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Deamination of Cytosine and Deamination of Cytosine and 5-methylcytosine5-methylcytosine
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Chemical MutagensChemical Mutagens
Intercalating AgentsIntercalating Agents EX, EX,
Ethidium Ethidium bromide, bromide, acridine acridine orangeorange
Can Can induce induce frameshiframeshift ft mutationmutationss
Uv Induced DimersUv Induced Dimers Thymine dimers Thymine dimers
and T-C dimersand T-C dimers Replication Replication
problemsproblems Interferes with the Interferes with the
ability of the T’s to ability of the T’s to base pair to the base pair to the opposite strand, opposite strand, and blocks DNA and blocks DNA replicationreplication
Other MutagensOther Mutagens Transposable elements—”jumping genes”.Transposable elements—”jumping genes”.
Major frameshift mutationsMajor frameshift mutations Factors in evolutionFactors in evolution
Mutator genes—mutations increase Mutator genes—mutations increase mutation rate. Four potent mutator genesmutation rate. Four potent mutator genes Mutant DNA pol III 3’Mutant DNA pol III 3’5’ exonuclease activity5’ exonuclease activity Mutant methylation enzymes(ex Mutant methylation enzymes(ex damdam)) Mutant enzymes in excision repair systemMutant enzymes in excision repair system Mutant enzymes in SOS systemMutant enzymes in SOS system
ReversionsReversions Mutations in an mutant can restore the Mutations in an mutant can restore the
wild type function (wild type function (reversion, back reversion, back mutation, or reverse mutationmutation, or reverse mutation))
Spontaneous or inducedSpontaneous or induced If mutation occurs at the site of the If mutation occurs at the site of the
original=original=True reversionTrue reversion Wild type restored by mutation at Wild type restored by mutation at
another site= another site= second site mutationsecond site mutation Second site in same gene= Second site in same gene= intragenic intragenic
suppressionsuppression Second site in another Second site in another gene=intergenic gene=intergenic
suppressionsuppression
Intragenic RevertantIntragenic Revertant TypesTypes
Same site Same site reversionreversion
Second site Second site revertant revertant
NOTE: shape NOTE: shape of R-groups of R-groups can also be a can also be a factor. EXfactor. EX
Reversion of Frameshift Reversion of Frameshift MutationsMutations
For reversion For reversion to be to be successfulsuccessful Reversion must Reversion must
be near original be near original site to reduce site to reduce # of aa altered# of aa altered
Section of Section of polypeptide polypeptide must be able to must be able to withstand withstand alteration alteration without without eliminating eliminating functionfunction
Intergenic SuppressionIntergenic Suppression Refers to a chnge in another gene Refers to a chnge in another gene
which suppresses or eliminates the which suppresses or eliminates the mutant phenotype. EXmutant phenotype. EX Multisubunit proteins—Mutation in one Multisubunit proteins—Mutation in one
subunit may be masked by mutation in subunit may be masked by mutation in another subunit (ex. restoring another subunit (ex. restoring hydrophobic patches)hydrophobic patches)
Suppression via suppressor tRNAsSuppression via suppressor tRNAs
Suppressor tRNASuppressor tRNA Nonsense mutation- aa codon Nonsense mutation- aa codon “Stop” “Stop”
Ex. AAGEx. AAGUAGUAG Some bacteria can “read through” these Some bacteria can “read through” these
mutations (though protein function may mutations (though protein function may be altered). HOW?be altered). HOW?
Mutant tRNA that has an anticodon that Mutant tRNA that has an anticodon that recognizes “Stop” as a reading codon.recognizes “Stop” as a reading codon. aa encoded depends on which tRNA is aa encoded depends on which tRNA is
mutatedmutated Not every suppressor restores normal Not every suppressor restores normal
functionfunction
Suppressor Mutants (cont’d)Suppressor Mutants (cont’d) EX. UUG (Leu)EX. UUG (Leu)UAG (Stop) (AUC anticodon)UAG (Stop) (AUC anticodon) A mutation in a tRNA resulting in “AUC” A mutation in a tRNA resulting in “AUC”
allows that tRNA to recognize “Stop”. allows that tRNA to recognize “Stop”. Can get suppression or partial Can get suppression or partial
suppressionsuppression NOTE: must be 2 copies of tRNA mutated. NOTE: must be 2 copies of tRNA mutated.
Why?Why? In any cell containing mutator, must In any cell containing mutator, must
also be a wild typealso be a wild type Suppressors allow survival, even if Suppressors allow survival, even if
sub-optimalsub-optimal
Termination of Translation in Termination of Translation in Suppressor StrainsSuppressor Strains
Problem: Must be a means of Problem: Must be a means of terminating translation.terminating translation.
HOW?HOW? Release factors still present, will Release factors still present, will
compete for the “Stop “ sitecompete for the “Stop “ site Many genes are double-Many genes are double-
terminatedterminatedEX. UAG-UAAEX. UAG-UAA
Photolyase
Fig. 20.39Fig. 20.40
O6-methylguanine methyl transferase
Repair by Direct Repair by Direct ReversalReversal
Excision Repair Excision Repair Most Most
common common repair repair mechanismmechanism
EX. Uvr EX. Uvr systemsystem
NOTE: NOTE: preferentiallpreferentially repairs y repairs dimers in dimers in essential essential regions of regions of genomegenome
UvRA recognizes UvRA recognizes damage and damage and binds w/UvrBbinds w/UvrB
UvRA released, UvRA released, UvrC bindsUvrC binds
UvrC nicks on UvrC nicks on both sides of both sides of damageUvrD damageUvrD unwinds regionunwinds region
Damaged strand Damaged strand releasedreleased
DAN pol IDAN pol I LigaseLigase
RecombinatioRecombinational Repairnal Repair
Sister Strand Sister Strand ExchangeExchange
Recombinational Repair, aka, Recombinational Repair, aka, Sister Strand ExchangeSister Strand Exchange
SOS Response SOS Response Is an inducible system of last resortIs an inducible system of last resort Also called error prone replication Also called error prone replication
because it inactivates the because it inactivates the proofreading function of DNA pol III. proofreading function of DNA pol III.
Turned on only when DNA damage is Turned on only when DNA damage is extremeextreme
Main players: recA and lexA and a Main players: recA and lexA and a battery of inducible enzymesbattery of inducible enzymes
SOS Response SOS Response