Download Prof. Kamakaka`s Lecture 12 Notes

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
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Most mutations are harmful in their effects; only rarely are
mutations beneficial.
A gene with one wild-type allele is monomorphic; a gene with
two or more wild-type alleles is polymorphic.
The vast majority of traits are determined by alleles of more
than one gene.
This means that most traits are multifactorial.
A Heterogeneous Trait is One That May be caused by
mutations in more than one gene.
Human deafness is an example of a heterogeneous trait:
mutations in any of at least 50 genes lead to deafness.
(How can one tell if two deaf individuals carry mutations in the same gene or mutations in different
genes?)
An important class of mutations are conditional mutations.
In these mutants the Environment affects Phenotype.
Conditional mutations are those that express their associated
phenotype only under some conditions (restrictive conditions)
and not others (permissive conditions).
Conditional lethal mutations are common.
Temperature-sensitive conditional mutations are invaluable in
genetic research.
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Frameshift mutations
A single base-pair deletion or insertion results in a change
in the reading frame
NNN AUG UUU AGC UUU AGC UUU AGC NNN
Met Phe Ser Phe Ser Phe Ser
Delete C
AUG UUU AGU UUA GCU UUA GC
Met Phe Ser Leu Ala Leu
Insert C
AUG UUU AGC CUU UAG CUU UAG C
Met Phe Ser Leu STOP
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Frameshift mutations
A single base-pair deletion or insertion results in a change
in the reading frame
NNNAUGUUUAGCUUUAGCUUUAGCNNN
…MetPheSerPheSerPheSer…
…CysLeuAlaLeuAlaLeu…
…ValStpLeuStpLeuStp…
Delete C
AUG UUU AGU UUA GCU UUA GC
Met Phe Ser Leu Ala Leu
Insert C
AUG UUU AGC CUU UAG CUU UAG C
Met Phe Ser Leu STOP
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Frameshift mutations- Deletion
A single base-pair deletion or insertion results in a change
in the reading frame
AUG UUU AGC UUU AGC UUU AGC
Met Phe Ser Phe Ser Phe Ser
Delete C
AUG UUU AGU UUA GCU UUA GC
Met Phe Ser Leu Ala Leu
Delete GC
AUG UUU AUU UAG CUU UAG C
Met Phe Ile Stp
Delete AGC
AUG UUU UUU AGC UUU AGC
Met Phe Phe Ser Phe Ser
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Frameshift mutations-Insertion
A single base-pair deletion or insertion results in a change
in the reading frame
AUG UUU AGC UUU AGC UUU AGC
Met Phe Ser Phe Ser Phe Ser
Insert C
AUG UUU AGC CUU UAG CUU UAG C
Met Phe Ser Leu STOP
Insert CC
AUG UUU AGC CCU UUA GCU UUA GC
Met Phe Ser Pro Leu Ala Leu
Insert CCC
AUG UUU AGC CCC UUU AGC UUU AGC
Met Phe Ser Pro Phe Ser Phe Ser
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Missense mutations
Missense mutations alters ONE codon so that it encodes
a different amino acid
UUU UUU UGC UUU UUU
Phe Phe Cys Phe Phe
WT
UUU UUU UGG UUU UUU
Phe Phe Trp Phe Phe
mut
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Consequences of Missense Mutations
Missense mutations alter one of the many amino acids
that make a protein
Its consequences depend on which amino acid is altered
Conservative mutations:
K to R
Nonconservative mutations:
K to E
Surface Vs buried
Mutations in globular domains Vs un structured tails
Silent mutations
Mutations in non-coding regions
Nonsense mutations
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Silent Mutations
Silent mutations do not alter the amino acid sequence!
The Genetic code is degenerate!
AUG UUU AGC UUU AGC UUU AGC
Met Phe Ser Phe Ser Phe Ser
WT
AUG UUC AGC UUU AGC UUU AGC
Met Phe Ser Phe Ser Phe Ser
Mut
Mutations that occur in introns are also silent
Mutations that occur in non-genic regions are often silent
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Mutations in non-protein coding regions
Mutations in the promoter, splicing junction or ribosome
binding site are also mutagenic
Reduced expression of mRNA might result in reduced levels
of proteins
OR
Increased expression of mRNA might result in increased
levels of protein
Mutations in splicing junctions may also be mutagenic
improperly spliced mRNA will result in the intron being
translated
Mutations in tRNA or aminoacyl-tRNA synthase are
mutagenic
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Nonsense mutations
Nonsense mutations alter one codon so that it now encodes
for a STOP codon
UUU UUU UGC UUU UUU
Phe Phe Cys Phe Phe
UUU UUU UGA UUU UUU
Phe Phe STOP
Nonsense mutations insert a stop codon which results in
premature termination
Truncated polypeptide usually results in loss of function
for polypeptide
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There are NO tRNAs in cells with anti-codons that
recognize STOP codons in mRNA
What happens if there is a mutation in the anti-codon
loop of a specific tRNA Gene that allows a tRNA to
recognize a stop codon
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Nonsense suppressor mutations!
These are the result of a mutation in the anti-codon loop of
a specific tRNA Gene
It allows the tRNA to recognize a nonsense codon and base
pair with it.
DNA
Gene encoding tRNATRP
Point mutation occurs in the anticodon loop OF THE tRNA
This allows this tRNA to base pair with a stop codon and ?
Trp
AUG
---UAC---UAG---UAA
Normal tRNA
Trp
Trp
AUG
AUC
--UAC---UAG---UAA
Mutant tRNA
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Nonsense suppressor
--- UUU UUU UAG UUU UUU ------- Phe Phe STOP
Trp-tRNA has mutation
In anticodon
This allows it to pair
with a stop codon
MetAla
Phe
Phe
Trp
AAA AUC
5’--- UUU UUU UAG UUU UUU UAA-----3’
--- Phe Phe Trp Phe Phe |
A mutant protein that is larger than normal will be synthesized!!
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Nonsense and Nonsense suppressor
--- UUU UUU CAG UUU UUU ------- Phe Phe Gln Phe Phe --Nonsense mutation
--- UUU UUU UAG UUU UUU ------- Phe Phe STOP
What will happen if an
individual carries both a
nonsense mutation in a gene
and a nonsense suppressor
mutation in the anticodon
loop of one of the trptRNA genes.
Trp
AUC
---UAG--MetAla
Phe
Phe
Trp
Phe
Phe
AAA AUC AAA AAA
5’--- UUU UUU UAG UUU UUU -----3’
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Phe Phe Trp Phe Phe
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Recombinant DNA
technology
When genes are mutated - proteins are mutatedDISEASE STATES OCCUR
Sickle cell Anemia
Globin
2 alpha globin chains
2 beta globin chains
Mol wt 16100 daltons xfour = 64650 daltons
Single point mutation in beta-globin
Converts Glu to Val at position 6
Need to know mutation
Need to look at genes of individuals
Genes lie buried in 6billion base pairs of DNA
(46 chromosomes).
Molecular analyses necessary
Take advantage of enzymes and reactions that naturally
occur in bacteria
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