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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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