l18 dna repair
TRANSCRIPT
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DNA REPAIR
ELEMENT 3LECTURE 18
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Significance of
mutation &repairmechanism
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1.Single basealteration
2.Two basealteration
3.Chain breaksDue to ionizing
radiations
4.Cross linkages betweenbases and cross linkagesbetween DNA and Protein
Depurination,Deamination,Alkylation,Insertion, Deletion,
Base substitution
UV light inducedTymine- Thymine dimer
4 types of DNADamage
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Depurination is removing of G or A fromdeoxyribose through hydrolysis of the N- glycosidiclinkage between them
Deamination of cytosine to uracil. [chemical change]--- not corrected C-G pair will be
converted to A-T pair [mutation]
Thymine dimers are formed by UV rays and ionizingradiation.
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DNA REPAIR
Damage to the master molecule must be repaired orthe organisms very life is threatened.
The cell spends a lot of energy on enzymes thatconstantly monitor DNA for mutations.
The proof reading function ofDNA polymerase II ofE.coli identifies copying error and corrects them.
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DNA REPAIR----CONT
In DNA damage, only one of the 2 strands needs tobe repaired.
Error in one strand can be repaired [bases removedand new ones added] by using the other strand as atemplate.
In this way, the correct bases are put in proper placto maintain a proper nucleotide sequence.
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REPAIR MECHANISM
The 4 basic steps in DNA repair are:
(1) Recognize
(2) Remove(3) Resynthesize(4) Religate
--- Enzymes are very specific
--- DNA Polymerase andligase
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DNA REPAIR SYSTEMS
Base Excision RepairNucleotide Excision Repair
Mismatch RepairDouble Strand Break Repair
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BASE EXCISION REPAIR
The amino groups ofcytosineadenine
guanine[EX.] C to U
Are susceptible tospontaneous
deamination
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Uracil DNA glycosylase recognizes the uracil and cleavesthe N- glycosidic bond creating apyrimidinic site. [AP site]
AP endonuclease nicksthe phosphodiester
backbone andexcises(cuts) thesugar-phosphate
residue
DNA polymerasereplaces
the correct base andligase rejoins the DNA
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NUCLEOTIDE EXCISION REPAIR
replace damaged DNA up to 30 bases. [bulky lesion]
Recognizes UV light damage [T-T dimers], damagecaused by smoking, and damage caused by
ionization radiation.
Once the lesion has been located an endonucleaseactivity UvrA, UvrB, UvrC cleaves the modified strand
on both sides of the distortion and the entire lesion isremoved.
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NUCLEOTIDE EXCISION REPAIR
replace damaged DNA up to 30 bases. [bulky lesion]
Recognizes damage byUV light damage [T-T dimers],
smokingionization radiation.
Once the lesion has been located an endonuclease
activity UvrA, UvrB, UvrC cleaves the modified strandon both sides of the distortion and the entire lesion isremoved.
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NUCLEOTIDE REPAIR
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MISMATCH REPAIR
Faulty DNA replication mismatched base pair[Ex] C is copied opposite to A instead of T
Specific proteins MutS which complexes with Mut L, MutHscan daughter strand and pass the mismatched information
to specific GATC endonucleases.
The parent template strand is methylated on GATC sequenceThe daughter strand is initially unmethylated
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MISMATCH REPAIR--CONT
he differential methylation
dentify the strand with wrongucleotide
he GATC endonuclease cuts
he strand bearing the mutation.
n exonuclease then digestshis strand from the GATC
his is followed by resynthesisnd religation.
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DOUBLE STRAND BREAK REPAIR
occurs due to ionization radiation or oxidative freeradical ions.Two proteins are involved---Ku and DNA-PK.These proteins bind to the free ends of DNA.This allows approximation of the separated endsDNA-PK phosphorylates Ku activates helicase,helps in unwinding.The unwound DNA forms base pairs, the extranucleotide tails are removed by exonuclease and thegaps are filled by ligase.
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DOUBLE STRAND BREAK REPAIR
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LEARNING OUTCOMES
At the end of the lecture, you should beable to:* Explain the various types of DNA
Damage* Describe the various repairmechanism to correct the DNA damage
* Explain the defects in DNA repairmechanism
Learning Outcomes
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References
1. Harpers Illustrated Biochemistry 26th edn,Chapter 36: DNA organization, Replicationand Repair
2. Lippincotts Illustrated Reviews Biochemistry 3rd ed, Chapter 29: DNAstructure and Replication
3. Textbook of Biochemistry (Vasudevan andSreekumari) 5th edn
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