DNA & Disease: DNA repair

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What are the three main types of DNA repair?
Reversal of Base Damage, Excision of Base Damage, and Repair of Double Strand Breaks
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What are the three types of excision of base damage repair?
MMR (Mismatch Excision Repair), NER (Nucleotide Excision Repair) and BER (Base Excision Repair)
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Describe MMR
Excision of nucleotides that are incorrectly paired with the (correct) nucleotide on the opposite DNA strand
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What causes mispairing?
Limited fidelity of DNA replication machinery
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Which heterodimer recognises the mispairing? how? what other heterodimer could do this?
MUTSalpha= MSH2 and MSH6, by recognising small loops. MUTSbeta= MSH2 and MSH3
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What then binds to this heterodimer?
PMS2 and MLH1 or MLH3
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What happens next?
exonuclease digests adjacent bases and the gap is filled by DNA polymerase and ligase
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Defects in MMR are primarily found in which proteins?
MSH2 and MLH1
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What do they cause?
primarily colon cancer, but also uterine ovarian and gastric
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Describe NER
Excision of approximately 30 bases around the damaged site, used to repair a wide variety of bulky lesions and associated with skin cancer
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list all the proteins in the complete repairosome
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What are the roles of a) XPC-HR23B b) TFIIH c) XPA and RPA
a) recruitment of other proteins b) unwinding DNA locally to create bubble c) recognition of DNA damage
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What is XPG recruited by?
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d) 3' incision, 3-5nts from damage site e) 5' incision, 24-25nts from damage site
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What is therefore the size of the fragment excised?
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What is RFC also known as? What does it do?
"molecular matchmaker"- loads PCNA trimeric circle onto DNA by dissociating the post-incision complex and facilitating association with DNA polE/delta + ligase
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What is the post-incision complex?
all but RPA
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What diseases are associated with defective excision repair?
Xeroderma Pigmentosum (XP) = defect in XPC , Cockayne's syndrome (CS) and Trichothiodystrophy (TTD) ---> all predisposed to skin cancer
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What does Base excision repair entail? What is it used for?
Excision of just one base. used to correct damage that isn't recognised by NER machinery
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What does DNa glycosylase do?
Recognises a single/small subset of chemically altered bases, and hydrolyses the N-glycosidic bond that links base to sugar-phosphate backbone
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What does AP endonuclease together with dRpase do?
removes remaining sugar-phosphate by hydrolysing phosphodiester bond
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What does AP site mean?
Apyramidic/Apurinic sites
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What is dRpase? what does it stand for?
an exonuclease that isn't inhibited by the presence of only sugar-phosphate instead of nt. DeoxyRibosePhospodiesterASE
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What do DNA polymerase and ligase then do?
Restore the DNA sequence
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What are the two forms of Ds-break repair?
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When does HR occur?
during meiosis
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What does HR require?
Extensive sequence homology and multiple proteins
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What does NHEJ require ?
Multi protein complex
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Which is more prone to errors?
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What is reversal of base damage also known as?
Direct repair
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What does it involve?
the breakage and repair of abnormal bonds between bases or nucleotides
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What fixes a) pyrimidine dimers b) 64PPs c) spore photoproduct lyase d) o6-methylguanine
a) DNA photolyase b) 64 PP lyase c) spore PP lyase d) O6 methylguanine DNA metyltransferase (transfers methyl group to cysteine residue on enzyme)
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What are those enzymes all found in?
a) all but placental mammals, b) all but e.coli/yeast/humans c) B. subtilis d) all
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Alternatively to repair... DNA damage can be?
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Describe how
Via translesion synthesis, in which replication machinery bypasses sites of damage
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What DNA polymerases does it use?
'sloppy' polymerases- η, ι
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Card 2


What are the three types of excision of base damage repair?


MMR (Mismatch Excision Repair), NER (Nucleotide Excision Repair) and BER (Base Excision Repair)

Card 3


Describe MMR


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Card 4


What causes mispairing?


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Card 5


Which heterodimer recognises the mispairing? how? what other heterodimer could do this?


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