Repair of DNA Mutations - Lecture 18 Flashcards
What are the types of repair mechanisms for mutations?
○ Direct repair
○ Excision repair
○ Mismatch repair
○ Nonhomologous end joining
What is direct repair?
○ A enzyme that corrects a nucleotide alteration caused by a mutagen
○ Is quite uncommon
What are examples of direct repair?
○ The ADA enzyme in E.coli can removed alkyl groups attached to position 4 of T and position 6 of G
○ The MGMT enzyme in humans can remove alkyl groups from position 6 of G
○ DNA photolyase in E.coli can repair base dimers formed from UV radiation
What is excision repair?
○ The damaged nucleotide is removed and the gap is filled by DNA synthesis
○ Base excision repair: a single altered base is removed
○ Nucleotide excision repair: a longer piece of DNA containing the altered bases is removed
How is base excision repair done in E. coli?
○ Begins with the removal of the damaged base by a DNA glycosylase enzyme
○ AP endonuclease + phosphodiesterase makes a gap by cleaving the sugar
○ DNA polymerase I + DNA ligase fills in the gap
How does the endonuclease work?
○ Damaged nucleotide causes helix distortion
○ UvrAB trimer attaches (UvrA is involved in recognition)
○ UvrA departs and UvrC attaches
○ UvrB and UvrC cuts the segment
○ DNA helicase II removes excised strand
○ UvrB bridges gap (protects) and UvrC detaches
What is mismatch repair?
○ Corrects errors in DNA replication
○ Parent strand contains the correct nucleotide and daughter strand contains the mismatch
How is the parent and daughter strand distinguished in E.coli?
Parent strand is methylated
How is mismatch repair carried out?
○ MutS recognises mismatch and binds to it
○ MutH binds to other side of MutS
○ MutH cuts the DNA
○ Exonuclease and Helicase II removed the segment
○ DNA polymerase I and DNA ligase fills in the gap
What is mismatch repair like in humans?
○ Methylation not involved
○ Base-base mismatches recongised by MutS like proteins
○ Different protein acts as an endonuclease
○ Human mismatch repair enzymes tightly associated with the replication fork so can recognise the daughter strand as it is being synthesised
What is nonhomologous end joining?
○ Correct DNA breaks
○ Telomeres mark natural ends
How is nonhomolgous end joining carried out in humans?
○ Ku proteins attach to the ends of DNA
○ Ku proteins attract one another and bring the broken DNA closer together
○ DNA ligase joins the two ends together
