Summary of DNA repair systems

Question 1

Base disincorporation (e.g. from tautomerization)

Answer 1

Proofreading finds/fixes this

proteins required: DNA polymerases with 3-5 exonuclease activity

Question 2

single-strand DNA nicks (radiation; spontaneous hydrolysis)

Answer 2

direct repair

proteins required: DNA ligase

Question 3

UV photoproducts: cyclobutane pyrimidine dimers, 6-4 photoproducts

Answer 3

direct repair

protein: DNA photolyase (but not in humans)

Question 4

O6 alkyl guanine products

Answer 4

direct repair

proteins required: MGMT

Question 5

abnormal bases in DNA (mostly dU); covalently modified bases (alkylation products); spontaneous deamination (C-U, 5’methylC to T)

Answer 5

base excision repair
proteins required: dna glycosidases
Apurinic (AP) endonuclease
DNA pol 1, dna ligase

Question 6

UV photoproducts and other lesions that cause significant structural changes (eg bulky covalent adducts)

Answer 6

nucleotide excision repair
abc exinuclease
dna pol 1
dna ligase

Question 7

base mismatches; short deletions and insertions; base modifications that distort double helix

Answer 7

DNA mismatch repair

need: methylates, MutH, MutL, MutS, exouclease 1; dna pol I or III; dna ligase