? • Basis of sexual reproduction • Accounts for individual genetic diversity • You are unique! You look a little bit like your mom and a little like your dad! • Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote (fertilized egg)

  13. Homologous Pair same genes, different alleles Here’s the key to your “uniqueness” Gene X SisterChromatids (same genes, same alleles) Homologous pairs separate in meiosis and therefore different alleles (versions of traits) separate. So many combos of traits are possible!

  14. from mom from dad child too much! meiosis reduces genetic content Meiosis Forms Haploid Gametes • Meiosis must reduce the chromosome # by half • Fertilization then restores the 2n number • 23 chromo from egg + 23 chromo from sperm = you 46! EGG SPERM YOU The right number! YOU EGG SPERM

  15. 46 MeiosisI Meiosis: 2-Part Cell Division Sister chromatids separate Homologous Pair separate MeiosisII Interphase Diploid (2x46 = 92) Diploid (46) Haploid (23) 1 replication of chromosomes is followed by 2 cell divisions (aka Interphase only happens once!)

  16. Meiosis: Reduction Division • 2 part cell division • Meiosis I • Interphase I • PMAT I • Meiosis II • PMAT II • End result: 4 genetically different haploid cells • 4 sperm or 1 egg Interphase Meiosis I Meiosis II

  17. Spindle fibers Nucleus Nuclear envelope Meiosis I: Reduction Division Chromo pair up Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid)

  18. Meiosis II: Reducing Chromo # Prophase II Metaphase II Telophase II Anaphase II 4 genetically different haploid cells

  19. Interphase I • SAME as MITOSIS • Chromosomes will double • G1, S, G2

  20. Prophase I • Homologs pair up and form tetrad (a pair of homologous chromosomes • Chromosomes condense. • Spindle forms. • Nuclear envelope disappears. • Crossing over occurs

  21. Tetrads Form in Prophase I Homologous chromosomes Pair up(each with sister chromatids)   Join to form a TETRAD Called Synapsis

  22. Crossing-Over occurs in Prophase I • Tetrad Forms • Definition: Pieces of chromosomes or genes are exchanged • Advantage of sexual reproduction = genetic variation!

  23. Crossing-Over It’s hard to predict what traits you’ll get from mom and dad because there is so many possible combinations! **THIS IS ONE SOURCE OF GENETIC VARIABILITY!**

  24. Genetic Variability is due to… • Crossing over • Independent Assortment • Random Segregation • Random Fertilization Meaning: You are unique for these 4 reasons!

  25. Independent Assortment • The way a pair of chromosomes lines up during metaphase is not dependent on other pairs. • Aka Mom’s chromosomes don’t necessarily all line up on one side • Random Segregation • Random chromatids separate into the newly forming eggs/sperm • Aka if you inherit mom’s hair color, you might get her brown hair trait OR her blonde hair trait

  26. Independent Assortment Random Segregation

  27. Metaphase I and Anaphase I Metaphase I Homologous pairs of chromosomes align along the equator Anaphase I -Homologs separate and move to opposite poles. -Sister chromatids remain attached at their centromeres.

  28. Telophase I and Cytokinesis Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two new diploid cells.

  29. Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome/gene. Meiosis II-occurs in 2 cells Gene X Only one homolog of each chromosome is present in the cell.

  30. Meiosis II: Reducing Chromo # Prophase II Metaphase II Telophase II Anaphase II 4 genetically different haploid cells

  31. Prophase II Nuclear envelope disappears Spindle fibers form No Crossing Over • Metaphase II • Chromosomes align along the equator

  32. Anaphase II Equator Pole Sister chromatids separate and move to opposite poles.

  33. Telophase II Nuclear envelope reforms. Chromosomes loosen into chromatin. Spindle breakdown. Cytokinesis breaks the cells into 2 new daughter cells

  34. Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

  35. Prophase II Prophase I Metaphase II Metaphase I Anaphase II Anaphase I Telophase II Telophase I Cytokinesis Cytokinesis

  36. Meiosis Animation

  37. Overview of Meiosis

  38. Oogenesis & Spermatogenesis THE PROCESS OF MAKING EGG AND SPERM

  39. Spermatogenesis • “Creation of sperm” • Testes • 2 divisions produce 4 viable haploid spermatids • Spermatids mature into sperm • Men produce about 250,000,000 sperm per day

  40. Spermatogenesis in the Testes Spermatid

  41. Spermatogenesis

  42. Oogenesis • “Creation of Eggs” • Ovary • 2 divisions produce 3 polar bodies that die + 1 viable egg • Polar bodies die because of unequal division of cytoplasm • Starting at puberty, if unfertilized, one immature oocyte matures into an ovum (egg) every 28 days  MENSTRUAL CYCLE

  43. Oogenesis in the Ovaries **Egg cells are special…only one egg is made every time meiosis occurs; the other 3 cells (polar bodies) that are made are much smaller & are discarded (Remember: the egg cell is the largest cell you’ll come across)

  44. 1st polar body may divide (haploid) a X a Polar bodies die a X X a X X Mitosis Meiosis I Meiosis II A A Oogonium (diploid) X Primary oocyte (diploid) X A Mature Egg (ovum) Oocyte A X Secondary oocyte (haploid) polar body (dies) Oogenesis

  45. Meiosis is IMPORTANT… Genetic Variability 1. Independent assortment (late pro/early meta I&II) -chromosomes line up randomly 2. Law of Segregation (Late meta/ana I&II) -spindle fibers attach randomly to chromosomes and separate. 3. Crossing-over (Pro I) pieces of sister chromatids are switched 4. Random fertilization Random sperm + random egg are combined

  46. What are the possibilities? ***Total possible chromosome combinations due to independent assortment = 2n [for humans = 223 = 8,388,608] ***Total possible chromosomally different zygotes due to fertilization = (223)2 = 70,368,744,000,000 ***Possible genetically different zygotes per couple if crossing-over occurs only once = (423)2 = 4,951,760,200,000,000,000,000,000,000 --advantageous b/c variability needed for evolution

  47. Comparison of Divisions

  48. 46 Single Chromosomes (23 Pairs) 46 Single Chromosomes (23 Pairs) 92 Single Chromosomes (46 Pairs) 46 Single Chromosomes (23 Pairs) 46 Single Chromosomes (23 Pairs) 23 singles 92 Single Chromosomes (46 Pairs) 46 Single Chromosomes (23 Pairs) 23 singles 46 Single Chromosomes (23 Pairs) 23 singles 23 singles What’s the difference between Mitosis & Meiosis ?

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