55 ch12meiosis2008

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  • Sexual reproduction is advantageous to species that benefit from genetic variability. However, since evolution occurs because of changes in an individual's DNA, crossing over and chromosome segregation is likely to result in progeny that are less well-adapted than their parents. On the other hand, asexual reproduction ensures the production of progeny as fit as the parent since they are identical to the parent. Remember the adage, “if it's not broken, don't fix it.” There are several hypotheses regarding the evolution of sexual reproduction. One is associated with repairing double-stranded DNA breaks induced by radiation or chemicals. The contagion hypothesis suggests that sex arose from infection by mobile genetic elements. The Red Queen hypothesis theorizes that sex is needed to store certain recessive alleles in case they are needed in the future. Along similar lines, eukaryotic cells build up large numbers of harmful mutations. Sex, as explained by Miller's rachet hypothesis, may simply be a way to reduce these mutations. The “whole truth” is likely a combination of these factors. Regardless of how and why, the great diversity of vertebrates and higher plants and their ability to adapt to the highly varied habitats is indeed a result of their sexual reproduction.
  • 55 ch12meiosis2008

    1. 1. Meiosis & Sexual Reproduction AP Biology 2007-2008
    2. 2. Cell division / Asexual reproduction  Mitosis  produce cells with same information  identical daughter cells  exact copies  clones  same amount of DNA  same number of chromosomes  same genetic information AP Biology Aaaargh! I’m seeing double!
    3. 3. Asexual reproduction  Single-celled eukaryotes yeast (fungi)  Protists   Paramecium  Amoeba  Simple multicellular eukaryotes  Hydra What are the disadvantages of asexual reproduction? AP Biology What are the advantages? budding budding
    4. 4. How about the rest of us?  What if a complex multicellular organism (like us) wants to reproduce?  joining of egg + sperm  Do we make egg & sperm by mitosis? No! What if we did, then…. 46 egg AP Biology + 46 92 sperm zygote Doesn’t work!
    5. 5. Human female karyotype 46 chromosomes 23 pairs AP Biology
    6. 6. Human male karyotype 46 chromosomes 23 pairs AP Biology
    7. 7. Homologous chromosomes  Paired chromosomes  both chromosomes of a pair carry “matching” genes  control same inherited characters  homologous = same information diploid 2n 2n = 4 AP Biology single stranded homologous chromosomes double stranded homologous chromosomes
    8. 8. How do we make sperm & eggs?  Must reduce 46 chromosomes → 23  must reduce the number of chromosomes by half 23 46 meiosis 46 AP Biology zygote 23 egg 46 23 23 sperm gametes fertilization
    9. 9. Meiosis: production of gametes  Alternating stages  chromosome number must be reduced haploid  diploid → haploid  2n → n  humans: 46 → 23  meiosis reduces chromosome number  makes gametes  fertilization restores chromosome number  haploid → diploid  n → 2n AP Biology diploid
    10. 10. Sexual reproduction lifecycle  2 copies  diploid  2n  1 copy  haploid  1n gametes AP Biology fertilization meiosis In the next generation… We’re mixing things up here! A good thing?  1 copy  haploid  1n gametes
    11. 11. Meiosis  Reduction Division    special cell division for sexual reproduction reduce 2n → 1n diploid → haploid  “two” → “half”  makes gametes  sperm, eggs Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are AP radically different. Do not confuse the two! Biology
    12. 12. Overview of meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1 metaphase 1 anaphase 1 n=2 n=2 prophase 2 metaphase 2 anaphase 2 telophase 2 n=2 telophase AP Biology 1
    13. 13. Double division of meiosis DNA replication Repeat I can’t after you! hear me AP Biology 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2
    14. 14. Preparing for meiosis  1st step of meiosis Duplication of DNA  Why bother?   meiosis evolved after mitosis  convenient to use 2n = 6 single stranded “machinery” of mitosis  DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) AP Biology M1 prophase 2n = 6 double stranded
    15. 15. Meiosis 1  1st division of meiosis 2n = 4 single stranded separates homologous pairs prophase 1 2n = 4 double stranded metaphase 1 2n = 4 double stranded synapsis tetrad reduction telophase 1 I can’t Repeat AP Biology after you! hear me 1n = 2 double stranded
    16. 16. Meiosis 2  2nd division of meiosis 1n = 2 double stranded separates sister chromatids prophase 2 What does this division look like? 1n = 2 single stranded AP Biology 1n = 2 double stranded metaphase 2 4 telophase 2
    17. 17. Steps of meiosis  Meiosis 1 interphase  prophase 1  metaphase 1  anaphase 1  telophase 1   Meiosis 2 prophase 2  metaphase 2  anaphase 2  telophase 2 AP Biology  1st division of meiosis separates homologous pairs (2n → 1n) “reduction division” 2nd division of meiosis separates sister chromatids (1n → 1n) * just like mitosis *
    18. 18. Meiosis 1 & 2 AP Biology
    19. 19. Trading pieces of DNA  Crossing over  during Prophase 1, sister chromatids intertwine  homologous pairs swap pieces of chromosome  DNA breaks & re-attaches synapsis tetrad AP Biology prophase 1
    20. 20. Crossing over  3 steps What are the advantages of crossing over in sexual reproduction? cross over  breakage of DNA  re-fusing of DNA   New combinations of traits AP Biology
    21. 21. Mitosis vs. Meiosis AP Biology
    22. 22. Mitosis vs. Meiosis  Mitosis       AP Biology 1 division daughter cells genetically identical to parent cell produces 2 cells 2n → 2n produces cells for growth & repair no crossing over  Meiosis       2 divisions daughter cells genetically different from parent produces 4 cells 2n → 1n produces gametes crossing over
    23. 23. Putting it all together… meiosis → fertilization → mitosis + development gametes 46 meiosis 46 23 egg 23 23 23 zygote fertilization AP Biology sperm 46 46 46 46 46 46 46 46 46 mitosis development
    24. 24. The value of sexual reproduction  Sexual reproduction introduces genetic variation  genetic recombination  independent assortment of chromosomes  random alignment of homologous chromosomes in Metaphase 1  crossing over  mixing of alleles across homologous chromosomes  random fertilization  which sperm fertilizes which egg?  Driving evolution  providing variation for natural selection metaphase1 AP Biology
    25. 25. Variation from genetic recombination  Independent assortment of chromosomes meiosis introduces genetic variation  gametes of offspring do not have same combination of genes as gametes from parents   random assortment in humans produces 223 (8,388,608) different combinations in gametes AP Biology from Mom from Dad offspring new gametes made by offspring
    26. 26. Variation from crossing over  Crossing over creates completely new combinations of traits on each chromosome  creates an infinite variety in gametes AP Biology
    27. 27. Variation from random fertilization  Sperm + Egg = ?  AP Biology any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations
    28. 28. Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers AP Biology Baldwin brothers Martin & Charlie Sheen, Emilio Estevez
    29. 29. Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) Vas deferens spermatids (haploid) spermatozoa  Spermatogenesis continuous & prolific process Cross-section of seminiferous tubule  each ejaculation = 100-600 million sperm AP Biology  MEIOSIS II
    30. 30. Egg production  Oogenesis     eggs in ovaries halted before Anaphase 1 Meiosis 1 completed during maturation Meiosis 2 completed unequal divisions after fertilization 1 egg + 2 polar bodies Meiosis 1 completed during egg maturation What is the advantage of this development system? Meiosis 2 completed AP Biology triggered by fertilization ovulation
    31. 31. Putting all your egg in one basket! Oogenesis germinal cell (diploid) primary follicles fallopian tube fertilization primary oocyte (diploid) MEIOSIS I secondary oocyte (haploid) first polar body MEIOSIS II after fertilization second polar body AP Biology ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum
    32. 32. Differences across kingdoms  Not all organisms use haploid & diploid stages in same way   which one is dominant (2n or n) differs but still alternate between haploid & diploid  must for sexual reproduction AP Biology
    33. 33. Any Questions?? What are the DISadvantages of sexual reproduction? AP Biology 2007-2008
    34. 34. Ghosts of Lectures Past (storage) AP Biology 2007-2008
    35. 35. AP Biology
    36. 36. AP Biology

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