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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 l.jpg
LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I

  • revisit DNA repair

  • chapter 15

    • overview

    • chromosome number

    • chromosome structure

    • humans


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GENERAL REVIEW

  • Friday December 8

  • 9 am – 12 noon

  • WHI 105

  • be prepared to ask & answer questions


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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


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BIOLOGICAL REPAIR

  • error-prone, during replication, single strand damage

    • SOS repair

    • error-prone DNA pols


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BIOLOGICAL REPAIR

  • error-free, post-replication, single strand damage

    • mismatch repair in prokaryotes

    • complementary template strand used to restore sequence


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BIOLOGICAL REPAIR

  • error-free, post-replication, double strand damage

    • homologous recombination

    • complementary sister chromatid used to restore sequence


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BIOLOGICAL REPAIR

  • error-prone, no replication, double strand damage

    • non-homologous end joining… trim & patch


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BIOLOGICAL REPAIR

  • error-prone, post-replication, double strand damage

    • crossing-over… gene conversion, either with or without associated strand exchange


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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


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MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over


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MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over


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MEIOTIC CROSSING-OVER

  • double-stranded break model of crossing-over


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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


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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


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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


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ROTATE PERSPECTIVE

BREAKS

MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

  • conversion

    • “horizontal breakage”


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BREAKS

MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

ROTATE PERSPECTIVE

  • recombination

    • “vertical breakage”


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MEIOTIC CROSSING-OVER

  • how to think about this problem...

BRANCH MIGRATION

thanks to Bill Engels, Univ. Wisconsin


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MEIOTIC CROSSING-OVER

  • how to think about this problem...

ROTATE PERSECTIVE

thanks to Bill Engels, Univ. Wisconsin


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OVERVIEW

  • 2 general questions to consider...

    • is the genome complete?

    • is the genome balanced?


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OVERVIEW

  • 3 classes of chromosome change


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CHANGES IN CHROMOSOME NUMBER

  • 2 classes of changes in chromosome sets

    • euploids / aberrant euploidy: whole sets

    • aneuploids / aneuploidy: partial sets


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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...


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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


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CHANGES IN CHROMOSOME NUMBER

  • polyploids (>2n)

    • common in plants, important in plant evolution

    • even #s most common n > 12

    • duplicated chromosome sets  new species


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CHANGES IN CHROMOSOME NUMBER

  • polyploids (>2n)

    • aberrant euploids are often larger than their diploid counterparts, e.g.:

    • tobacco leaf cells 

    • oysters 


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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


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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


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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


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CHANGES IN CHROMOSOME NUMBER

  • autopolyploids

    • diploid (2n)  2 (spontaneous)  tetraploid (4n) or

    • diploid (2n) + colchicine (disrupt microtubules) 


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CHANGES IN CHROMOSOME NUMBER

  • autopolyploids

    • tetraploids  diploid gametes &  usually viable

      • some trivalent / univalent combinations  aneuploid gametes & offspring


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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


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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


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CHANGES IN CHROMOSOME NUMBER

  • allopolyploids in nature

    • importance in production of new species


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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


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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


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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


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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


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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


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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


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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


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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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