Mutation and polymorphism

1
Mutation and polymorphism mutation change
in the nucleotide sequence or arrangement in
DNA genomic mutation change in chromosome
number chromosome mutations change in
chromosome structure gene mutations change in
an individual gene
2
Mutation and polymorphism mutation change
in the nucleotide sequence or arrangement in
DNA genomic mutation change in chromosome
number chromosome mutations change in
chromosome structure gene mutations change in
an individual gene mutation may occur in a
somatic cell or in a germ cell germ cells give
rise to egg and sperm
3
genomic mutation change in chromosome number
error in separation in mitosis or
meiosis non-disjunction results in
aneuploidy occurs in 1 in 25-50 meiotic cell
divisions a change in chromosome number may
have serious consequences spontaneous
abortions in many (most?) cases monosomy
and trisomy Turners syndrome - 45, X
Kleinfelters syndrome 47, XXY
4
chromosome mutation change in chromosome
structure part of chromosome
is duplicated deleted inverted translocated
(moved to another spot on the same
chromosome or another chromosome) occurs in
1 in 1700 cell divisions rarely transmitted
to the next generation may cause
transformation of cell to a cancer cell
5
gene mutation change in nucleotide sequence in
a gene may involve a single base pair to
thousands of base pairs due to an error in
replication or failure of repair mechanism (most
often replication errors, failure of
proof-reading) occurs in 1 in 10 million
base pairs during replication often
invisible may be spontaneous or may be
induced spontaneous are generally replication
errors induced are due to exposure to mutagen

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gene mutation change in nucleotide sequence in
a gene DNA change loss of a base or
change in base structure may be
spontaneous may be caused by
mutagens x-rays, gamma rays or UV irradiation
causes loss of a base or changes base
structure these changes are not as
readily repaired as replication errors
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molecular basis of mutation point mutations
involve one to a few base pairs point
mutations generate alleles different expressions
of a single gene 2 alleles at a given locus
polymorphism locus is polymorphic e.g.,
ABO blood groups individual with two
identical alleles homozygous individual
with two different alleles heterozygous
alleles present constitute the genetic makeup of
individual genotype genetic composition at a
locus phenotype appearance of the individual
genotype not always evident from phenotype
8
molecular basis of mutation point mutations
involve one to a few base pairs base pair
substitutions (nucleotide substitutions) transiti
ons purine replaces purine A-T ? G-C
transversions purine replaces pyrimidine A-T
? C-G or T-A
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molecular basis of mutation transitions more
frequent hot-spots in DNA cytosine is
methylated esp. at C on 5 side of a G
5----CG-----3 deamination of methylated
cytosine occurs C converted to T
5---CG---3 5---TG---3
3---GC---5 3---GC---5 DNA replication or
repair 5---CG---3 5---TG---3
3---GC---5 3---AC---5
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effect of base pair substitutions (nucleotide
substitutions) missense mutations change
the codon different amino acid
inserted generate an incorrect stop
codon affect transcription reduce amount
of mRNA (alter RNA polymerase binding
site) affect mRNA processing abolish
splicing site provide alternate splicing site
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insertions and deletions may involve a small
to a large portion of a gene or entire
gene often detectable with molecular
techniques Southern blot or PCR
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Southern blot DNA cut with restriction
enzymes bacterial enzymes which cut DNA at
specific sequences sequences usually 6 base
pairs long, palindromes
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Southern blot
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Southern blot ASO allele-specific
oligonucleotide
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PCR polymerase chain reaction generate
large amounts of DNA of interest test with
allele-specific probes
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insertions and deletions may involve a small
to a large portion of a gene or entire gene
small deletions or insertions affect only
a small number of base pairs frameshift
mutations one to two base pairs changes the
codons from the mutation point onward ----ACG
GCAUUUCGCACG--- ----ACGGCAUUUCCGCACG--- ----ACGC
AUUUCGCACG-----
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insertions and deletions large deletions
more common than insertions insertions may
be due to L1 family of repetitive DNA L1 copied
into RNA, RNA copied into free DNA free DNA
inserts anywhere in genome some cases of
hemophilia A have an L1 region inserted,
inactivating the gene
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insertions and deletions large deletions
caused by recombination errors errors in
crossing over in meiosis I, chromosomes
pair very tight pairing brings homologous
regions together breakage and exchange may occur
between homologs crossing over
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insertions and deletions repeated sequences
may align, followed by crossing over
inversion
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crossing over breakage exchange
generally reciprocal
21
crossing over breakage exchange
unequal crossing over leads to deletion on one
chromosome leads to duplication on the other
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duplications newly discovered
mutations Huntington disease fragile X
syndrome trinucleotide repeat in the coding
region of a gene or in a transcribed but not
translated region trinucleotide expands
and interferes with gene expression - generates
an abnormal protein - alters mRNA transcription
or processing ---ABCABCABCDEFGHIJ--- (3
ABC) ---ABCABCABCABCABCDEFGHIJ--- (5ABC)
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Genetic diversity many changes dont affect
gene function - dont alter protein structure or
activity - are in non-coding regions of the
genome result is genetic diversity generates
genetic polymorphism
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Genetic polymorphism within every 1000 base
pairs, there is about 1 difference between any 2
individuals alleles different version of a
DNA sequence in a gene locus when alleles are
common (more than 1 of the chromosomes in the
population), the alleles constitute a genetic
polymorphism (if less than 1 - rare variants)
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Genetic polymorphism ABO blood typing system
and Rh factor RBC have a surface antigen with
two variants A B phenotypes O A B
AB gene on chromosome 9 3 alleles A B O
A B are codominant, O is recessive product of
A adds N-acetylgalactosamine to H
protein product of B adds galactose to H
protein no product from O no modification of H
protein genes A B 4 base pair difference
amino acid differences change enzyme gene O
1 base pair difference, a deletion
frameshift mutation
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Genetic polymorphism Rh factor - plays a
role in hemolytic disease in neonates
and transfusions phenotypes Rh and Rh-
gene on chromosome 1 Rh have polypeptide on
RBC Rh- dont have the polypeptide
hemolytic disease in neonates Rh- mother forms
antibodies to Rh factor during pregnancy
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Genetic polymorphism serum protein
polymorphism a1-antitrypsin enzyme
elastase from leukocytes breaks down elastin
a1-antitrypsin blocks elastase activity
deficiency early onset emphysema gene for
a1-antitrypsin on chromosome 14 several rare
alleles each a different version of the
protein Z allele medically significant
several common alleles M1, M2 and M3
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genetic diversity changes in nucleotide
sequence in non-coding areas are not visible in
protein changes in nucleotide sequence may
alter cleavage sites of restriction enzymes
changes sizes of DNA fragments everyone has
different distribution of cleavage sites for
restriction enzymes gives variation in
Southern blots
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Restriction Fragment Length Polymorphism RFLP
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Restriction Fragment Length Polymorphism RFLP
31
RFLP based on change in restriction cleavage
site based on insertion/deletion between
cleavage sites variable amount of DNA one
class due to insertion of tandem repeat of a
10-100 bp sequence minisatellite
class known as Variable Nucleotide Tandem
Repeat VNTR
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VNTRVariable Nucleotide Tandem Repeat DNA
fingerprinting three sets of twins two
identical one fraternal
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microsatellite markers