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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
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
error-prone, during replication, single strand damage SOS repair error-prone DNA pols
BIOLOGICAL REPAIR
error-free, post-replication, single strand damage mismatch repair in
prokaryotes complementary template
strand used to restore sequence
BIOLOGICAL REPAIR
error-free, post-replication, double strand damage homologous
recombination complementary
sister chromatid used to restore sequence
BIOLOGICAL REPAIR
error-prone, no replication, double strand damage non-homologous end joining… trim & patch
BIOLOGICAL REPAIR
error-prone, post-replication, double strand damage crossing-over… gene conversion, either with or
without associated strand exchange
BIOLOGICAL REPAIR
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
double-stranded break model of crossing-over
MEIOTIC CROSSING-OVER
double-stranded break model of crossing-over
MEIOTIC CROSSING-OVER
double-stranded break model of crossing-over
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...
MEIOTIC CROSSING-OVER
AAAAaaaa
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
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
how to think about this problem...
MEIOTIC CROSSING-OVER
BRANCH MIGRATION ROTATE PERSPECTIVE
BREAKS conversion
“horizontal breakage”
MEIOTIC CROSSING-OVER
BRANCH MIGRATION ROTATE PERSPECTIVE
BREAKS
how to think about this problem...
recombination
“vertical breakage”
MEIOTIC CROSSING-OVER
BRANCH MIGRATIONthanks to Bill Engels, Univ. Wisconsin
how to think about this problem...
MEIOTIC CROSSING-OVER
ROTATE PERSECTIVEthanks to Bill Engels, Univ. Wisconsin
how to think about this problem...
2 general questions to consider... is the genome complete?
is the genome balanced?
OVERVIEW
3 classes of chromosome change
OVERVIEW
CHANGES IN CHROMOSOME NUMBER
2 classes of changes in chromosome sets euploids / aberrant euploidy: whole sets aneuploids / aneuploidy: partial sets
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...
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
CHANGES IN CHROMOSOME NUMBER
polyploids (>2n) common in plants, important in plant evolution even #s most
common n > 12 duplicated
chromosome sets new species
CHANGES IN CHROMOSOME NUMBER
polyploids (>2n) aberrant euploids are
often larger than their diploid counterparts, e.g.:
tobacco leaf cells oysters
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
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
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
CHANGES IN CHROMOSOME NUMBER
autopolyploids diploid (2n) 2 (spontaneous) tetraploid (4n) or diploid (2n) + colchicine (disrupt microtubules)
CHANGES IN CHROMOSOME NUMBER
autopolyploids tetraploids diploid gametes & usually viable
some trivalent / univalent combinations aneuploid gametes & offspring
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
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
CHANGES IN CHROMOSOME NUMBER
allopolyploids in nature importance in production of new species
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
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
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
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
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
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
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
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