Transcript Mutations, Mutagenesis, and Repair

Mutations, Mutagenesis,
and Repair
Chapter 10
The Problem
 DNA
extremely long, fragile
 Subject to both physical and
chemical damage
 Consequences could be lethal for
organism or offspring
Mutation
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A heritable change in the base sequence
of DNA
Point mutation- change in a single base
position
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}
Additions
Frameshift mutations
Deletions
Substitutions
Transitions
 Transversions
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Multiple mutations
Consequences of Mutation
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Silent Mutation---base change, no amino acid
change
Neutral Mutation--- Base change resulting in aa
change that does not affect protein function
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EX. Apartic acid (D) Glutamic acid (E)
Missense mutation---altered codon, new aa with
different chemical properties. Function affected.
Nonsense mutation---base pair substitution
results in a stop codon (and shorter polypeptide)
Frameshift mutations—additions or deletions.
Peptide may be longer or shorter.
Sense mutation?
Other Terms
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Conditional Mutation—wild type function
except under certain (permissive)
conditions
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Ex. Temperature sensitive mutants show
mutant phenotype only at certain
temperatures
Leaky mutations— a missense amino acid
change that reduces but doesn’t eliminate
protein function
Mutagenesis
 The
process of mutation
 Mutagen—anything that promotes
ort causes mutations
 Chemical
 Physical
Mutation-Causes
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Incorrect base pairing due to tautomeric
shifts
Removal of nitrogenous bases
Alteration of nitrogenous bases
Addition or deletion of nucleotides
Single strand breaks
Double strand breaks
Crosslinking—covalent linkage between
bases
Spontaneous Mutations
Arise without mutagenic agents. DNA pol
has proofreading function, can remove
mismatched base
 Even if DNA pol misses a mismatch other
systems can recognize and repair it.
 Recognition?
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Hemimethylation-allows enzymes to
distinguish between parent and daughter
strands.
Spontaneous Mutations
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Tautomeric shifts during replication.
Depurination—if a purine base is lost from
C-1 of deoxyribose, will get apurinic site.
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Odds of misincorporation on the daughter
strand=75%
Enzymes specific for this type of mutation
have evolved
Deamination.
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CU
AHypoxanthine
} Altered H-bonding
Tautomers and Mutation
Normal base
pairing
Rare imino forms
of adenine and
cytosine
Rare enol forms
of thymine and
guanine
Back
Deamination of C and A
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CU
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3 Hbonds
w/G2
H-bonds
w/A
AHypoxanthine
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2 Hbonds
w/G3
H-bonds
w/C
Removing and Replacing Uracil
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Uracil automatically removed from DNA by
uracil N-glycosylase
AP Endonuclease cuts 5’
to
apurinic site
Sugar phosphate removed
by phosphodiesterase
DNA pol I adds correct base
Ligase seals
Base Excision Repair (BER)
Base Excision Repair (BER)
P
P
P
P
P
P
A
G
G
C Uracil DNA glycosylase
A
T
U
C
G
T
P
P
P
P
P
G
P
P
G
C
C
G
P
P
P
AP endonuclease
P
P
A
P
G
T
G
C
P
P
C
C
P
P
DNA polymerase I
G
P
DNA ligase
P
P
A
P
G
T
P
G
C
G
P
P
5 Methyl Cytosine Deamination
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Easily
recognized
and
corrected
What about
5-methyl
cytosine?
Is there a
problem?
Always
remove T
from a GT
pair
?
Deamination of Cytosine and 5methylcytosine
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Induced Mutations
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Caused by exposure to a mutagen
Causes
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Exposure to base analogs
Chemical mutagens
Intercalating agents
Uv- radiation
Transposable elements
Mutator genes
Exposure to Bases Analogs
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Base analogs—
substances that are
similar to and can
substitute for
standard bases
Examples—AZT, 5bromouracil (5-BU)
and 2-aminopurine
(2-AP)
5 Bromouracil
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The Problem: 5
THE PROCESS
bromouracil
A·T
Replication in
assumes the enol
presence of BrU
form at a much
A·BrU
higher frequency
Tautomeric shift
than T
if it replaces T, will
A·BrU*
probably get a
Replication
mutation due to
A·T + G·BrU*
tautomerization
Replication
during replication
Result: A·T G·C
A·T + A·T + G·BrU* + GC