DNA repair I

Question 1

mistakes by DNA polymerase

Answer 1

fixed by proofreading and mismatch repair

Question 2

spontaneous DNA damage

Answer 2

base loss; deamination e.g. uracil formation in DNA

Question 3

chemical or radiation attack

Answer 3

tobacco smoke, UV; thymine dimers, ionising radiation and ROS form single and ds DNA breaks and 8-oxo guanine. Topoisomerases- reversible breaks

Question 4

what is mismatch repair?

Answer 4

repairs DNA replication errors

Question 5

methyltransferase

Answer 5

reverses O^6-MeG methylation

Question 6

base exicision

Answer 6

abasic sites, base changes , nicks

Question 7

nucleotide excision repair

Answer 7

bulky lesions, thymine dimers

Question 8

double strand DNA break repair

Answer 8

• homologous recombinaiton (HR)
• non homologous end joining (NHEJ)

Question 9

cells have multiple DNA polymerases

Answer 9

bacteria (5)
eukaryotes (16)

Question 10

DNA mismatch repairs

Answer 10

• repairs replication errors
• relies on signalling distinguishing parental and newly synthesis DNA strands
• uses complementary DNA strand to correc
• inherited mutation in mismatch repair genes = mutator phenotype and hereditary non-polyposis colon cancer in men

Question 11

what is MutS?

Answer 11

a mismatch DNA repair protein

Question 12

what does MutS do?

Answer 12

kinks and binds DNA region bearing the mismatch

Question 13

what are inherited defects of mismatch repair involved in?

Answer 13

cancer
- results in predisposition to a form of colon cancer; Lynch syndrome (aka; hereditary non-polyposis coli cancer),

Question 14

what is HNPCC characterised by?

Answer 14

extended microsatellite repairs.
mismatch repair is a key anti-cancer mechanism

Question 15

spontaneous damage to DNA

Answer 15

• purine and pyrimidine base loss through hydrolysis; produces an abasic site
• alteration of base structure through demaination; changes base pairing preferences to produce mutations following replication

Question 16

depurination

Answer 16

loss of DNA base by hydrolysis of glycosidic bond between sugar and base in DNA.
10,000 purine bases lost per cell per 24 hours

Question 17

depyrimidination

Answer 17

loss of 500 pyrimidine bases per cell per 24hours

Question 18

base deamination

Answer 18

excess in protein is consumed = removal of an amine group, then coverted into ammonia and expelled by urination

Question 19

DNA damage via ROS

Answer 19

ROS reacts with nitrogenous bases and deoxyribose = significant oxidative reactions.
leads to; mutations, carcinogenics, apoptosis, necrosis and hereditary diseases

Question 20

what is MGMT?

Answer 20

methylguanine methyl transferase
- DNA repair enzyme

Question 20

reversal of guanine methylation by MGMT process

Answer 20

MGMT flips out methylated base, and in stoichiometric reaction, methyl group is transferred irreversibly to active site cystein 145 restoring guanosine structure.
Therefore, MGMT is a ‘suicide’ enzyme, only carries out one reaction turnover

Question 21

what does MGMT reduce?

Answer 21

cancer risk from DNA methylation agents

Question 22

what do base excision repairs do?

Answer 22

fix single altered bases and abasic sites

Question 23

what do human cells have that allows repair?

Answer 23

many DNA glycosylates that selectively release altered bases = abasic site allowing repair
- Uracil N-glycosylase acts by uracil base-flipping mechanism

Question 24

how are bulky DNA adducts removed?

Answer 24

nucleotide excision repair
- helix distorting adduct present > cleavage of DNA fragment ~24nt on 5’side and 5nt on 3’ side of adduct > filling in by pol-delta or epsilon, PCNA and RPA > closure by ligase

Question 25

nucleotide excision repair in nontranscribed region (bulk of DNA)

Answer 25

initially distortion recognised
1. open bubble strucuture formed around a lesion in a reaction that uses ATP-dependent helicase activities of two subunits of TFIIH and involves RPA and XPA.
2. formation of this open complex = specific sites for cutting on the 3’ side by the XPG nuclease and thenon 5’side
3. after a 24- to 32- residue olignucleotide is release, gap is filled in by PCNA-dependent POL epsilon or delta, sealed by a DNA ligase (LIG1)

Question 26

what do mutations in different NER components show?

Answer 26

eight complementation groups identified in xeroderma pigmentosum, a recessive predisposition to skin cancer but not solid tumors.
- 25% of XP cases do not involve faulty NER but rather defect in translesion DNA polymerase

Question 27

what do translesion DNA polymerases allow?

Answer 27

DNA replication past bulky lesion such as thymine dimers.