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MSc Medical Biochemistry,
Transitions(pyrimidine-to-pyrimidine and purine-to-purine)
Transversions(pyrimidine-purine and purine-to-pyrimidine)
Insertions and deletions (a nucleotide or a small number of nucleotides)
★point mutations: mutations that alter a single nucleotide
In E .coli three proteins ( Mut S , Mut L & Mut H ) are rrequired for recognition of the mutation & nicking of the strand . Other cellular enzymes ligase , polymerase & SSBs remove & replace the strand .
MutS scans the DNA, & recognize the mismatch or the distortion in the DNA backbone .
an abasic site
Often mispair with thymine
Reactive oxygen species
O2-, H2O2, OH•
G modification (alkylation & oxidation)
(apurinic/apyrimidinic; recognizes missing base)
Incapable of base-pairing and cause the DNA
polymerse to stop during replication
Thymine dimer by ultraviolet light
1.UvrA and UvrB scan DNA to identify a distortion
2. UvrA leaves the complex,and UvrB melts DNA locally round the distortion
3. UvrC forms a complex with UvrB and creates nicks to the 5’ side of the lesion
4. DNA helicase UvrD releases the single stranded fragment from the duplex, and DNA Pol I and ligase repair and seal the gap
Transcription coupled DNA repair:
nucleotide excision repair system is capable of rescuing RNA polymerase that has been arrested by the presence of lesions in the DNA template