Download - LECTURE 21 LARGE-SCALE CHROMOSOME CHANGES I
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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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error-free, pre-/no replication, single strand damage
(a) direct chemical reversal of damaged base e.g., photorepair of UV-induced T-dimer
(b) base excision & replacement, DNA glycosylases
(c) segment excision & replacement prokaryotes: exinuclease, DNA pol I, ligase eukaryotes: transcription-coupled “repairisome”
BIOLOGICAL REPAIR
(b & c) complementary template strand used to
restore sequence
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error-prone, during replication, single strand damage SOS repair error-prone DNA pols
BIOLOGICAL REPAIR
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error-free, post-replication, single strand damage mismatch repair in
prokaryotes complementary template
strand used to restore sequence
BIOLOGICAL REPAIR
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error-free, post-replication, double strand damage homologous
recombination complementary
sister chromatid used to restore sequence
BIOLOGICAL REPAIR
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error-prone, no replication, double strand damage non-homologous end joining… trim & patch
BIOLOGICAL REPAIR
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error-prone, post-replication, double strand damage crossing-over… gene conversion, either with or
without associated strand exchange
BIOLOGICAL REPAIR
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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
MEIOTIC CROSSING-OVER
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double-stranded break model of crossing-over
MEIOTIC CROSSING-OVER
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double-stranded break model of crossing-over
MEIOTIC CROSSING-OVER
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double-stranded break model of crossing-over
MEIOTIC CROSSING-OVER
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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...
MEIOTIC CROSSING-OVER
AAAAaaaa
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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 not repaired
MEIOTIC CROSSING-OVER
AAAaaaaa
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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
MEIOTIC CROSSING-OVER
AAaaaaaa
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how to think about this problem...
MEIOTIC CROSSING-OVER
BRANCH MIGRATION ROTATE PERSPECTIVE
BREAKS conversion
“horizontal breakage”
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MEIOTIC CROSSING-OVER
BRANCH MIGRATION ROTATE PERSPECTIVE
BREAKS
how to think about this problem...
recombination
“vertical breakage”
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MEIOTIC CROSSING-OVER
BRANCH MIGRATIONthanks to Bill Engels, Univ. Wisconsin
how to think about this problem...
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MEIOTIC CROSSING-OVER
ROTATE PERSECTIVEthanks to Bill Engels, Univ. Wisconsin
how to think about this problem...
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2 general questions to consider... is the genome complete?
is the genome balanced?
OVERVIEW
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3 classes of chromosome change
OVERVIEW
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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... 1st e.g.: cabbage + radish (both 2n = 18) n + n gametes
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 + n2 gametes viable 2n hybrids
n1 + n2 gametes sterile 2n hybrids + colchicine viable 2n1 + 2n2 = 4n amphidiploid (double diploid)
fusion of 2n1 + 2n2 cells 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