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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.

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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
slide2

GENERAL REVIEW

  • Friday December 8
  • 9 am – 12 noon
  • WHI 105
  • be prepared to ask & answer questions
slide3

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

slide4

BIOLOGICAL REPAIR

  • error-prone, during replication, single strand damage
    • SOS repair
    • error-prone DNA pols
slide5

BIOLOGICAL REPAIR

  • error-free, post-replication, single strand damage
    • mismatch repair in prokaryotes
    • complementary template strand used to restore sequence
slide6

BIOLOGICAL REPAIR

  • error-free, post-replication, double strand damage
    • homologous recombination
    • complementary sister chromatid used to restore sequence
slide7

BIOLOGICAL REPAIR

  • error-prone, no replication, double strand damage
    • non-homologous end joining… trim & patch
slide8

BIOLOGICAL REPAIR

  • error-prone, post-replication, double strand damage
    • crossing-over… gene conversion, either with or without associated strand exchange
slide9

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
slide10

MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over
slide11

MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over
slide12

MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over
slide13

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

slide14

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

slide15

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

slide16

ROTATE PERSPECTIVE

BREAKS

MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

  • conversion
    • “horizontal breakage”
slide17

BREAKS

MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

ROTATE PERSPECTIVE

  • recombination
    • “vertical breakage”
slide18

MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

thanks to Bill Engels, Univ. Wisconsin

slide19

MEIOTIC CROSSING-OVER

  • how to think about this problem...

ROTATE PERSECTIVE

thanks to Bill Engels, Univ. Wisconsin

slide20

OVERVIEW

  • 2 general questions to consider...
    • is the genome complete?
    • is the genome balanced?
slide21

OVERVIEW

  • 3 classes of chromosome change

slide22

CHANGES IN CHROMOSOME NUMBER

  • 2 classes of changes in chromosome sets
    • euploids / aberrant euploidy: whole sets
    • aneuploids / aneuploidy: partial sets
slide23

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...
slide24

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
slide25

CHANGES IN CHROMOSOME NUMBER

  • polyploids (>2n)
    • common in plants, important in plant evolution
    • even #s most common n > 12
    • duplicated chromosome sets  new species
slide26

CHANGES IN CHROMOSOME NUMBER

  • polyploids (>2n)
    • aberrant euploids are often larger than their diploid counterparts, e.g.:
    • tobacco leaf cells 
    • oysters 
slide27

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
slide28

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
slide29

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
slide30

CHANGES IN CHROMOSOME NUMBER

  • autopolyploids
    • diploid (2n)  2 (spontaneous)  tetraploid (4n) or
    • diploid (2n) + colchicine (disrupt microtubules) 
slide31

CHANGES IN CHROMOSOME NUMBER

  • autopolyploids
    • tetraploids  diploid gametes &  usually viable
      • some trivalent / univalent combinations  aneuploid gametes & offspring
slide32

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
slide33

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
slide34

CHANGES IN CHROMOSOME NUMBER

  • allopolyploids in nature
    • importance in production of new species
slide35

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
slide36

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
slide37

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
slide38

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

slide39

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
slide40

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
slide41

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
slide42

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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