Download Deletion loops in polytene chromosomes

Chapter 14
Chromosomal Rearrangements
and Changes in Chromosome
Number Reshape Eukaryotic
Genomes
Outline of Chapter 14
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Rearrangements of DNA sequences within and
between chromosomes
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Deletions
Duplications
Inversions
Translocations
Movements of transposable elements
Changes in chromosome number
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Aneuploidy: monosomy and trisomy
Monoploidy
Polyploidy
Deletions
remove
genetic
material
from
genome
Fig. 13.2
Phenotypic consequences of
heterozygosity
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Homozygosity
for deletion is
often but not
always lethal
Heterozygosity
for deletion is
often
detrimental
Fig. 13.3
Deletion heterozygotes affect
mapping distances
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Recombination between homologues can only occur if both
carry copies of the gene
Deletion loop formed if heterozygous for deletion
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Identification of deletion location on chromosome
Genes within can not be separated by recombination
Fig. 13.4 a
Deletion loops in polytene
chromosomes
Fig. 13.4 b
Deletions in heterozygotes can
uncover genes
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Pseudodominance shows a deletion has
removed a particular gene
Fig. 13.5
Deletions can be used to locate genes
Deletions to
assign genes to
bands on
Drosophila
polytene
chromosomes
Complementation
tests crossing
deletion mutants
with mutant
genes of
interests
 Deletion
heterozygote
reveals
chromosomal
location of
mutant gene

Fig. 13.6
Deletions to locate genes at the
molecular level
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Fig. 13.7 a
Labeled probe hybridizes to wild-type
chromosome but not to deletion
chromosome
Molecular mapping of deletion
breakpoints by Southern blotting
Fig. 13.7 b, c
Duplications add material to the
genome
Fig. 13.8 a,b
Duplication loops form when chromosomes pair in
duplication heterozygotes

In prophase I, the duplication loop can
assume different configurations that
maximize the pairing of related regions
Fig. 13.8 c
Duplications can affect phenotype

Novel phenotypes
More gene copies
 Genes next to
duplication
displaced to new
environment
altering expression

Fig. 13.9
Unequal crossing over between duplications
increases or decreases gene copy number
Fig. 13.10
Fig. 13.10
Summary of duplication and
deletion effects on phenotpye
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Alter number of genes on a chromosome and may
affect phenotype of heterozygote
Heterozygosity create one or three gene copies and
create imbalance in gene product altering
phenotypes (some lethal)
Genes may be placed in new location that modifies
its expression
Deletions and duplications drive evolution by
generating families of tandemly repeated genes
Inversions reorganize the DNA
sequence of a chromosome

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Produced by half
rotation of
chromosomal regions
after double-stranded
break
Also rare crossover
between related genes
in opposite orientation
or transposition
Fig. 13.11a,b
An inversion can affect phenotype if
it disrupts a gene
Fig. 13.11 c
Inversion heterozygotes reduce the
number of recombinant progeny

Inversion loop in
heterozygote forms
tightest possible
alignment of
homologous regions
Fig. 13.12
Gametes produced from pericentric and
paracentric inversions are imbalanced
Fig. 13.13
Pericentric inversion
(cont’d)
Paracentric inversion
(cont’d)
Fig. 13.13
cont’d
Inversions suppress recombination
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Balancer chromosomes carry both a
dominant marker D and inversions
(brackets) that prevent recombination with
experimental chromosome.
Heterozygous parent will transmit balancer
or experimental chromosome.
Dominant mutation has an easily
distinguished phenotpye (e.g., curly wing)
Translocations attach on part of a
chromosome to another
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Translocation – part of one chromosome
becomes attached to nonhomologous
chromosome
Reciprocal translocation – two different parts of
chromosomes switch places
Robertsonian translocations can
reshape genomes
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Reciprocal exchange between acrocentric
chromosomes generate large metacentric
chromosome and small chromosome
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Tiny chromosome may be lost from organism
Fig. 13.16
Leukemia patients have too many blood cells
Fig. 13.17
Heterozygosity for translocations diminishes
fertility and results in pseudolinkage
Fig. 13.18 a.b
Three possible segregation patterns in a translocation
heterozygote from the cruciform configuration
Fig. 13.18 c
Pseudolinkage – because only alternate segregation patterns produce
viable progeny, genes near breakpoints act as if linked
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Semisterility
results from
translocation
heterozygotes
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< 50% of gametes
arise from
alternate
segregation and
are viable
Fig. 13.18 d
Translocation Down syndrome
translocation of chromosome 21 is small and thus produces
viable gamete, but with phenotypic consequence
Fig. 13.19
Transposable elements move from
place to place in the genome
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1930s Marcus Rhoades and 1950s Barbara McClintock –
transposable elements in corn
1983 McClintock received Nobel Prize
Found in all organisms
Any segment of DNA that evolves ability to move from one
place to another in genome
Selfish DNA carrying only information to self-perpetuate
Most 50 – 10,000 bp
May be present hundreds of time in a genome
LINES, long interspersed element in mammals
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SINES, short interspersed elements in mammals
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~ 20,000 copies in human genome up to 6.4kb in length
~ 300,000 copies in human genome
~ 7% of genome are LINES and SINES
Retroposons generate an RNA that encodes a
reverse transciptase like enzyme

Two types
Poly-A tail at 3’
end of RNA-like
DNA strand
 Long terminal
repeat (LTRs)
oriented in same
direction on either
end of element

Fig. 13.23 a
Fig. 13.23 b
The process of LTR transposition
Fig. 13.23
Transposons encode transposase enzymes that
catalyze events of transposition
Fig. 13.24 a
P elements in Drosophila
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After excision of P element transposon, DNA
exonucleases first widen gap and then repair it
Repair uses sister chromatid or homologous
chromosome as a template
P strains of Drosophila have many copies of P
elements
M strains have no copies
Hybrid dysgenesis – defects including sterility,
mutation, and chromosomal breakage from cross
between P and M strains
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Promotes movement of P elements to new positions
Genomes often contain defective
copies of transposable elements
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Many TEs sustain deletions during
transposition or repair
If promoter needed for transcription
deleted, TE can not transpose again
Most SINES and LINES in human genome
are defective TEs
Nonautonomous elements – need activity of
nondeleted copies of same TE for movement
Autonomous elements – move by themselves
TEs can generate mutations in adjacent genes
spontaneous mutations in white gene of Drosophila
Fig. 13.25
TEs can generate chromosomal rearrangements and
relocate genes
Fig. 13.26
The loss or gain of one or more
chromosomes results in aneuploidy
Autosomal aneuploidy is harmful to the organism
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Monosomy usually lethal
Trisomies – highly deleterious
Trisomy 18 – Edwards syndrome
 Trisomy 13 – Patau syndrome
 Trisomy 21 – Down syndrome
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Humans tolerate X chromosome aneuploidy
because X inactivation compensates for dosage
Fig. 13.27
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Mitotic nondisjunction
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Failure of two sister chromatids to separate during mitotic
anaphase
Generates reciprocal trisomic and monosomic daughter cells
Chromosome loss
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Produces one monosomic and one diploid daughter cell
Fig. 13.28
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Mosaics – aneuploid and normal tissues that lie
side-by-side
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Fig. 13.28 b
Aneuploids give rise to aneuploid clones
Gynandromorph in Drosophila results from female
losing one X chromosome during first mitotic
division after fertilization
Fig. 13.29
Euploid individuals contain only
complete sets of chromosomes
Monoploid organisms contain a single copy of
each chromosome and are usually infertile
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Monoploid plants have many uses
Visualize recessive traits directly
 Introduction of mutations into individual cells
 Select for desirable phenotpyes (herbicide
resistance)
 Hormone treatment to grow selected cells
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Fig. 13.30
Treatment with colchicine converts back to diploid
plants that express desired phenotypes
Fig. 13.30 c
Polyploidy has accompanied the
evolution of many cultivated plants
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1:3 of flowering plants are polyploid
Polyploid often increases size and vigor
Often selected for agricultural cultivation
Tetraploids - alfalfa, coffee, peanuts
 Octaploid - strawberries
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Fig. 13.31
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Fig. 13.32
Triploids are
almost always
sterile
Result from union
of monoploid and
diploid gametes
Meiosis produces
unbalanced
gametes
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Tetraploids are often source of new species
Failure of chromosomes to separate into two
daughter cells during mitosis in diploid
Cross between tetraploid and diploid creates
triploids – new species, autopolyploids
13.33 a
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Fig. 13.33 b
Maintenance of
tetraploid species
depends on the
production of gametes
with balanced sets of
chromosomes
Bivalents- pairs of
synapsed homologous
chromosomes that
ensure balanced
gametes
Fig. 13.33 c
Some polyploids have agriculturally desirable traits
derived from two species
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Amphidiploids created by
chromosome doubling in
germ cells
e.g., wheat – cross
between tetraploid wheat
and diploid rye produce
hybrids with desirable
traits
Fig. 13.34