Transcript Deamination of Cytosine and 5

Deamination of Cytosine and 5methylcytosine
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Chemical Mutagens
Intercalating Agents
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EX,
Ethidium
bromide,
acridine
orange
Can
induce
frameshift
mutations
Uv Induced Dimers
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Thymine dimers
and T-C dimers
Replication
problems
Interferes with the
ability of the T’s to
base pair to the
opposite strand, and
blocks DNA
replication
Other Mutagens
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Transposable elements—”jumping genes”.
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Major frameshift mutations
Factors in evolution
Mutator genes—mutations increase mutation
rate. Four potent mutator genes
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Mutant
Mutant
Mutant
Mutant
DNA pol III 3’5’ exonuclease activity
methylation enzymes(ex dam)
enzymes in excision repair system
enzymes in SOS system
Reversions
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Mutations in an mutant can restore the wild
type function (reversion, back mutation, or
reverse mutation)
Spontaneous or induced
If mutation occurs at the site of the
original=True reversion
Wild type restored by mutation at another
site= second site mutation
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Second site in same gene= intragenic suppression
Second site in another gene=intergenic
suppression
Intragenic Revertant
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Types
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Same site
reversion
Second site
revertant
NOTE: shape
of R-groups
can also be a
factor. EX
Reversion of Frameshift Mutations
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For reversion to
be successful
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Reversion must
be near original
site to reduce #
of aa altered
Section of
polypeptide
must be able to
withstand
alteration
without
eliminating
function
Intergenic Suppression
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Refers to a chnge in another gene which
suppresses or eliminates the mutant
phenotype. EX
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Multisubunit proteins—Mutation in one
subunit may be masked by mutation in
another subunit (ex. restoring hydrophobic
patches)
Suppression via suppressor tRNAs
Suppressor tRNA
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Nonsense mutation- aa codon  “Stop”
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Some bacteria can “read through” these
mutations (though protein function may
be altered). HOW?
Mutant tRNA that has an anticodon that
recognizes “Stop” as a reading codon.
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Ex. AAGUAG
aa encoded depends on which tRNA is
mutated
Not every suppressor restores normal
function
Suppressor Mutants (cont’d)
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EX. UUG (Leu)UAG (Stop) (AUC anticodon)
A mutation in a tRNA resulting in “AUC” allows
that tRNA to recognize “Stop”.
Can get suppression or partial suppression
NOTE: must be 2 copies of tRNA mutated.
Why?
 In any cell containing mutator, must also
be a wild type
Suppressors allow survival, even if suboptimal
Termination of Translation in
Suppressor Strains
 Problem: Must be a means of
terminating translation.
 HOW?
 Release
factors still present, will
compete for the “Stop “ site
 Many genes are double-terminated
 EX.
UAG-UAA
Repair by Direct
Reversal
Photolyase
O6-methylguanine
methyl transferase
Fig. 20.39
Fig. 20.40
Excision Repair
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Most common
repair
mechanism
EX. Uvr
system
NOTE:
preferentially
repairs dimers
in essential
regions of
genome
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UvRA recognizes
damage and binds
w/UvrB
UvRA released,
UvrC binds
UvrC nicks on both
sides of
damageUvrD
unwinds region
Damaged strand
released
DAN pol I
Ligase
Recombinational
Repair
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Sister Strand
Exchange
Recombinational Repair, aka, Sister
Strand Exchange
SOS Response
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Is an inducible system of last resort
Also called error prone replication because
it inactivates the proofreading function of
DNA pol III.
Turned on only when DNA damage is
extreme
Main players: recA and lexA and a battery
of inducible enzymes
SOS Response