Download Genetic Mutations SDK Nov 2, 2012

Genetic Mutations
SDK
October 8, 2013
OBJECTIVES
By the end of this session the student should be able to:
 Define Mutation
 Frequency of mutations in normal individuals
 Classify different types of mutation
 Explain the mechanism of mutation
 Explain the role of mutation in biodiversity
 Explain how mutations can cause severe diseases
 Give examples of deletions, duplications, and insertions in genes
 Define trinucleotide repeat expansions and how they cause
neurological diseases
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What is a gene mutation?
Replacement or change of a nucleotide base with another,
in one or both strands, or addition or deletion of a base
pair in a DNA molecule .
Mutations are changes in genetic material(Nitrogenous
bases) – changes in DNA code – thus a change in a
gene(s)
In gene mutations, the DNA code will have a base
(or more) missing, added, or exchanged in a codon.
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Gene mutation out come
• Mutations can lead to missing or malformed
proteins, and that can lead to disease.
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Types of Mutations
 Germ-line mutations .Mutations that are inherited
from parents are called germ-line mutations.
 Acquired mutations. Mutations that are acquired
during your lifetime are called acquired mutations
 Some mutations happen during cell division, when
DNA gets duplicated.
 Still other mutations are caused when DNA gets
damaged by environmental factors, including UV
radiation, chemicals, and viruses.
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How common are mutations?
Mutations occurs at a frequency of about 1
in every 1 billion base pairs
Everybody has about 5-10 potentially
deadly mutations in our genes- in each cell
of our body!
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When everyone has mutations, Why
they are not always seen
Diseases
by because
just
one
of a
•They
Most
genetic
diseases
arecopy
recessive,
areinherited
notcaused
always
seen
the
mutation
which
means
that
ainperson
must
inherit
defective
gene
are not
manifested
with
the
may have
occurred
a section
of DNA
thattwo
copies of
the mutated
gene to inherit a
exception
ofdoesn’t
make a protein.
disorder.
Huntington's
disease, which is rare and
• This is one
reason are
that marriage
between
afflicted
carriers
more likely
toclose
die
relatives
is
discouraged;
two
genetically
before reproducing.
similar adults are more likely to give a child
two copies of a defective gene.
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Mutations Outcome
The affected gene may still function.
Mutations may be harmful.
Mutations may be beneficial.
Mutations may have no effect on the organism.
 Mutations are a major source of genetic variation in a
population increasing biodiversity.
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Mutations a cause of Biodiversity
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Does all mutations passed on to next
generation?
NO
Only mutations in gametes (egg & sperm) are
passed onto offspring(Germline Mutation).
Mutations in somatic cells (body cells) only
affect the body in which they occur and are not
passed onto offspring.
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Brain Work 1
A mutation may happen in any gene.
TRUE OR FALSE?
TRUE
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Spontaneous and Induced Mutations
Spontaneous: Occur spontaneously without
obvious reason.
Induced mutations: caused by mutagens.
Mutagens are the agent that causes the DNA
code to change (mutate)
X-Ray,
Chemicals,
UV light,
Radiation, etc
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Brain Work 2
Which of the following may cause mutations?
A.
B.
C.
D.
Coffee
UV light (sun light)
Hair gel
Vaccines
UV Light (Sun Light)
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Types of Mutations
1. Point mutations. A point mutation is a simple change in
one base of the gene sequence.
• Original
• Point Mutation
The fat cat ate the wee rat.
The fat cat ate the wet rat.
2. Frame shift mutations. one or more bases are inserted
or deleted
Original.
Frame Shift
T h e f a t c a t a t e t h e we e r a t
The fat caa tet hew eer at
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Morphological Types of Point mutations
1. Transitions. Transitions occur when a
 Purine is converted to a purine
(A to G or G to A)
 Pyrimide is converted to a pyrimidine (T to C or C to T)
2. Transversion. A transversion results when
 Purine is converted to a pyrimidine (A to C or G to T)
 Pyrimidine is converted to a purine. (T to A or C to G)
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Types of Mutations according to their effects on
the protein (or mRNA).
Silent Mutations. Mutation in a codons that produce same amino
acid. These mutations affect the DNA but not the protein.
Therefore they have no effect on the organism’s phenotype.
CUU
CUC
Missense Mutations. Missense mutations substitute one amino
acid for another. Example. HbS, Sickle Cell Hemoglobin, is a
change in the beta-globin gene, where a GAG codon is converted to
GUG.
GAG
GUG
Nonsense mutations. convert an amino acid into a stop codon.
The effect is to shorten the resulting protein. Sometimes this has
only a little effect, however, often nonsense mutations result in
completely non-functional proteins.
UUU
UAA\ UGA
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Frame-shift
 In a frameshift mutation one or more bases are inserted, or
deleted.
 Because our cells read our DNA in three letter words,
adding or removing one letter changes each subsequent
word.
 This type of mutation can make the DNA sequence
meaningless .
 For example:
 Original=
T H E FAT C AT AT E T H E W E E R AT
 FRAMESHIFT= T H E FAT C A A T E T H E W E E R AT.
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Brain Work 3
Mutations are a natural part of the cellular
process reproduction. The cell has tools that
catch and repair 99.9% of mutations.
TRUE OR FALSE?
TRUE
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Brain Work 4
Most mutations are caught and repaired in the cell.
TRUE or FALSE?
TRUE
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Classical Types of Point mutation Mutations
Point mutation occurs when the base sequence of a
codon is changed. (ex. GCA is changed to GAA)
There are 3 types:
•Substitution
•Deletion
•Insertion
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Also called
frameshift
mutations
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Substitution
A substitution is a mutation that exchanges one
base for another (i.e. a change in a single
“chemical letter” such as switching an A to G.
For example:
CTGGAG
CTGGGG
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Substitution Mutations
Normal DNA: CGA – TGC – ATC
Alanine – Threonine - stop
Mutated DNA: CGA – TGC – TTC
Alanine – Threonine - Lysine
What
This
iswill
a substitution
happen
to the
the
mutation
amino
What
The
has
adenine
happened
was replaced
to
DNA?
with
acids?
thymine
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The cat ate the rat
The hat ate the rat
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Clinical Examples
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Sickle Cell Anemia
• Sickle cell anemia is the result of a (substitution) point
mutation in codon 6 of the -globin gene resulting in
the substitution of amino acid glutamic acid by valine
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Sickle Cell Anemia
Under conditions of low oxygen tension, such as
Following exercise or
In an atmosphere containing a low oxygen level,
The following changes occur:




The haemoglobin agglutinates to form insoluble rod-shaped polymers
Red blood cells become distorted and sickle-shaped
The sickle-shaped cells rupture easily causing haemolytic anaemia
The sickle shaped cells tend to block capillaries interfering with the blood flow
to various organs.
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 Thalassaemia
Substitution of C by U in mRNA that is coding  globin
chain of 146 amino acid.
Resulted in the formation of a stop signal UAG in place
CAG of glutamate in codon number 39.
 This result in a shortened globin chain containing only
39 instead of the normal 146 amino acids in the -globin
protein chain.
This protein is functionally useless and is equivalent to
absence of -globin gives clinical symptoms of 
thalassaemia,
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 Thalassaemia
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 Thalassemias
Beta thalassaemia is a genetic disorder in which there is
lack of beta globin.
It may be the result of:
 Deletion of the whole gene so that beta globin cannot not produced
(designated  o )
 Deletion of the promoter region so that transcription cannot occur
(designated  o )
 Deletion of a large part of the gene resulting in a grossly abnormal or
reduced synthesis functional protein (designated  + )
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Clinical Features of  -Thalassaemia
 Haemoglobin A (α2  2) cannot be produced
 Hb F (α2 g2) is produced even in adults
 Hb A2 (α 2 d2) formation is increased
 Eerythrocytes are microcytic (small) due to lack of normal
haemoglobin
 Erythrocytes rupture easily causing severe haemolytic anaemia,
requiring repeated blood transfusions
 The bone marrow expands trying to compensate by increasing
haemopoiesis.
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Clinical Features of  -Thalassaemia
 The bones of the face and skull are thickened causing a
characteristic facial appearance
 The spleen and liver enlarge because haemopoietic tissue forms in
them
 Excess iron accumulates in the blood and is deposited in the heart,
liver, pancreas and other organs (this is because of repeated
transfusions while no blood is actually lost from the body)
 Children have delayed growth and development and are prone to
repeated infections
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Point Mutation In alpha-Globin Gene
“Elongated α Globin Chain, Haemoglobin Constant spring \Wayne
Hb”
 Here the stop codon UAA at position 142 in the alpha (-)
globin gene was substituted by the codon for glutamine.
 Translation of the protein thus continued until a stop codon
was encountered at codon 173.
 The -globin was considerably elongated, resulting in a
variant of haemoglobin termed Haemoglobin Constant spring\
Wayne Hb.
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Elongated α Globin Chain Or Haemoglobin Wayne
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Insertion
Insertions are mutations in which extra base
pairs are inserted into a new place in the DNA.
CTGGAG
C T G G C C TA G
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Insertion Mutations
Normal DNA: CGA – TGC – ATC
Alanine – Threonine – stop
Mutated DNA: CGA – TAG – CAT – C
Alanine – Isoleucine – Valine
This
An
is adenine
an
insertion
was mutation,
inserted
thereby
also
a type
What
will
happen
to the
amino
What
has
happened
pushing all
the other bases
over.a frame.
of frameshift
mutation
acids?
to the
DNA?
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Insertion Mutations
The cat ate the rat
The cca tat eth era t
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Haemophilia A
 An X-linked recessive disorder in which blood clotting does not
occur due to deficiency of clotting factor VIII.
 In most cases the mutation is the result of insertion of a large
segment, consisting of about 3800 bp, in the coding region of the
factor VIII.
 This results in total inactivation of the protein.
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Haemophilia A
 Inherited as a sex linked recessive trait with bleeding
manifestations only in males.
 Genes which control factor VIII and IX production are
located on the x chromosome
 Affected male marries a normal female: none of sons will be
affected, all daughters will be carriers
 Female carrier marries normal male: 50% chance sons will be
affected and 50% chance daughters will be carriers
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Deletion
 Deletions involve removal of one or more base pairs.
 They vary greatly in size from deletion of a single base
to deletion of a whole gene.
 The clinical effects often depend on the size and
location of the deleted part of the gene.

CTGGAG
CT AG
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Deletion Mutations
Normal DNA: C G A – T G C – AT C
Alanine – Threonine – stop
Mutated DNA: C G A – T C A - T C
Alanine – Serine
A guanine
was deleted,
thereby pushing all
What
has happened
What
to the
the will
bases happen
down a frame.
toa the
DNA?
This is called
deletion
mutation, also a
amino
acids?
type
of frameshift
mutation.
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Muscular Dystrophy
Deletions of Dystrophin Gene
Dystrophin is a protein that is an important component of
skeletal muscle.
The dystrophin gene is located on the p arm of the
X chromosome (Xp21.2).
It is a very large gene spanning 2.5 million bp of genomic
DNA and consists of 79 exons coding for a protein of
approximately 3600 amino acids (11kb).
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Muscular Dystrophy
Deletions of Dystrophin Gene
Deletion of the whole or most of the dystrophin gene
Dystrophin may not be produced at all
Or produce in in abnormal forms,
Resulting in Duchenne muscular dystrophy
 This is a severe X-linked recessive disorder that affects boys and is
transmitted by carrier females. X linked Recessive disorder.
 In affected boys there is almost complete lack of dystrophin, muscle
weakness beginning in childhood and increasing progressively in severity so
that the individual is wheel-chair bound at the age of about 15 years.
 Death usually ensues in the early twenties due to respiratory muscle
involvement.
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X linked Recessive disorder
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Becker Muscular Dystrophy(BMD)
 Deletions involving a small non-critical part of the gene result
in altered dystrophin.
 This causes the clinical condition of Becker muscular
dystrophy
 in BMD muscle weakness begins in adolescence
and is very slowly progressive, and affected
individuals may lead an almost normal life.
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Cystic Fibrosis.
 Cystic fibrosis (CF) is a genetic condition that affects many organs
in the body: especially the lungs, pancreas and sweat glands.
 Cystic fibrosis is caused by a mutation in the Cystic Fibrosis Transmembrane Regulator (CFTR) gene, that is located on chromosome
7.
 This gene Produces a trans-membrane protein that regulates the
flow of chloride ions into the cells.
 The most common mutation is termed the ∆508 mutation, which is
a deletion of a single codon at position number 508 in exon 10
of the CFTR gene.
 Homozygotes(both parents need to be the carriers of the defective
gene) for a ∆508 mutation have cystic fibrosis disease.
Gene That Encodes CFTR
 The gene that encodes the CFTR protein is found on
the human chromosome 7, on the long arm at position
q31.2.
 Mutations consist of replacements, duplications,
deletions or shortenings in the CFTR gene.
 This may result in proteins that may not function,
work less effectively, are more quickly degraded, or
are present in inadequate numbers
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Cystic Fibrosis.
 The defective gene produces a defective protein leading to a
blockage in the transportation of the salt, thus leading to
production of thick, sticky mucus.
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Presentation
 The secretion of very thick, sticky mucus causes
 Obstruction of the bronchi and predisposing to pulmonary infections,
 Pancreatic duct obstruction leads to problems with digestion.
 Intestinal and liver problems and
 When it blocks the sweat glands, it leads to loss of excessive salt
through sweat. This leads to imbalance of minerals within the body.
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Paroxysmal Nocturnal
Hemoglobinurea
• Paroxysmal nocturnal hemoglobinuria is a disorder of
blood cells in which absence of specific molecule(GPI
anchor protein, CD55 [Decay Accelerating Factor
(DAF)], and CD59 [Membrane Inhibitor of
Complement Lysis (MIRL)] on the surface of the cells
(particularly RBC) leads to premature destruction of
the cells by the complement system.
• This destruction is intermittent (paroxysmal).
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Paroxysmal Nocturnal
Hemoglobinurea
 GPI anchor protein on the surface of red blood cells
produced by the bone marrow stem cells
 This is caused by a mutation of PIG-A gene,
 PIG-A gene is present on the X chromosome important in
making GPI protein anchors.
 Defect makes the red cells in susceptible to destruction
by the complement system.
Paroxysmal Nocturnal
Hemoglobinurea
 The PIG-A mutation occurs in a bone marrow stem cell.
 All the blood cells made by this defective stem cell are deficient in
GPI-anchored proteins. (glycosyl-phosphatidylinositol GPI).
 The GPI-anchored proteins are present on the surface of red blood
cells that protect red cells from the activity of the complement
system.
 When they are absent , no protection from complement and this
lead to intravascular haemolysis.
Genetics
• PIGA gene(phosphatidylinositol glycan class A) is
present in in the X chromosome and can have several
mutations, from deletions to point mutations.
• The genetic mutation leading to the inability to
synthesize the glycosyl-phosphatidylinositol (GPI)
anchor proteine.
RBC Lysis
Normal RBCs
CD59
PNH RBC
Complement
Activation
Intact RBC
Chronic Hemolysis
Lysed PNH RBCs and free hemoglobin in the plasma
Role of CD55 &CD 59
Lectin Pathway
Carbohydrate structure
Pathogen/Damaged cell
Classical Pathway
Antibody bound to antigen
Immune complexes
Alternative Pathway
Microbial membranes
Bacterial LPS
C3 convertase
C4b2a, C3bBb
C3
C3a
C3b
Weak anaphylatoxin
CD55
C5 convertases
C4b2a3b, C3bBb3b
CD55
C5
C5a
C6,C7,C8,C9
CD59
C5b-9
Microorganism Destruction
Red Blood Cell Lysis
Platelet Activation
Inflammation
Cell Activation
Paroxysmal Nocturnal
Hemoglobinurea
• The term "nocturnal" refers to the belief that
hemolysis is triggered by acidosis during sleep and
activates complement to hemolyze an unprotected
and abnormal RBC membrane.
• However, this observation was later disproved.
Hemolysis has been shown to occur throughout the
day and is not actually paroxysmal, but the urine
concentrated overnight produces the dramatic change
in color.
Deletion of 6 codons in the -globin gene resulting
in a variant Hemoglobin
The codons 92 to 97 of the -globin gene are deleted.
This results in a shortened -globin protein that produces
a haemoglobin variant termed Haemoglobin Gun Hill.
In homozygotes it produces mild clinical; symptoms.
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Codon no
Codon
Amino acid
91
CUG
Leucine
92
CAC
Histidine
93
UGU
Cysteine
94
GAC
Aspartic acid
95
AAG
Lysine
96
CUG
Leucine
97
CAC
Histidine
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Frame shift mutations
Frame shift mutations involve a deletion or insertion of
one or two base pairs within a coding sequence of a
gene.
As the coding message is read in triplets codons and
deletions will altered the the reading frame of mRNA
This results in a non-sense sequence of amino acids till
stop codon.
Original=
Frame-shift=
T H E FAT C AT AT E T H E W E E R AT
T H E FAT C A A T E T H E W E E R AT
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Frame Shift Mutations(Deletion)
An example occurs in the -globin gene in which one
nucleotide of codon 39 is deleted leads to altered sequence.
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Frame Shift Mutations(Insertion)
Insertion of a sequence of bases into a coding sequence of
a gene.
 Sometimes a whole gene sequence may be duplicated.
1. Hereditary motor and sensory neuropathy type I
A DNA segment at locus 17p11 is duplicated.
2. Tay-Sachs Disease
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Tay-Sachs Disease
 Tay-sachs disease is an autosomal recessive disorder
 This genetic defect is located in the HEXA (hexosaminidase)
gene, which is found on chromosome 15.
 The hexa gene makes part of an enzyme called betahexosaminidase A
 This enzyme helps break down a fatty substance called GM2
ganglioside in nerve cells.
 Mutations in the HEXA gene disrupt the activity of betahexosaminidase A, preventing the breakdown of the fatty
substances.
 As a result, the fatty substances accumulate to deadly levels in the
brain and spinal cord.
 The buildup of GM2 ganglioside causes progressive damage to the
nerve cells.
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Tay-Sachs Disease
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Tay-Sachs Disease
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Tay-Sachs Disease
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Tay-Sachs Disease S\S
Loss of hearing
Physical and mental retardation
Seizures
Dementia
And most noticeably detected by the red dots it causes
on the retina of an individuals eye
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Trinucleotide Repeat Expansions
 Trinucleotides are triplets of nucleotides that are repeated
 The number of repeats varies in different individuals.
 Trinucleotide repeats is - - - CAG CAG CAG CAG CAG - - -  Trinucleotide repeats are widespread in the genome, and may
occur in exons, introns, promoter sequences or non-coding
regions.
 They are perfectly normal and occur in all individuals
 However a mutation arises when the repeats become unstable and
namelyisan11increase
 undergo
Normal expansion,
range of (CAG)n
to 34 in the number of repeats
as they are transmitted from one generation to the next.
 Huntington's disease appear in individuals in whom the number
 When the number of repeats exceeds a certain limit, clinical
of repeats is greater than 37.
symptoms occur.
 An example is the huntingtin gene, which, when mutated, causes
Huntington's disease.
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Huntington's Disease
Autosomal Dominant Inheritance
Due to an excess of C-A-G nucleotide repeats(>37) in
the HTT gene on the short arm of chromosome 4 forms
(4p 16.3)Huntington's protein.
 Huntington's protein has increase number of glutamine
(polyglutamine).
Excess of repeats causes the protein to form aggregates
that are deposited within the neurons causing neuronal
degeneration
Affects brain and spinal cord, especially the basal
ganglia.
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Trinucleotide Repeat Expansions
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Clinical Manifestations
 Most commonly appear in mid-forties
 If appear at a younger age, more severe
 Manifestations occur because of wasting away of brain cells.
1. Sudden jerky, involuntary movements throughout body
2. Difficulties with balance and coordination
3. Dysphasia
4. Hesitant/slurred speech
5. Progressive dysfunction of intellectual and thought
processes (dementia)
6. Cognitive deficits
a. Working memory loss
b. Reduced capacity to plan, organize, and sequence
7. Restlessness, irritability
8. Depression or Euphoria
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What happens to a person who has a mutation?
 Most of the time the mutation is harmless because
95% sections of DNA do not code for protein (junk
DNA).
 But sometimes the mutations can cause disorders
such as Huntington’s disease and sickle cell anemia
etc.
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Brain Work 5
Which of these is NOT a type of mutation.
a) Point mutation
b) Flyaway mutation
c) Frameshift mutation
d) Nonsense mutation
Flyaway Mutation
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Brain Work 6
1. THE CAT SAW THE FAT RAT
2. THE CAT SAW THE RAT
The change in Statement 1 to form Statement 2 is most
similar to what type of mutation?
A. Insertion
 The correct answer is B deletion.
B. Deletion
C.Because
the sentence is missing the word fat
Substitution
occurs in deletion as it is the removal of a
D.which
Frameshift
section of DNA.
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Brain Work 7
5 ′ A G A U C G A G U 3 ′ → 5 ’A C A U C G A G U 3 ′
The chain above represents three codons.
Which of the following changes would be expected
in the amino acid chain if the mutation shown
above occurred?
A.
B.
C.
D.

The amino acid sequence would be shorter
than expected.
The identity of one amino acid would change.
The amino acid sequence would remain
unchanged.
The identities of more than one amino acid
The
would change.
correct answer is B because
according to the codon
chart if G is switched by a C then one amino acid is
affected because now instead of coding for Arginine it
now codes for Threonine.
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Thank You
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