Repairmechanisms.pptVIP

Repair mechanisms 1. Reversal of damage 2. Excision repair 3. Mismatch repair 4. Recombination repair 5. Error-prone repair 6. Restriction-modification systems 1. Reversal of damage Enzymatically un-do the damage a) Photoreactivation b) Removal of methyl groups Photolyase breaks apart pyrimidine dimers 1a. Photoreactivation Photolyase: binds a pyrimidine dimers and catalyzes a photochemical reaction Breaks the cyclobutane ring and reforms two adjacent T’s 2 subunits, encoded by phrA and phrB. Conversion of 6-O-methyl-dG back to dG 1b. Removal of methyl groups 6-O-methylguanine methyltransferase Recognizes 6-O-methylguanine in DNA, removes the methyl group Transfers the methyl group to an amino acid of the enzyme “suicide” mechanism Encoded by ada gene in E. coli 2. Excision repair General Process: remove damage (base or DNA backbone) ss nick/gap provides 3’OH for DNA Pol I initiation DNA ligase seals nick Nucleotide excision repair: Cut out a segment of DNA around a damaged base. Base excision repair: Cut out the base, then cut next to the apurinic/apyrimidinic site, and let DNA Pol I repair Nucleotide excision repair Discovery of mutants defective in DNA repair polA mutants are defective in repair UvrABC excision repair Cleavage and helicase Fill in with polymerase and ligate Mutations in excision repair in eukaryotes can cause xeroderma pigmentosum (XP) 2b. Base excision repair Excision and filling in by DNA PolI 3. Mismatch repair Action of DNA polymerase III (including proofreading exonuclease) results in 1 misincorporation per 108 bases synthesized. Mismatch repair reduces this rate to 1 change in every 1010 or 1011 bases. Recognize mispaired bases in DNA, e.g. G-T or A-C base pairs These do not cause large distortions in the helix: the mismatch repair system apparently reads the sequence of bases in the DNA. Role of methylation in discriminating parental and progeny strands dam methylase acts on the A of GATC (note that this sequence is symmetical or pseud