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LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I

LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I. revisit DNA repair chapter 15 overview chromosome number chromosome structure humans. GENERAL REVIEW. Friday December 8 9 am – 12 noon WHI 105 be prepared to ask & answer questions. BIOLOGICAL REPAIR.

Samuel
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LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I

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  1. LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I • revisit DNA repair • chapter 15 • overview • chromosome number • chromosome structure • humans

  2. GENERAL REVIEW • Friday December 8 • 9 am – 12 noon • WHI 105 • be prepared to ask & answer questions

  3. BIOLOGICAL REPAIR • error-free, pre-/no replication, single strand damage • direct chemical reversal of damaged base e.g., photorepair of UV-induced T-dimer • base excision & replacement, DNA glycosylases • segment excision & replacement prokaryotes: exinuclease, DNA pol I, ligase eukaryotes: transcription-coupled “repairisome” (b & c) complementary template strand used to restore sequence

  4. BIOLOGICAL REPAIR • error-prone, during replication, single strand damage • SOS repair • error-prone DNA pols

  5. BIOLOGICAL REPAIR • error-free, post-replication, single strand damage • mismatch repair in prokaryotes • complementary template strand used to restore sequence

  6. BIOLOGICAL REPAIR • error-free, post-replication, double strand damage • homologous recombination • complementary sister chromatid used to restore sequence

  7. BIOLOGICAL REPAIR • error-prone, no replication, double strand damage • non-homologous end joining… trim & patch

  8. BIOLOGICAL REPAIR • error-prone, post-replication, double strand damage • crossing-over… gene conversion, either with or without associated strand exchange

  9. MEIOTIC CROSSING-OVER • initiated by double-stranded chromosome breakage • between 2 homologous non-sister chromatids • no gain or loss of genetic material • 2 steps • double stranded breakage • heteroduplex DNA formed, derived from non-sister chromatids on homologous chromosomes

  10. MEIOTIC CROSSING-OVER • double-stranded break model of crossing-over

  11. MEIOTIC CROSSING-OVER • double-stranded break model of crossing-over

  12. MEIOTIC CROSSING-OVER • double-stranded break model of crossing-over

  13. MEIOTIC CROSSING-OVER • evidence first from aberrant ratios observed in fungi • aberrant asci have > 4 copies of on genotype • extra copies changed through gene conversion • 5:3 ratio from non-identical sister spores in meiosis • with heteroduplex... A A A A a a a a

  14. MEIOTIC CROSSING-OVER • evidence first from aberrant ratios observed in fungi • aberrant asci have > 4 copies of on genotype • extra copies changed through gene conversion • 5:3 ratio from non-identical sister spores in meiosis • with heteroduplex notrepaired A A A a a a a a

  15. MEIOTIC CROSSING-OVER • evidence first from aberrant ratios observed in fungi • aberrant asci have > 4 copies of on genotype • extra copies changed through gene conversion • 6:2 ratio from non-identical sister spores in meiosis • with heteroduplex repaired A A a a a a a a

  16. ROTATE PERSPECTIVE BREAKS MEIOTIC CROSSING-OVER • how to think about this problem... BRANCH MIGRATION • conversion • “horizontal breakage”

  17. BREAKS MEIOTIC CROSSING-OVER • how to think about this problem... BRANCH MIGRATION ROTATE PERSPECTIVE • recombination • “vertical breakage”

  18. MEIOTIC CROSSING-OVER • how to think about this problem... BRANCH MIGRATION thanks to Bill Engels, Univ. Wisconsin

  19. MEIOTIC CROSSING-OVER • how to think about this problem... ROTATE PERSECTIVE thanks to Bill Engels, Univ. Wisconsin

  20. OVERVIEW • 2 general questions to consider... • is the genome complete? • is the genome balanced?

  21. OVERVIEW • 3 classes of chromosome change   

  22. CHANGES IN CHROMOSOME NUMBER • 2 classes of changes in chromosome sets • euploids / aberrant euploidy: whole sets • aneuploids / aneuploidy: partial sets

  23. CHANGES IN CHROMOSOME NUMBER • “ploidy” terminology • monoploid (n): 1 chromosome set (abnormal) • haploid (n): 1 chromosome set (normal) • euploid (>1n): >1 chromosome set • polyploid (>2n): >2 chromosome sets • triploid, tetraploid, pentaploid, hexaploid...

  24. CHANGES IN CHROMOSOME NUMBER • monoploids (n) • some insects are haplo-diploid (e.g. bees) • males develop from unfertilized eggs • their gametes form by mitosis • not found in most animals • due to recessive mutations = genetic load • masked by wild-type alleles in diploids • surviving monoploids are sterile in most animals

  25. CHANGES IN CHROMOSOME NUMBER • polyploids (>2n) • common in plants, important in plant evolution • even #s most common n > 12 • duplicated chromosome sets  new species

  26. CHANGES IN CHROMOSOME NUMBER • polyploids (>2n) • aberrant euploids are often larger than their diploid counterparts, e.g.: • tobacco leaf cells  • oysters 

  27. CHANGES IN CHROMOSOME NUMBER • 2 types of polyploids, multiple chromosome sets originating from different sources • autopolyploids: • 1 species • chromosomes fully homologous • allopolyploids: • 2 related species • chromosomes only partially homologous

  28. CHANGES IN CHROMOSOME NUMBER • autopolyploids • diploid (2n)  tetraploid (4n)... • fusion of gametes: n + 2n triploid (3n) • triploids (& all odd# n)  aneuploid gametes • 1 or 2 chromosomes / each type  2° meiocyte

  29. CHANGES IN CHROMOSOME NUMBER • autopolyploids • triploids  aneuploid gametes &  usually sterile • P ½ for each chromosome type • as n , P (balanced gametes) ...e.g.: • if n 10, P (2n gamete)  (1/2)10 0.001

  30. CHANGES IN CHROMOSOME NUMBER • autopolyploids • diploid (2n)  2 (spontaneous)  tetraploid (4n) or • diploid (2n) + colchicine (disrupt microtubules) 

  31. CHANGES IN CHROMOSOME NUMBER • autopolyploids • tetraploids  diploid gametes &  usually viable • some trivalent / univalent combinations  aneuploid gametes & offspring

  32. CHANGES IN CHROMOSOME NUMBER • what are the genotypic & phenotypic probabilities in the progeny of a P cross A/A/A/a A/A/A/a? • P gametes: P(A/A) = P(A/a) = ½, P(a/a) = 0 • F1 genotypes: P(A/A/A/A) = (½)2 = ¼ P(A/A/A/a) = 2(½)2 = ½ P(A/A/a/a) = (½)2 = ¼ • F1 phenotypes: all A • A/A/a/a? • A/a/a/a? • autopolyploids

  33. CHANGES IN CHROMOSOME NUMBER • allopolyploids • useful for agriculture... blend characteristics of 2 plants... 1ste.g.: cabbage + radish (both 2n = 18) • n + ngametes  sterile 2n diploid • sterile 2n diploid + colchicine  fertile 4n = 36 amphidiploid

  34. CHANGES IN CHROMOSOME NUMBER • allopolyploids in nature • importance in production of new species

  35. CHANGES IN CHROMOSOME NUMBER • allopolyploids synthesized in the laboratory • sometimes, n1+ n2gametes  viable 2n hybrids • n1+ n2gametes  sterile 2n hybrids + colchicine  viable 2n1+ 2n2= 4n amphidiploid (double diploid) • fusion of 2n1+ 2n2cells  4n tetraploid

  36. CHANGES IN CHROMOSOME NUMBER • agriculture • diploids mask expression of recessive traits • monoploids express recessive traits; retain desirable, dispose of deleterious • monoploid culture  select  double chromosomes

  37. CHANGES IN CHROMOSOME NUMBER • agriculture • diploids mask expression of recessive traits • monoploids express recessive traits; retain desirable, dispose of deleterious • monoploid culture  select  double chromosomes • can also use method with mutagenesis to generate new varieties with desirable traits, e.g.: • pesticide resistance • drought tollerance

  38. CHANGES IN CHROMOSOME NUMBER • agriculture • autotriploids, e.g. bananas (3n = 33) • sterile, seeds nearly absent • autotetraploids, e.g. grapes • bigger • allopolyploids, e.g. wheat, cotton, many others DIPLOID TETRAPLOID

  39. CHANGES IN CHROMOSOME NUMBER • polyploid animals • less common than in plants • sterility is the main barrier for this process •  polyploid animals are often parthenogenic • lower invertebrates, some crustaceans, fish, amphibians & reptiles • triploid & tetraploid Drosophila have been synthesized in the lab

  40. CHANGES IN CHROMOSOME NUMBER • aneuploidy • + or - 1 or 2 chromosomes • diploids • 2n + 1  trisomic / trisomy • 2n - 1  monosomic / monosomy • 2n - 2  nullosomic / nullosomy • haploids • n + 1 disomic / disomy • sex chromosomes require specific notation, e.g., XXX, X0, XYY, etc

  41. CHANGES IN CHROMOSOME NUMBER • aneuploidy • by nondisjuction = abnormal segregation • meiotic (2 ways)  whole organism affected • normal disjuction aided by crossing over • mitotic  mosaic patches affected

  42. CHANGES IN CHROMOSOME NUMBER • aneuploidy • gene balance ~ gene dosage affects • gene products function in a balanced coctail • imbalance affects physiological pathways • important genes may be haplo- or triplo-abnormal • X-chromosome expression level same in males & females because of dosage compensation • fruit flies - males have hyperactive X • mammals - females have only 1 transcriptionally active X

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