Download Prof. Kamakaka`s Lecture 11 Notes

Mutations
1
What is a gene?
Prokaryotic Genes
PROMOTER
3’
5’
antisense
---TTGACAT------TATAAT-------AT-/-AGGAGGT-/-ATG CCC CTT TTG TGA
---AACTGTA------ATATTA-------TA-/-TCCTCCA-/-TAC GGG GAA AAC ATT
sense
3’
(-35)
(-10)
RIBOSOME
BINDING
SITE
5’
5’
3’
U-/-AGGAGGU-/-AUG CCC CUU UUG UGA
Met Pro leu leu stp
When ALL OF THESE RULES ARE SATISFIED THEN THEN A
PIECE OF DNA WILL GENERATE A RNA WHICH WILL BE READ
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AND TRANSLATED INTO A PROTEIN.
Reading the genetic code
5’
3’
A T G T T T A A A T A G C C C
G G G C T A T T T A A A C A T
3’
5’
5’
3’
A T G T T T A A A T A G C C C
5’
3’
A U G U U U A A A U A G C C C
G G G C T A T T T A A A C A T
3’
5’
5’
3’
A U G U U U A A A U A G C C C
U A C A A A U U U S T P
3
Met
Phe
Lys
No Gaps
5’
3’
A U G U U U A A A U A G C C C
U A C A A A U U U S T P
Met
Phe
Lys
5’
3’
A U G U U U A A A U A G C C C
U A C
A A U U U A
Met
Asn
Leu
4
No overlaps
5’
3’
A U G A A A C C C U A G C C C
U A C U U U G G G S T P
Met
Lys
Pro
5’
3’
A U G A A A C C C U A G C C C
U A C U U U G G
Met
Lys Trp
5
The GENETIC CODE
The code is a three letter code.
Second letter
U
C
CUU
CUC
CUA
CUG
A
AUU
AUC
AUA
AUG
G
GUU
GUC
GUA
GUG
Phe
Leu
Leu
Ile
Met
Val
A
UCU
UCC
UCA
UCG
UAU
UAC
UAA
UAG
CCU
CCC
CCA
CCG
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG
Ser
G
Tyr
STOP
His
Pro
CAU
CAC
CAA
CAG
Asn
Thr
AAU
AAC
AAA
AAG
Ala
GAU
GAC
GAA
GAG
Gln
Lys
Asp
Glu
UGU
UGC
UGA
UGG
CGU
CGC
CGA
CGG
AGU
AGC
AGA
AGG
GGU
GGC
GGA
GGG
U
C
A
G
Cys
STOP
Trp
U
C
A
G
Arg
U
C
A
G
Ser
Arg
U
C
A
G
Gly
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Third letter
First letter
U
UUU
UUC
UUA
UUG
C
The code
3
5
1
9
2
amino
amino
amino
amino
amino
acids are specified by 6 different codons
acids are specified by 4 different codons
acid is specified by 3 different codons
acids are specified by 2 different codons
acids are specified by 1 different codons
The degeneracy arises because
More than one tRNA specifies a given amino acid
A single tRNA can base-pair with more than one codon
tRNAs do not normally pair with STOP codons
Ser
Ser
Ser
AGG
AGU
UCG
----UCC------UCA------AGC
Ser
Ser
AGG
AGG
----UCC------UCA------
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The Genetic Code
Properties of the Genetic code:
1- The code is written in a linear form using the nucleotides
that comprise the mRNA
2- The code is a triplet: THREE nucleotides specify
ONE amino acid
3- The code is degenerate: more than one triplet specifies
a given amino acid
4- The code is unambiguous: each triplet specifies only
ONE amino acid
5- The code contains stop signs- There are three different stops
6- The code is comma less
7- The code is non-overlapping
8
xxxxxx
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Generation of mutations
Spontaneous mutations
Replication induced mutations of DNA
Usually base substitutions (Most errors are corrected)
Meiotic crossing over can induce mutations
Small additions and deletions AND Large changes as well
Environment induced changes
Exposure to physical mutagens - Radioactivity or chemicals
Depurination (removal of A or G)
Repair results in random substitution during replication
Deamination (removal of amino group of base)
Cytosine--uracil--bp adenine--replication--
(nitrous acid)
Oxidation (oxoG)
guanine--oxoguanine--bp adenine--replication -Base analog incorporation during replication BU-T
Intercalating agents
10
Methods used to study mutations
Gross chromosomal changesdeletions, insertions, inversions, translocations
Cytology- microscopy- karyotype
Small mutations
Small deletions, insertions and point mutations
Recombinant DNA technologies
11
Mutation rate
There are approximately 1013 cells in the human body
Each cell receives 10,000 DNA lesions per day (Lindahl and
Barnes 2000).
Most pervasive agent is UV. 100,000 lesions per exposed cell
per hour (Jackson and Bartek 2009).
Ionizing agents (X-rays/g-rays) are most toxic because they
generate double strand breaks (Ward 1988).
Chromosome instability (gain or loss of entire segments) is
frequent - 40% of imbalances are entire arm imbalance while
45% are terminal segment imbalance (double strand break, nondysjunction etc)
Sequencing 179 humans as part of the 1000 genome project:
On average, each person is found to carry approximately 250
to 300 loss-of-function variants in genes of which 50 are in
genes previously implicated in inherited disorders.
1.3 million short indels (1-10,000 bp) were identified and
20,000 large (>10,000 bp) variants were identified.
Variation detected by the project is not evenly distributed
across the genome: certain regions, containing repetitive
sequences (sub-telomeres etc), show high rates of indels.
12
Sequencing the whole genomes of a family (2010 Science 328
636).
98 crossovers in maternal genome
57 crossovers in paternal genome
Mutation rate is 1x10-8 per position per haploid genome
(human genome is 3x109 bp)
It was calculated that there are ~70 new mutations in each
diploid human genome
Some sites such as CpG sites mutate at a rate 11 times
higher than other sites
Exome sequencing of 2440 individuals (Science 2012 337 40)
Each person has ~100 loss of function mutations (~35
nonsense).
20 loss of function mutations are homozygous
Some alterations in sequence concentrate in specific
geographic populations
Rare changes are population specific and their frequencies
vary for each geographic population
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Chromosomes and chromosome rearrangements
Cytogenetics is the study of genetics by visualizing
chromosomes. This area of research is germane to several
areas of biological research.
Cytogenetics has been fundamental to understanding the
evolutionary history of a species (for example, although the
Chimp and the human are morphologically very different, at the
level of the chromosome (and DNA sequence) they are
extremely similar.
H = human
C= chimp
G = Gorilla
O = Orang utang
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Karyotype
Chromosomes are classified by size, centromere position and
banding pattern:
Shown below is the human karyotype (description of the
chromosome content of a given species)
Karyotype is the chromosome description of length, number,
morphology.
Karyotype analysis is extremely important in medicine.
Alternations in karyotypes are linked to birth defects and many
human cancers.
Metacentric- centromere in the middle
Acrocentric- centromere off center
telocentric centromere at one end
15
Banding patterns
Specialized stains produce unique banding patterns along each
chromosome. Banding patterns are extremely useful for
detecting abnormalities in chromosome structure.
For many of the chromosome stains- the molecular basis of
the banding patterns is unclear. Nonetheless these techniques
remain fundamental in many areas of genetic research
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MU to bp
Genetic maps are based on recombination frequencies and
describe the relative order and relative distance between
linked genes.
Remember genes reside on chromosomes.
So what we would like to know is where are the genes located
on the chromosomes
22% Rf = 22MU
What does this mean in terms of chromosomes and DNA?
17
Physical maps
Physical maps provide information concerning the location of
genes on chromosomes
Where are the genes on chromosomes?
Cytological studies have been successfully used to map genes to
specific regions of a chromosome.
For example in Drosophila in some cells the chromosomes
become highly replicated and exhibit very characteristic
banding patterns:
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In situ hybridization
Salivary glands
Squash on slide
Denature/Stain polytene chromosomes
label gene probe (you can only use this method if you have the
gene cloned)
Hybridize probe to polytene chromosomes
Autoradiography
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Chromosome loss
Chromosome instabilityElevated gain or loss of complete chromosomes
Frequent in tumors
Frequent in in vitro fertilized embryos
Gross chromosomal rearrangements during in vitro fertilization.
40% of embryos carried entire chromosome imbalance
Gain or loss of segments of chromosomes
CNV- copy number variation of chromosome segments
5% of individuals genome displays CNV
Synuclein gene CNV is involved in Parkinsons
Many cancers- malignant cells most often gain additional copies
of chromosome segments- genes in these segments are misexpressed or mis-express other genes)
Microarray hybridization of DNA from tissues of identical twinsdifferences at several loci seen (Bruder et al 2008)
Microarray hybridization of DNA from different tissues of single
individual (Piotrowski et al., 2008)
Gross chromosomal rearrangements during in vitro fertilization
55% of embryos carried terminal imbalance (sub-telomere loss)
(Vanneste et al., 2009) -microarray based screen of IVF 35
embryo
20
Gross chromosomal changes
The Cri du chat syndrome in humans is a result of a deletion in
the short arm of chromosome 5. This was determined by
comparing banding patterns with normal and Cri du Chat
individuals
Types of chromosome rearrangements that can be studied by
karyotype analysis:
GROSS CHROMOSOMAL CHANGES
Deletions, Duplications, Inversions, Translocations
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DDIT
Normal Chromosome
A____B____C________D____E____F
Deletions (deficiency)
A____B____C________D____F
Duplications
A____B____C________D____E____E____F
Inversions
A____B____C________E____D____F
Translocation
A____B____C________D____E____F
A____B____C________D____L
H____I____J________K____L
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H____I____J________K____E____F
Insertion and deletions are frequent: Sequencing 179 humans
as part of the 1000 genome project:
On average, each person is found to carry approximately
250 to 300 loss-of-function variants in genes of which 50
are in genes previously implicated in inherited disorders.
20,000 large structural variants were identified and 1.3
million short indels were identified.
Variation detected by the project is not evenly distributed
across the genome: certain regions, such as subtelomeric
regions, show high rates of variation.
1 in 500 children have reciprocal translocation but Such
translocations are usually harmless. (However gametes
produced by the children will have defects).
1 in 50 children have inversions (small and large). The
heterozygous inversion carrier generally show no adverse
phenotype (but produce abnormal meiotic products from
crossing-over in the inversion loop).
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Deletions
Deletions are often detected cytologically by comparing
banding patterns between the normal and the partially
deleted chromosomes
Deleted
segment
Chromosome no
female
deletion
chromosome1
Band
46,XX, del(1)(q24q31)
Female with a deletion of chromosome 1 on the long arm (q)
between bands q24 to q31.
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In many instances deletions are too small to be detected
cytologically. In these instances genetic/molecular techniques
are used.
Since cytological deletions remove a contiguous set of genes,
there is a high probability that an essential gene will be
deleted. Therefore deletions will survive as heterozygotes
and not homozygotes.
A____B________C____D
Normal
A____B________C____D
A____________C____D
A____________C____D
A____________C____D
A____B________C____D
Homologous deletion
(Lethal?)
Heterologous deletion
(NOT Lethal)
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Consequences of deletions
A+_____B+_____C+___________D+
Normal
A+_____B+_____C+___________D+
B+
A+____/ \_____C+___________D+
A+___________C+___________D+
In individuals heterozygous
for the deletion, pairing is
disrupted in the regions
surrounding the deletion.
Therefore recombination is
also significantly reduced in
these regions.
Genotype
A+_____b______c____________D+
Normal
A+_____B+_____C+___________D+
A+____b______c____________D+
A+___
_____C+___________D+
A deletion on one
homologue unmasks
recessive alleles on the
other homologue. The
effect is called pseudodominance.
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Deletions in X
Females in Drosophila
XX
Males in Drosophila
XY or XO
Deletion series
phenotype
sick
dead
sick
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Changes in chromosome structure
Deletions:
1.
Hemizygosity from large deletions results in lethalityeven the smallest cytologically defined deletions take out
tens of 1,000's of bps and are likely to remove essential
genes.
2. Organisms can tolerate hemizygosity from small but not
large deletions. The reason for this is not entirely clear
and is placed under the rubric of disrupting the overall
ratio of gene products produced by the organism
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Deletion mapping
Deficiency mapping or deletion mapping:
This provides a means of rapidly mapping a new mutation
A deficiency or deletion is the loss of a contiguous series
of nucleotides
ATGATCGGGCCCATCAAAAAAAAAAAATCATCCCCCGGGG
DELETION
ATGATCGGGCCCATC
CATCCCCCGGGG
ATGATCGGGCCCATC|CATCCCCCGGGG
Defined deficiencies are very useful for mapping genes
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Deficiency mapping
Say we have 6 sites defined by point mutations within
the rosy gene
---1-----2-----3-----4-----5-----6
------------------------------------------2--------------------------------------------4-------------------------DDDDDDDDDD---------Can we get intragenic recombinants that will restore
normal rosy gene?
ry2 and ry4?
Y
ry2 and the deletion?
Y
ry4 and the deletion
N
Say we isolate a new ry mutation you call it ry(zany)
You cross it to the deletion and do not find any
Recombinants
Where does ry(z) map
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Deficiency mapping
Say we have 6 sites defined by point mutations within
the rosy gene
---1-----2-----3-----4-----5-----6
------------------------------------------2--------------------------------------------4-------------------------DDDDDDDDDD---------Can we get intragenic recombinants that will restore
normal rosy gene?
ry2 and ry4?
YES
ry2 and the deletion?
YES
ry4 and the deletion?
No
Say we isolate a new ry mutation you call it ry(z)
You cross it to the deletion and do not find any
recombinants
Where does ry(z) map?
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Deficiency mapping
Generate a heterozygote
Gene point mutant/deletion mutant
Ask if you get intragenic recombinants
Heterozygote will be pseudodominant
The single point mutation will be observed over the deletion
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Multiple deficiencies
Specific deletions can define a series of regions within
a gene
Gene
---1-----2-----3-----4-----5-----6----7----8--
---------------------------------------------DDDDDDDDDDDDDDDDDDDDDDDDDDDDD----------------------------------DDDDDDDDDDDDDDDDDDDDDD------
These two deletions define 4 regions within the gene
I
II
III
IV
Now say a newly isolated mutation does not produce
normal recombinants with both deletions
To which region does it map?
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Multiple deficiencies
Multiple deletions can define a series of regions within
a gene
Gene
---1-----2-----3-----4-----5-----6----7----8--
---------------------------------------------DDDDDDDDDDDDDDDDDDDDDDDDDDDDD----------------------------------DDDDDDDDDDDDDDDDDDDDDD------
These two deletions define 4 regions within the gene
 4-7
 1-5
I
+
-
II
-
III
+
IV
+
+
+ = If a mutation maps to this region, normal
recombinant flies are produced
- = If a mutation maps to this region, normal
recombinant flies are NOT produced
Now say a newly isolated white mutation does not produce
normal recombinants with both deletions
To which region does it map?
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Duplications
A____B____C________D____E____F
A____B____C________D____E____E____F
normal
Duplication
Individuals bearing a duplication possess three copies of the genes
present in the duplicated region.
In general, for a given chromosomal region, organisms tolerate
duplications much better than deletions.
46,XY, dup(7)(q11.2q22)
Male with a duplication of chromosome 7 on the long arm (q) 35
between bands 11.2 to 22
Tandem duplications- Important class of duplications!!!
This is a case in which the duplicated segment lies adjacent to
the original chromosomal segment
A B C D ------ A B C B C B C B C D
Once a tandem duplication arises in a population, even more
copies may arise because of asymmetrical pairing at meiosis.
Remember when the homologs pair during prophase of meiosis
I, they line up base-pair for base pair. Duplications lead to
mistakes in this pairing mechanism
36
Proper pairing:
A____B____C____B____C____D____E
A____B____C____B____C____D____E
A____B____C____B____C____D____E
A____B____C____B____C____D____E
Inappropriate pairing:
A____B____C____B____C____D____E
A____B____C____B____C____D____E
A____B____C____B____C__-----------__D____E
A____B____C____B____C__-----------__D____E
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Tandem duplications expand by mistakes in meiosisI during
pairing
A
b
B
c
C
d
D
e
E
B
a
C
b
c
38
Tandem duplications expand by mistakes in meiosis during
pairing
Paired non-sister chromatids
B
C
A
B
C
D
E
a
b
c
d
e
b
c
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What happens if you get a crossover after mis-pairing in
meiosisI?
A
B
C
B
C
D
A
B
C
B
C
D
b
c
b
c
a
b
a
B
c
b
c
d
d
C
A
B
C
D
E
a
b
c
d
e
b
c
A
B
C
B
C
D
A
B
C
B
C
B
A
B
C
D
A
B
C
B
C
D
C
D
40
The four meiotic products of a crossover between regions B
and C:
A-B-C-B-C-D-E
A-B-C-D-E
A-B-C-B-C-B-C-D-E
A-B-C-B-C-D-E
This process may repeat itself many times, such that a small
fragment of the genome is repeated 10,000 times.
41
An example of this is near the centromeres of the Drosophila
genome:
If you look at the DNA sequence in this region it consists of
small 5-10 bp sequences (AATAC)n repeated 1,000s of times.
It is believed to have arisen from unequal crossing over.
Repetitive DNA- cell does not like it- They try to reduce
recombination of repetitive DNA by packaging the DNA with
proteins to form heterochromatin- cold spots of recombination
along the chromosome
42
Duplications provide additional genetic material capable of
evolving new function. For example in the above situation if
the duplication for the B and C genes becomes fixed in the
population- the additional copies of B and C are free to evolve
new or modified functions.
This is one explanation for the origin of the tandemly
repeated globin genes in humans. Each of these has a unique
developmental expression pattern and provides a specialized
function.
The hemoglobin in fetus has a higher affinity for oxygen since
it acquires its oxygen from maternal hemoglobin via
competition
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Two distinct globin chains (each with its individual heme
molecule) combine to form hemoglobin.
One of the chains is designated alpha. The second chain is
called "non-alpha".
The fetus has a distinct non-alpha chain called gamma. After
birth, a different non-alpha globin chain, called beta, pairs
with the alpha chain. The combination of two alpha chains and
two non-alpha chains produces a complete hemoglobin molecule.
The genes that encode the alpha globin chains are on
chromosome 16. Those that encode the non-alpha globin
chains are on chromosome 11.
The alpha gene complex is called the "alpha globin locus",
The non-alpha complex is called the "beta globin locus".
The expression of the alpha and non-alpha genes is closely
balanced by an unknown mechanism. Balanced gene expression
is required for normal red cell function. Disruption of the
balance produces a disorder called thalassemia.
The closely linked
globin genes may have
originally arisen from
tandem duplication.
44
Alternatively some duplicated genes accumulate mutations and
are no longer expressed (these are akin to junked cars along the
highway).
These are known as pseudogenes. One of the genes in the
hemoglobin cluster is a pseudogene.
-Gg-Ag-*--
pseudogene
Unequal crossing over among the tandemly repeated hemoglobin
gene cluster is the explanation for some inherited blood diseases.
Hemoglobin lepore
45
Alternatively some duplicated genes accumulate mutations and
are no longer expressed (these are akin to junked cars along the
highway).
The beta-globin gene cluster in humans contains 6 genes, called
epsilon (an embryonic form), gamma-G, gamma-A (the gammas
are fetal forms), pseudo-beta-one (an inactive pseudogene),
delta (1% of adult beta-type globin), and beta (99% of adult
beta-type globin. Gamma-G and gamma-A are very similar,
differing by only 1 amino acid.
These are known as pseudogenes. One of the genes in the
hemoglobin cluster is a pseudogene.
-Gg-Ag-*--
pseudogene
Unequal crossing over among the tandemly repeated hemoglobin
gene cluster is the explanation for some inherited blood diseases.
Hemoglobin lepore anemia
-Gg-Ag--
-Gg-Ag--
-Gg-Ag46
***
•If mispairing in meiosis occurs, followed by a crossover between
delta and beta, the hemoglobin variant Hb-Lepore is formed.
• This is a gene that starts out delta and ends as beta. Since
the gene is controlled by DNA sequences upstream from the
gene, Hb-Lepore is expressed as if it were a delta. That is, it
is expressed at about 1% of the level that beta is expressed.
• Since normal beta globin is absent in Hb-Lepore, the person
47
has severe anemia.
Inversion
Chromosomes in which two breaks occur and the resulting
fragment is rotated 180 degrees and reinserted into the
chromosome.
Inversions involve no change in the amount of genetic material
and therefore they are often genetically viable and show no
abnormalities at the phenotypic level.
Gene fusions may occur
Inversions are defined as to whether they span the centromere
Paracentric inversions do not span the centromere:
A
B
A
C
B
D
D
E
C
E
Pericentric inversions span the centromere:
A
C
B
D
E
In a pericentric inversion one break is in the short arm and one in the
long arm. Therefore an example might read 46,XY,inv(3)(p23q27).
48
A paracenteric inversion does not include the centromere and an
example might be 46,XY,inv(1)(p12p31).
Homologs which are heterozygous for an inversion have
difficulties pairing in meiosis.
During pairing homologous regions associate with one another.
Consequently individuals heterozygous for an inversion will form
a structure known as an inversion loop.
Crossover within inverted region?
A---B---C---D---E---F---G
A’--B’---C’---D’--E’---F’---G’
A---B---C---D---E---F---G
A’--B’---C’---E’---D’--F’---G’
A
B
C
A’
B’
C’
D
E
D’
E’
F
G
‘F
G’
49
The consequence of crossover within a paracentric inversion
a-b-c
d-e
a-b-c
a-b-c
f-g
d-e
e-d
f-g
f-g
During meiosis, pairing leads to formation of an inversion loop
This is a problem if crossing over occurs within the inversion
A
B
C
A’
B’
C’
D
E
D’
E’
F
G
‘F
G’
A-B-0-C-D-E’-C’--0--B’-A’
dicentric-fragmentation
G-F-E-D’-F’-G’
acentric- no segregation
50
The consequence of crossover within a pericentric inversion
(one that spans the centromere).
a-b-c
d-e
a-b-c
a-b-c
f-g
d-e
e-d
f-g
f-g
During meiosis, pairing leads to formation of an inversion loop
This is a problem if crossing over occurs within the inversion
A
B
C
A’
B’
C’
D
E
D’
E’
F
G
‘F
G’
A-B-C-D-0-E’-C’-B’-A’
fragment
G-F-E-0-D’-F’-G’
fragment
51
•Paracentric inversion crosses
over with a normal chromosome,
the resulting chromosomes are an
acentric, with no centromeres,
and a dicentric, with 2
centromeres.
•The acentric chromosome isn't
attached to the spindle, so it gets
lost during cell division, and the
dicentric is usually pulled apart
(broken) by the spindle pulling the
two centromeres in opposite
directions. These conditions are
lethal.
•Pericentric inversion crosses
over with a normal chromosome,
the resulting chromosomes are
duplicated for some genes and
deleted for other genes. (They do
have 1 centromere apiece though).
•The gametes resulting from
these do not produce viable
progeny.
•
•Thus, either kind of inversion
has lethal results when it crosses
over with a normal chromosome.
•The only offspring that survive
are those that didn't have a
crossover or crossed over in
regions outside the inversion.
•Thus when you count the
offspring you only see the noncrossovers, so it appears that
52
crossing over has been
suppressed.
What are the consequences of crossing-over in an individual
homozygous for an inversion?
Genotype for normal individual
A
B
0
C
D
E
F
G
A
B
0
C
D
E
F
G
Genotype of an individual heterozygous for an inversion:
A
B
0
C
D
E
F
G
A
B
0
C
F
E
D
G
Genotype of an individual homozygous for an inversion:
A
B
0
C
F
E
D
G
A
B
0
C
F
E
D
G
53
Translocations
A segment from one chromosome is exchanged with a segment
from another chromosome.
Chromosome 1
A
B
C
D
E
F
----------------------0--------------------------------------------0----------------------A
B
C
D
E
F
Chromosome 2
O
P
Q
R
S
T
----------------------0--------------------------------------------0----------------------O
P
Q
R
S
T
Reciprocal translocation
A
B
C
D
S
T
----------------------0----------------------O
P
Q
R
E
F
----------------------0-----------------------
This is more specifically called a reciprocal translocation and
like inversions (and unlike duplications and deficiencies) no
genetic material is gained or lost in a reciprocal translocation.
54
Non-reciprocal translocations may also occur
long arms of chromosome 7 and
21 have broken off and
switched places. So you can
see a normal 7 and 21, and a
translocated 7 and 21.
This individual has all the
material needed, just switched
around (translocated), so they
should have no health
problems. However there can
be a problem when this person
has children.
Remember that when the
gametes are made, each parent
gives one of each chromosome
pair. What would happen if this
person gave the normal seven
and the 21p with 7q attached?
There are three copies of 7q
instead of two. And there is
only one copy of 21q
t(11;18)(q21;q21) translocation between chromosomes 11 and 18
at bands q21 and q21
Philadelphia chromosome: t(9;22)(q34;q11).
55
As with inversions, individuals heterozygous for a reciprocal
translocation will exhibit abnormalities in chromosome pairing
A
B
C
D
E
F
----------------------0--------------------------------------------0----------------------A
B
C
D
S
T
O
P
Q
R
S
T
----------------------0--------------------------------------------0----------------------O
P
Q
R
E
F
Notice this individual has the normal amount of genetic material
(two copies of each gene).
However it is rearranged.
If the translocated fragment contains a centromere, you could get
dicentri and acentric chromosomes
How will translocated chromosomes pair in meiosis?
56
N1
F
F
E
E
T1
A
B
C
D
R
Q
P
O
A
B
C
D
R
Q
P
O
T2
S
S
T
T
N2
Homologous regions associate with one another.
These chromosomes will follow Mendel's rule of independent of
assortment. In this instance one must focus on the centromere
There are three possible patterns of segregation.
Normal Pairing of 10
chromosomes in maize
57
Chr8-9 translocation
Alternate segregation:
ｷ
N1 and N2 segregate to one pole
ｷ
T1 and T2 segregate the other pole
These gametes have the normal haploid gene content: one copy
of each gene and are normal
Adjacent segregation:
ｷ
N1 and T1 segregate to one pole
ｷ
T2 and N2 segregate to the other pole
These gametes are anueploid: they are missing some genes and
duplicated for other genes.
Adjacent segregation
ｷ
N1 and T2 segregate to one pole
ｷ
N2 and T1 segregate to one pole
Therefore, in a translocation heterozygote, some of the
gametes are viable and some are inviable.
N1
F
F
E
E
T
1
A
B
C
D
R
Q
P
O
A
B
C
D
R
Q
P
O
T
2
S
S
T
T
58
N2
Reciprocal translocations result in genes that are known to map
to different chromosomes but behave as linked genes.
Under normal circumstances genes E and R assort independently
because they are on different chromosomes. However in a
translocation they will behave as closely linked genes and
segregate together.
N1
F
F
E
E
T1
A
B
C
D
R
Q
P
O
A
B
C
D
R
Q
P
O
T2
S
S
T
T
N2
59
Translocations (and inversion) breakpoints sometimes disrupt an
essential gene. That is the break occurs in the middle of a
gene.
In fact because of this, a number of specific translocations are
causally associated with specific human cancers.
The inherited disease Duchenne muscular dystrophy was mapped
through a translocation that specifically disrupted this gene.
60
Karyotypes and chromosome rearrangements
The Philadelphia chromosome:
This is a translocation involving chromosome 9 and 22
Individuals bearing this chromosome develop chronic
myelogenous leukemia.
First example of a chromosome translocation associated with
a human disease.
1
61
Glevec and the Philadelphia chromosome
Abl is a tyrosine kinase.
Function of the normal BCR gene product is not clear.
In chronic myelogenous leukemia, the Philadelphia chromosome
leads to a fusion protein of abl with bcr (breakpoint cluster
region), termed bcr-abl.
This is now a continuously active tyrosine kinase.
Glevec inhibits the abl protein of cancer and non-cancer cells
but cells normally have additional redundant tyrosine kinases
which allow them to continue to function. Tumourogenesis
however is entirely dependent on Bcr-Abl and so these cells get
inactivated.
62
abl/bcr
Fusion protein
Chronic myelogenous and acute lymphotic leukem
ALK/NPM Fusion
Large cell lymphomas
HER2/neu Fusion
Breast and cervical carcinomas
MYH11/CBFB Fusion
Acute myeloid leukemia
ML/RAR
Acute premyelocytic leukemia
Fusion
ERG/TMPRSS2
Fusion
prostate cancer
Gene fusion -prostate cancer -ERG merges with a prostatespecific gene called TMPRSS2. ERG is a transcription factors
63