ls3-lec12-dna-damagerepair

ls3-lec12-dna-damagerepair - DNA DAMAGE (lecture 12) Source...

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Unformatted text preview: DNA DAMAGE (lecture 12) Source of damage What/where? How? (mechanism) How is this weird? (impact on replica.on + possible outcomes Result Direct repair mechanism? E.coli Humans UV light (EM radia.on) X‐rays or γ‐rays (EM radia.on) EMS “ethyl‐ methane sulfonate” (en vironmental carcinogen) Spontaneous (intrinsic) Damage: Cause: Pyrimidine dimers UV light Base oxidaJon X‐rays or γ‐rays AlkylaJon EMS (environmental carcinogen) DeaminaJon Spontaneous C U Looks like: X‐ray or gamma‐ray . Very specific (fewer cases) DNA REPAIR (Most cases) DIRECT REPAIR EXCISION REPAIR [only base; not disturbing sugar‐phosphate backbone] BASE excision repair (BER) [remove en2re nucleo2de(s); ie “bulky base” damage] NUCLEOTIDE excision repair (NER) Eukaryo.c E.coli only Photolyase enzyme repairs pyrimidine dimers; light ac.vated Many organisms Repair alkyla.on of guanine O6; enzyme O6‐methylguanine methyltransferase Prokaryo.c (E.coli) Eukaryo.c Prokaryo.c (E.coli) _____________ = excinuclease DNA polymerase I DNA ligase __________________ Mediated by XP proteins (A to G) ____, ____have excinuclease fxn 2 types differ in how damage is detected GG‐NER “Global Genome” ______ senses damage TC‐NER “ Txn Coupled” ______ _______ senses damage AP endonuclease (humans: ____) (humans: __________) DNA polymerase I DNA polymerase I DNA ligase DNA ligase DNA DAMAGE (lecture 12) Source of damage What/where? Pyrimidine dimers (C or T) (adjacent bases) How? (mechanism) Cross‐link adjacent pyrimidine bases = adjacent on SAME STRAND • Ionizing x or γ rays free radicals (from H2O) • Free radicals = highly reac.ve • Oxida.on (‐OH) How is this weird? • Bases hydrogen bond to complementary bases (on opposite DNA strand) • Covalent bonding forms the phosphate‐sugar backbone (impact on replica.on + possible outcomes Result Direct repair mechanism? E.coli Humans UV light (EM radia.on) Can’t “read” dimer 1. block replica.on cell death 2. Random base inser.on; wrong base muta.on Yes Photolyase enzyme; light ac.vates it No (default = excision) X‐rays or γ‐rays (EM radia.on) EMS “ethyl‐ methane sulfonate” (en vironmental carcinogen) OxidaJon of bases Change chemical structure of base gets misread Base pair conversion (misread G for T); GCTA Yes – when ac.ng on O6 of guanine Via O6‐methylguanine methyltransferase is a “suicide enzyme”; makes an irreversible complex wi/substrate (self‐ sacrifice) AlkylaJon • Guanine N7 • Adenine N3 • Electrophile (chemical) donates alkyl group • Addi.on of alkyl groups Abnormal base pairing! Chemical structure of base change site of hydrogen bonding Base pair conversion (transi.on) Ie. GC AT Spontaneous (intrinsic) DeaminaJon of bases • Removal of an amine group (NH2) carbonyl • subs.tu.on cytosine uracil Uracil is not a DNA base!! Base pair conversion CG UA Damage: Cause: Pyrimidine dimers UV light Base oxidaJon X‐rays or γ‐rays AlkylaJon EMS (environmental carcinogen) DeaminaJon Spontaneous C U Looks like: X‐ray or gamma‐ray . Very specific (fewer cases) DNA REPAIR (Most cases) DIRECT REPAIR PhotoreacJvaJon E.coli only Photolyase enzyme repairs pyrimidine dimers; light ac.vated EXCISION REPAIR [only base; not disturbing sugar‐phosphate backbone] Suicide enzyme Many organisms Repair alkyla.on of guanine O6; enzyme O6‐methylguanine methyltransferase BASE excision repair (BER) [remove en2re nucleo2de(s); ie “bulky base” damage] NUCLEOTIDE excision repair (NER) Eukaryo.c Prokaryo.c (E.coli) Eukaryo.c Prokaryo.c (E.coli) uvrABC endonuclease = excinuclease DNA polymerase I DNA ligase DNA glysosylase Mediated by XP proteins (A to G) XPF, XPG have excinuclease fxn 2 types differ in how damage is detected GG‐NER “Global Genome” XPC senses damage TC‐NER “ Txn Coupled” RNA polymera se senses damage AP endonuclease (humans: APEI) DNA phosphodiesterase DNA polymerase I (humans: DNA pol β) DNA polymerase I DNA ligase DNA ligase ...
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