Organisms use DNA Repair Systems to Counteract Mutations Flashcards
what enzyme catches 99% errors in DNA repair by proofreading
DNA Polymerase
what mechanism may be activated to cope with errors that remain after proof reading e.g base-base mismatches, small insertions, deletions
mismatch repair mechanism (MMR)
e.g of some cancers MMR defects are found in
leukemias
lymphomas
ovary/prostate
when does post replication repair respond
after damaged DNA has escaped repair and failed to be completely replicated
how might DNA polymerase leave an unreplicated gap on newly synthesised strand
may stall at lesion and skip over it
what protein creates matching gap on donor strand that can be filled by repair synthesis
RecA protein
what’s the repair that reverses UV damage called
photo reactivation repair
how can UV damage to E. coli DNA be partially reversed
if cells are exposed to visible light (mostly blue range)
what enzyme does photoreactivation repair need
photolyase protein
what must the photolyase enzyme do to cleave thymine dimers bonds
absorb a photon of blue light
who lacks photo reactivation repair
humans
what are the 3 steps of base mechanisms involved in excision repair
damage/error is recognised and clipped out
DNA polymerase adds nucleotides
DNA ligase seals
what clips out error, leaving gap on 1 side of helix
an endonuclease
what end of the clipped DNA does DNA polymerase add nucleotides to
free 3’OH end
what enzyme closes the gap in base and nucleotide excision repair
DNA ligase
2 types of excision repair
base excision repair
nucleotides excision repair
what recognises an inappropriately paired base in 1st step of base excision repair
DNA glycosylases
what do DNA glycosylases first do in base excision repair
cut glycosidic bond between target base sugar, forming an apurrinic/apyrimidic site
what enzyme recognises sugar with missing base and makes cut in phosphodiester backbone of site
AP endonuclease
what does nucleotide excision repair repair
bulky DNA regions that alter/ distort double helix
briefly explain nucleotides excision repair in humans
proteins recognise damaged dna and recruit more repair proteins to site
forms excision repair complex to remove long DNA fragment with lesion in from strand
gap filled by DNA polymerase + sealed by DNA ligase
