DNA REPAIR MECHANISM : DNA REPAIR MECHANISM Presented by
Birendra kumar H.
MS Ramaiah College, Banglore Flow of the Seminar : Flow of the Seminar Damage
Basic mechanism Of Repairing DNA
Base Excision Repair
Nucleotide excision repair
Double stand break
diseases How DNA are Damage : How DNA are Damage Mismatched bases
Polymerase error rate about 1 in 104
Deamination of C to U leading to mismatch
Missing bases. Hydrolysis of purine-deoxyribose bond leading to AP-site.
Structural damage. Dimer formation.
Broken phosphodiester bonds. Chemicals or radiation
REPAIR MECHANISMS NECESSARY FOR SURVIVAL Types of DNA Damage Summarised : Types of DNA Damage Summarised Types of Repair Mechanisms : Types of Repair Mechanisms Damage Reversal- removal of dimers
Double Strand Break- Recombination
SOS Repair Mechanisms- Last line of Defense Proteins Involve in Repair Mechanisms : Proteins Involve in Repair Mechanisms Slide 7: The MutS protein of Escherichia coli
MutS is responsible for recognizing and binding to base pair mismatches, and recruits other key proteins (MutH and MutL) required for repair to the mismatch site. ATP Proteins involve in Base Excision : Proteins involve in Base Excision UvrA and UvrB scan DNA to identify a distortion
UvrA leaves the complex,and UvrB melts DNA locally round the distortion
UvrC forms a complex with UvrB and creates nicks to the 5’ side of the lesion
DNA helicase UvrD releases the single stranded fragment from the duplex, and DNA Pol I and ligase repair and seal the gap Slide 9: Excision repair........ OR Nucleotide excision repair : Nucleotide excision repair Two excinucleases (excision endonucleases) bind DNA at the site of bulky lesion.
One cleaves the 5’ side and the other cleaves the 3’ side of the lesion, and the
DNA segment is removed by a helicase.
DNA polymerase fills in the gap and
DNA ligase seals the nick. Damage Reversal : Damage Reversal Photoreactivation (the enzyme DNA potolyase captures energy from light ) Mismatch Repair : Mismatch Repair Mismatch repair deals with correcting mismatches of the normal bases; that is, failures to maintain normal base pairing (A･T, C･G)
Recognition of a mismatch requires several different proteins including one encoded by MSH2.
Cutting the mismatch out also requires several proteins, including one encoded by MLH1. Double Strand Break Mechanisms : Double Strand Break Mechanisms Damage in the DNA template can lead to DSB formation during
replication The SOS Hypothesis : The SOS Hypothesis Radman 1974 originally proposed an inducible repair system
Requires RecA and a regulator system Repair normally at low level
lexA gene identified as a regulator
But NO increased UV mutagenesis (ie 30 dimers produces no extra mutants). Higher doses required
LOW DOSE - Error-free repair
HIGH DOSE - Error repair INDUCED
LexA is an autoregulated repressor
Represses level of activity of many genes
Collectively called DNA Inducible (din) genes
Includes uvrA,B,C,D and sfi etc...
RecA protease activity; Cleaves LexA
Also CI repressor inducing lysis Slide 15: SOS System in E.coli Low level expression HIGH level expression Diseases : Diseases cellular ultraviolet sensitivity
Werner syndrome (premature aging, retarded growth)
Bloom syndrome (sunlight hypersensitivity)
colon cancer Slide 17: Xeroderma pigmentosum Autosomal recessive mutations in several complementation groups
Extreme sensitivity to sunlight
Predisposition to skin cancer (mean age of skin cancer = 8 yrs vs. 60 for normal population)