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Mutations
 mutations – errors in the DNA
 can have a bad resultant effect
 can have no effect
 can have a positive resultant effect
Mutations are usually not an issue because
we have two copies of each gene.
Mutations
Changes in DNA result in:
1.
silent mutations
2.
missense mutations
3.
nonsense mutations
Silent Mutations
 silent mutation – has no effect on the function of the
cell; sequence of amino acids is not affected
 often found in the non-coding regions (introns)
 If introns in mRNA are spliced out, then the mutation
never surfaces
 Some amino acids can be coded for by multiple
codons. (referred to as redundancy in the genetic
code
 It is possible that one change in the base pair will still
produce the same amino acid
Missense Mutation
 missense mutation – nucleotide sequence is
changed such that a different amino acid is coded for
 protein function may or may not be affected
Sickle cell anemia is a result of a missense mutation.
Sickle Cell Anemia
Wild-type hemoglobin DNA
3
Mutant hemoglobin DNA
5
C T
T
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
G U A
The mutant mRNA has
a U instead of an A in
one codon.
3
5
T
C A
mRNA
mRNA
G A
A
5
3
5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Sickle Cell Anemia
Nonsense Mutations
 nonsense mutation – nucleotide sequence is
changed such that an amino acid is replaced with an
early stop codon
 short proteins are made; usually digested by
proteases
 nonsense mutations usually are lethal
DNA:
5' - ATG ACT CAC CGA GCG CGA AGC TGA - 3‘
3' - TAC TGA GTG GCT CGC GCT TCG ACT - 5'
Template
Strand
mRNA: 5' - AUG ACU CAC CGA GCG CGA AGC UGA - 3'
Protein:
Met Thr His Arg Ala Arg Ser Stop
DNA:
5' - ATG ACT CAC TGA GCG CGA AGC TGA - 3‘
3' - TAC TGA GTG ACT CGC GCT TCG ACT - 5‘
mRNA: 5' - AUG ACU CAC UGA GCG CGU AGC UGA - 3‘
Protein:
Met Thr His Stop
Mutation
Type of DNA Change:
Point Mutations
 point mutation – nucleotide changes where one base
pair (bp) is replaced by another bp
Point mutations are also known as substitution
mutations.
Types of DNA Change
Point Mutation (Base-Pair Substitution)
2. Frameshift Mutation
3. Chromosomal Mutation
1.
Silent
mutation
Type of DNA Change:
Frameshift Mutation
 frameshift mutations – changes in the DNA
sequence which modify the translation reading frame
1. Deletions – removal of one or more NTs
2. Insertions – addition of one or more NTs
Type of DNA Change:
Chromosomal Mutations
Sections of chromosomes can break off and reattach
abnormally.
1. Translocation mutations
2. Inversion mutations
Chromosomal Mutation
 translocation mutations – large portions of DNA are
exchanged
Translocation
Chromosomal Mutations
 inversion mutations – sections of a chromosome are
reversed
Causes of Mutations
1.
spontaneous errors due to enzymes
2.
induced errors due to mutagenic agents



3.
UV radiation
X-rays
chemical (Agent Orange)
transposable elements
Spontaneous Errors
 guanine may react with
water to form 8-oxo
guanine
 8-oxo G pairs up with
adenine during
replication
Visualization of Mutations
Transposable Elements
 some DNA sequences
naturally move
themselves around
the genome -
transposons
Classwork/Homework
 Pg. 263 #1-8