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©2001 Timothy G. Standish
RepairRepair
Maintaining the integrity of DNA
©2001 Timothy G. Standish
DNA ModificationDNA Modification Maintaining DNA integrity is vital to its function A number of mechanisms exist to ensure that the sequence of
nucleotides is maintained in DNA Some of these mechanisms involve the chemical modification
of DNA after replication The most common modification is methylation, in which a
methyl group is added to bases on DNA Methylation functions in:
– Distinguishing between a cells DNA and foreign DNA– Distinguishing between old and new DNA strands– Controlling Gene Expression
©2001 Timothy G. Standish
MethylationMethylation 5-Methylcytosine is the most commonly methylated
nulceotide in E. coli.
61
2
34
N O
NH2
ON
NH2
NCytidine
5
N O
NH2
N O
CH3
N
5-MethylcytidineMethylation
NH2
4-Methylcytosine is less common, but is also known.
©2001 Timothy G. Standish
N
N
Adenine
N
N
NH2
MethylationMethylation 6-Methyladenine is another common methylated
nulceotide.
1
34
76
28
9
5Methylation N
N
N6-Methyladenine
N
N
NHH3C
©2001 Timothy G. Standish
E. coliE. coli Methylation Systems Methylation Systems Three methylation systems are known in E. coli:
1 dcm system - Methylates cytosine - Function is unknown
2 dam system - Methylates adenine - Functions in distinguishing new strands of DNA, is involved in control of replication, marks DNA strands for repair and influences transposon activity
3 hsd system - Methylates adenine (cytosine in some bacteria) - Creates specific methylation patterns marking a bacteria’s own DNA and distinguishing it from other species or pathogens’ DNA
©2001 Timothy G. Standish
Destroying Foreign DNADestroying Foreign DNA Methylase enzymes methylate specific bases in specific sequences of
DNA Only the cells own DNA is methylated at a given sequence Thus it is possible to differentiate between the cells DNA and DNA that
has been introduced into a cell by a virus or from some other source In bacteria, restriction enzymes are paired with methylases that
recognize the same sequences Restriction enzymes will not cut methylated DNA Thus restriction endonucleases cut up foreign DNA, but not the cells
DNA Working with methylases, REs restrict bacteriophages to only one host
bacterial strain.
©2001 Timothy G. Standish
Mutation And RepairMutation And Repair Maintaining the integrity of genetic material is vital to
the survival of organisms Somatic cell mutations are known to lead to cancers in
multicelled eukaryotes Mutations in gametes are passed to offspring and most
commonly will result in decreased fitness Elaborate systems for prevention and repair of
mutations are known in prokaryotes and are believed to exist in eukaryotes although, in eukaryotes, these systems have not yet been well characterized
©2001 Timothy G. Standish
MutationsMutations Mutation = A random change in the genetic material of a cell Two major types of mutations:1 Macro mutations:
– Chromosome number mutations
– Addition or deletion of large chunks of DNA
– Movement of large chunks of DNA
2 Point mutations:– Changes in only one or two bases in a gene
Not all mutations result in phenotypic change
©2001 Timothy G. Standish
Micro or Point MutationsMicro or Point Mutations Two major types of Micromutations are recognized:
1 Frame Shift - Loss or addition of one or two nucleotides
2 Substitutions - Replacement of one nucleotide by another one. There are a number of different types:– Transition - Substitution of one purine for another purine, or
one pyrimidine for another pyrimidine (more common)– Transversion - Replacement of a purine with a pyrimidine
or vice versa (less common)
©2001 Timothy G. Standish
Frame Shift MutationsFrame Shift Mutations
5’AGUC-AUG-ACU-UUG-GUA-GUU-GAC-UAG-AAA3’
3’AGTTCAG-TAC-TGA-AAC-CAT-CAA-CTG-ATCATC5’
3’AGTTCAG-TAC-TGA-ACA-CCA-TCA-ACT-GATCATC5’
5’AGUC-AUG-ACU-UGU-GGU-AGU-UGA-CUAGAAA3’
Met Thr Cys Gly Ser
Met Thr ValVal ValLeu
Frame shift mutations tend to have a dramatic effect on proteins as all codons down stream from the mutation are changed and thus code for different amino acids. As a result of the frame shift, the length of the polypeptide may also be changed as a stop codon will probably come at a different spot than the original stop codon.
©2001 Timothy G. Standish
Purine to Pyrimidine
Transversion
Pyrimidine to Pyrimidine
Transition
Substitution MutationsSubstitution Mutations
3’AGTTCAG-TAC-TGA-ATA-CCA-TCA-ACT-GATCATC5’
3’AGTTCAG-TAC-TGA-ACA-CCA-TCA-ACT-GATCATC5’
5’AGUC-AUG-ACU-UGU-GGU-AGU-UGA-CUAGAAA3’
Met Thr Cys Gly Ser
3’AGTTCAG-TAC-TGA-AAA-CCA-TCA-ACT-GATCATC5’
3’AGTTCAG-TAC-TGA-ACA-CCA-TCA-ACT-GATCATC5’
5’AGUC-AUG-ACU-UGU-GGU-AGU-UGA-CUAGAAA3’
Met Thr Cys Gly Ser
5’AGUC-AUG-ACU-UAU-GGU-AGU-UGA-CUAGAAA3’
Met Thr Gly SerTyr
5’AGUC-AUG-ACU-UUU-GGU-AGU-UGA-CUAGAAA3’
Met Thr Gly SerPhe
©2001 Timothy G. Standish
Transitions Vs TransversionsTransitions Vs Transversions Cells have many different mechanisms for preventing
mutations These mechanisms make mutations very uncommon Even when point mutations occur in the DNA, there may
be no change in the protein coded for Because of the way these mechanisms work, transversions
are less likely than transitions Tranversions tend to cause greater change in proteins than
transitions
ValMutant -globin
H2NOH
OH
CO
H2CH
CCH2
C
O Acid
GluNormal -globin
TC T
Normal -globin DNA
AG AmRNA
TC A
Mutant -globin DNA
AG UmRNA
The Sickle Cell Anemia MutationThe Sickle Cell Anemia Mutation
©1998 Timothy G. Standish
H2NOH
CO
H3CH
CCH
CH3
Neutral Non-polar
©2001 Timothy G. Standish
Weakness
Tower skull
Impairedmental function
InfectionsEspeciallyPneumonia
Paralysis Kidneyfailure
Rheumatism
Sickle Cell Anemia:Sickle Cell Anemia:A Pleiotropic TraitA Pleiotropic TraitMutation of base 2 in globin codon 6 from A to T
causing a change in meaning from Glutamate to Valine
Mutant globin is produced
Red blood cells sickle
Heart failure
Pain andFever
Braindamage
Damage to other organs
Spleen damage
Anemia
Accumulation of sickledCells in the spleen
Clogging of smallBlood vessels
Breakdown ofRed blood cells
©2001 Timothy G. Standish
Repair SystemsRepair Systems Direct repair - Uncommon: Direct reversal or removal of damage Excision repair - Common: Recognition of damage followed by
cutting out of damaged strand and replacement with a new strand Mismatch repair - Detection of mismatched bases followed by
excision and replacement of one, generally the one on the new strand
Tolerance systems - Important in higher eukaryotes: Used when DNA is damaged so that replication cannot proceed normally. May involve many errors
Retrieval systems - Important in prokaryotes “Recombination repair” damaged sections of DNA are filled in using recombination
©2001 Timothy G. Standish
Direct RepairDirect Repair The best characterized system of direct repair is
widespread and found in everything from plants to E. coli
DNA strongly absorbs ultraviolet light, this energy may be dissipated by joining adjacent pyrimidines (ie thymine) together to form pyrimidine dimers
Photoreactivation of pyrimidine dimers is achieved by the detection of dimers by a light dependant enzyme that then uses light energy to reverse the reaction and separate the pyrimidines
In E. coli a single enzyme, photolyase (the phr gene product), is responsible for this process
Thymine DimersThymine Dimers
Thymine
Thymine
H
P
O
HO
O
O
CH2
OH
H
P
O
OH
HO
O
O
CH2
O
O
H
H
P OH
O
O
CH2
O
O
H
H OH
P
O
OH
O
O
CH2
NH2
N
N
N
CH 3
O
O
HNN
N
NH2
N
N
N
CH 3
O
O
HNN
N
UVUV LightLight
UVUV LightLight
Thymine DimersThymine Dimers
Thymine
Thymine
OHH
P
O
HO
O
O
CH2
OH
H
P
O
OH
HO
O
O
CH2
O
H OH
O
OCH2
NH2
N
N
N
N
NH2
N
N
N
CH 3
O
O
HNN
N
O
H
H
P
O
O
CH2
O
O
H
P OHOCH 3
O
O
HNN
Ph
otolyaseP
hotolyase
LightLightLightLight
Thymine DimersThymine Dimers
Thymine
Thymine
H
P
O
HO
O
O
CH2
OH
H
P
O
OH
HO
O
O
CH2
O
O
H
H
P OH
O
O
CH2
O
O
H
H OH
P
O
OH
O
O
CH2
NH2
N
N
N
CH 3
O
O
HNN
N
NH2
N
N
N
CH 3
O
O
HNN
N
Ph
otolyaseP
hotolyase
Thymine DimersThymine Dimers
Thymine
Thymine
H
P
O
HO
O
O
CH2
OH
H
P
O
OH
HO
O
O
CH2
O
O
H
H
P OH
O
O
CH2
O
O
H
H OH
P
O
OH
O
O
CH2
NH2
N
N
N
CH 3
O
O
HNN
N
NH2
N
N
N
CH 3
O
O
HNN
N
©2001 Timothy G. Standish
MutationMutationWhen Mistakes Are MadeWhen Mistakes Are Made
5’ 3’
5’
DNAPol.
5’
5’ 3’
5’ 3’
5’
DNAPol.
DNAPol.
Mism
atch
3’ to 5’ Exonuclease activity
©2001 Timothy G. Standish
Thim
ine
Dimer
5’ 3’
3’ 5’
MutationMutationExcision RepairExcision Repair
3’
5’ 3’
5’
5’ 3’
3’ 5’
DNAPol.
DNAPol.
Ligase
Endo-Nuclease
Ligase
Nicks
©2001 Timothy G. Standish
MacromutationsMacromutations Four major types of Macromutations are
recognized:
1 Deletions - Loss of chromosome sections
2 Duplications - Duplication of chromosome sections
3 Inversions - Flipping of parts of chromosomes
4 Translocations - Movement of one part of a chromosome to another part
©2001 Timothy G. Standish
Macromutation - DeletionMacromutation - Deletion
ChromosomeCentromere
A B C D E F G H
Genes
E F
A B C D G H
©2001 Timothy G. Standish
Macromutation - DuplicationMacromutation - Duplication
A B C D E F E F G H
ChromosomeCentromere
A B C D E F G H
Genes
E F
Duplication
©2001 Timothy G. Standish
Macromutation - InversionMacromutation - InversionChromosome
Centromere
A B C D F E G H
Genes
A B C D E F G H
Inversion
©2001 Timothy G. Standish
Macromutation - TranslocationMacromutation - Translocation
A B E F C D G H
ChromosomeCentromere Genes
A B C D E F G H